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    FAQ or Answers to Frequently Asked Questions                  Section 26
    This is a file containing answers, tips, hints and guidelines associated 
    with recurring  questions asked by photographers.   If you would like to 
    add a tidbit of knowledge to  this list just send it to   ANDPPH@rit.edu 
    who will gladly add it to this collection. For complete table of content
    send message to   ritphoto@rit.edu   with  FAQ$txt  in the Subject: line
                    These files are available in SECTIONS. 
             This is Section 26 and its contents are listed below.
      26.01  -< Aluminum Frames - where to buy them? >- 
      26.02  -< Polarizing and UV filters - Q and A >-
      26.03  -< Diffraction, Depth of Field, Color Temp. and Common Sense >-
      26.04  -< Guide for Forte Films used in Kodak Developers >-
      26.05  -< Making Duplicate Slides with Enlarger >- 
      26.06  -< Duplicating Slides - Procedures and Films >-
      26.07  -< 18% Gray Card Reference Articles >-
      26.08  -< Cross-processing - what goes on? >-
      26.09  -< Commercial Silver Recovery Units FYI >- 

Note 26.01          -< Aluminum Frames - where to buy them? >-
   >some years ago I bought a batch of aluminum frames for a show, now
   >I've lost my source file. anyone know of a good cheap, fast source
   >for 11 x 14 and 16 x 20 frame pieces?
I use Contemporary Frame Co., Dept O, 346 Scott Swamp Rd, Box 514, Farmington,
CT 06032.  For information, call 1-203-677-7787, or to order, call
1-800-243-0386.  Their minimum credit card order was $25 the last time I
ordered, which was about a year ago.  I don't recall how fast they delivered,
but if they have what you want in stock, I'm sure they will expedite it for a
price.  (But then they may cease to be considered 'cheap'.)
The frames I usually get, their style 905 silver ones, cost about $6.61 for a
16x20, and that includes the necessary hardware.  Locally, I'd pay $15 or more
for the same thing.
John Snell  
Try for framing:
Westfall Framing 1-800-874-3164 (FL 800-334-1652)
United Mfrs      1-800-645-7260 (Lots of supplies for a framer)
                 They have a nice simple frame for 5.80 (16x20) Hdwre included.
Both take credit cards. 
K. Mosley (mosley@artsci.wustl.edu)
Frames can be purchased from American Frame Company - (800)537-0944
Marshall Kragen  mkragen@access.digex.net
yet another source is:
Light Impressions
439 Monroe Ave.
Rochester NY 14603-0940
FAX 1-800-828-6216
Note 26.02          -< Polarizing and UV filters - Q and A >-
This file contains the answers to frequently asked questions about polarizing
and UV/skylight filters. I saw so many questions about these filters on news,
I thought it wouldn't hurt to make it a FAQ.
If anyone has comments on this faq or useful supplements, please E-mail them to
                 or    RAG@octopus.et.TUDelft.nl
All the information useful to other news-readers will be added to this FAQ,
mentioning the name of the one who sent it to me. You can also E-mail to the
above address if you still have unanswered questions about filters. Should 
there still be questions that are frequently asked, I'll add them to this FAQ.
This file may be copied and distributed freely, as long as you keep in intact.
So let me get the credit for the time I spent on it and do not just copy parts
of it, for chances are that I will get questions that are already answered in 
this FAQ.
I will try to keep this FAQ in rec.photo.*, posting the new version when this
one is removed from the list.
 update september 28, 1994
- The answer to Q2 of the polarizing filters has been replaced by the FAQ on
  circular polarizers written by TOM DAVIS (davis@bedlam.asd.sgi.com)
- DAVID JACOBSON (jacobson@cello.hpl.hp.com) suggested I should go for the
  wave theory on light instead of the particles. I tried. He also had some
  useful comments I have added to this FAQ, and he pointed out an error in A5
  on the polarizers. I was wrong on the amount of light you loose, but it has
  been corrected now.
- RICHARD KARASH (rkarash@world.std.com) also had some comments on the circular
  polarizer, but they were already in the circular polarizer FAQ by Tom Davis.
- Via news JEFF SPIRER (jffspirer@aol.com) and Dr. GROVER LARKINS
  (larkinsg@solix.fiu.edu) also commented on the polarizers, mostly about the
  circular vs. linear question, and dr. Larkins commented on my advice of
  overexposing film when in doubt of the light. This is corrected. The other
  comments on the circular polarizer were already corrected in the circular
  polarizer faq by Tom Davis.
 update september 29, 1994
- LUCA DE ALFARO (luca@cs.stanford.edu) sent in a test about determining if you
  really need a circular polarizer. Jeff Spirer sent a comment on this, so I
  decided to unite both of them in Q6 and A6 on polarizers.
 update october 3, 1994
- JOE CALI (Joe.Cali@anu.edu.au) sent in an explanation of the so called
  "Brewsters Angle". This is added to the FAQ in Q7 and A7.
 update october 4, 1994
- Rev. Dr. PHIL HERRING (revdoc@wumpus.uow.edu.au) pointed out that even if you 
  use a manual camera, a linear polarizer might disturb your light meter. This
  is added to A2 on polarizers
* I want to thank all of them for their contribution to this FAQ. *
    - Polarizing Filters -
Q1: What does a polarizing filter do?
A1: In order to answer this question, you'll first have to know something about
    light. Light can be regarded as a flow of particles (called photons), or as
    an electromagnetic wave. I'll try to stick to the wave theory. So, light
    can be regarded as a waveform, oscillating in an arbitrary direction 
    perpendicular to its direction of motion. There will be waves oscillating
    up/down, oscillating left/right and all that's in between those two.
    A polaroid filter only allows to pass the component of the oscillation that
    is directed in the polarizing direction of the filter.
    So only the waves that are oscillating in the polarizing direction can pass
    the filter unattenuated. All other waves will attenuate according to the
    Amplitude after polarizing = Amplitude before polarizing * cos (theta),
    where theta is the angle between the oscillating direction of the wave and
    the polarizing direction of the filter.
    The overall effect is blocking out half of the available light, and
    'directing' the other half.
Q2: What is the difference between a linear and a circular polarizer?
A2: The main points are:
    (1) If you use a standard linear polarizer with cameras that use auto-focus
        and/or auto-exposure, you may have problems.
        This filter may also cause trouble with manual cameras, if you're using
        TTL light measurement. A little dependent on the camera, your meter
        might go crazy.
        (It is not said you _will_ get problems, only that you might get them.)
    (2) A circular polarizer can be used on all cameras, and will work the same
        as a linear polarizer with respect to darkening the sky, eliminating
        glare, et cetera -- circular polarizers are just more expensive.
    A circular polarizer is just a linear polarizer followed by a quarter-wave
    plate set at 45 degrees to the axis of polarization.
    A quarter-wave plate is made of a material in which light polarized in one
    particular direction travels more slowly than light polarized in the
    perpendicular direction.  A quarter-wave plate is just thick enough that
    after passing through it, light polarized in one direction is delayed 90
    degrees (or one-quarter wavelength) relative to light polarized in the
    other direction.
    Since the quarter-wave plate is set at 45 degrees to the polarization, you
    can think of the incoming light as having two equal components in the
    principal directions of the quarter-wave plate.  After passing through the
    plate, one component is delayed 90 degrees, and the resulting light is
    circularly polarized.
    The idea is to use a linear polarizer up front to get rid of some linearly
    polarized light you don't want (glare off shiny surfaces, for example, will
    have a large linearly polarized component), and then it "stirs up" the
    result so you don't have linearly polarized light bouncing around in the
    A problem with linearly polarized light in your camera, for example, is
    that when you bounce it off a mirror at (near) Brewster's angle, it may be
    (nearly) completely eliminated.  If the light meter measures the light
    after it bounces off a mirror, the amount of light arriving at the meter
    may be drastically different than the amount of light that will arrive at
    the film with no bounce, since the mirror has flipped out of the way.
    Of course, a quarter-wave plate is only exactly a quarter wave for one
    frequency of light.  That frequency is usually chosen to be a yellow in
    about the middle of the visible spectrum so that on the average, the light
    will be circularly polarized with various degrees of elliptical
    polarization mixed in.  I suppose if you were photographing something that
    was primarily red, or primarily violet, your metering might be slightly
    off, even using a circular polarizer.
    And of course, since there's another chunk of material in the way (the
    quarter-wave plate), there will be slighly more degradation of the image
    with a circular than with a linear polarizer.
    Another nice way to think of circular polarization is to imagine a wave
    travelling down a rope where you hold one end and the other end is tied to
    a wall.  If you shake your end back and forth along a line, the waves will
    all lie in a plane.  You can shake your end in any direction perpendicular
    to the rope, and the only change will be in the direction of the
    polarization.  Now start moving your end around in a circle, and circular
    waves will move down the rope.  This corresponds to circular polarization.
    If you move your hand in an ellipse with various eccentricities, you'll get
    the equivalent of elliptical polarization (with various eccentricities).
    If you're wondering whether your polarizer is circular or not, look through
    your polarizer at a mirror and look at how dark the polarizer is that the
    guy in the mirror is holding.  Reverse the polarizer in your hand so the
    other side of the glass is pointing toward the mirror.  With a circular
    polarizer, one direction will be significantly darker than the other. With
    a linear polarizer, both sould be the same.  The reason is that linearly
    polarized light will still be linearly polarized in the same direction
    after bouncing off the mirror.  Clockwise circularly polarized will be
    counter-clockwise after bouncing off a mirror, and will be cancelled when
    it comes back.
    So if you hold a circular polarizer as if your eye is the camera (with the
    side that's normally screwed into the camera nearest your eye), it'll
    appear light in the mirror.  If you flip it over it should appear almost
    Some manufacturers (B+W and Heliopan, for example) sell a so-called
    Kaesemann polarizer which is even more expensive.  A Kaesmann type has the
    foil stretched and held under constant tension in all directions.  To do
    this it is necessary to totally edge seal the filter in glass rather than
    just bind the glasses and foil with an adhesive. This type of polarizer is
    available in linear, circular and in warmtone types.
    Its advantages are that the polarizing effect is slightly greater, the
    filter is "tropicalized" so it is immune to moisture, fungus, etc and
    it is very, very flat. So it will not adversely effect the sharpness of
    longer lenses. For this reason Heliopan only supplies Kaesmann type
    polarizers in sizes from 82mm up.
Q3: What can I use a polarizing filter for?
A3: The manufacturers will have us believe that you can block out any unwanted
    reflection in glass, water etc. You can *not* block out the reflections in
    metallic surfaces, since they do not polarize the light.
    But, although the manufacturers are right for the greater part, you will
    have to use the polarizer in the right way to get the above effect. This
    means, you will have to take your picture in a direction perpendicular to
    the sun (i.e. the line sun-reflecting surface has to be perpendicular to
    the line camera-surface), as is illustrated below:
    O  sun
       \      o  you
        \    /
         \  /
       ---------  reflecting surface
    You will be able to block out the unwanted reflections this way, dependent
    on the direction of the filter. When you're standing perpendicular to the
    sun, the effect will be maximum, slowly decreasing as you move in line
    with the reflecting surface and the sun. Then the effect will become zero.
    You can also use a polarizer to control the colour of the sky, ranging from
    light blue to dark blue/grey. Since the sky is reflected light and hence
    polarized, you can deepen the blue by removing light reflected of dust in
    the atmosphere(haze). In this way, you are able to let the clouds almost
    disappear or make them better visible. This also works best when you are
    standing on a line perpendicular to the line sun-earth.
Q4: What is the best way to work with a polarizer?
A4: It depends on what you are planning to do. When taking pictures of
    reflecting surfaces, it will give you the possibility to remove the 
    reflections, thus creating a 'better' picture than without the filter. You
    can also use the polarizer to create more contrast in your pictures. The
    best way to find out what you can do with a polarizer is just try it. Use a
    roll of slide film (can't be corrected or ruined during printing) and take
    pictures of the things you normally take pictures of, but now use the
    following system: Take four or five picctures in a row of the same object,
    preferably with just a short period of time between them. The first picture
    should be taken without filter, just for reference. Then start with the 
    filter in an arbitrary position, take a picture, rotate the filter a little
    (about 15 to 20 degrees ) and take the next picture, rotate the filter
    again etc. until you have four or five pictures. Then move on to another
    situation and repeat the above sequence. After developing the film, you
    will see quite remarkable differences between the various positions of the
    filter. Do try to start with the filter in the same starting position each
    time you start on a new series of four or five pictures.
    David Jacobson suggested you just look through the viewfinder of your
    camera, but that won't give you information on what influence the polarizer
    has on the autoexposure or autofocus of your camera (All cameras are equal,
    but some are more equal than others - after George Orwell). Also, there are
    compact cameras and TLR's with a possibility to append filters to them, and
    it is not much use looking through the viewfinder in those situations,
    since you're not looking through the filter. I admit, this is a situation
    that will not occur very often, but I wouldn't say it's impossible.
Q5: What effect does a polarizer have on taking pictures?
A5: As said before, a polarizer can influence the colours in your picture by
    darkening them, it can block out unwanted reflections and it can disturb
    your AF measuring beam or autoexposure (only linear polarizers). Also,
    because it will block about at least half the available light, it will slow
    your film down 1.5 to 2 stops, so if you are using a separate light meter,
    set your ISO dial 1.5 to 2 stops lower to correct for the loss of light.
    (You can also try measuring the amount of light through the filter with
    your light meter, but this is not a very accurate way of calibrating it).
    In this case, just try a few pictures, you'll soon find out what correction
    to use in your particular case.
    In case of doubt: A little overexposure is not as bad as underexposure, so
    if you want to be on the safe side, use the 2 stops correction. Warning:
    this is, if you're using normal film. Slides like to be underexposed a
    little if you're not sure on the amount of light.
    One other comment: the 1.5 to 2 stops is *not* valid for every polarizer.
    Most of them will indeed take 1.5 or 2 stops, but it may happen that your
    polarizer only takes 1 stop, or takes as much as 3 to 3.5 stops. It is 
    totally dependent on the brand and kind of equipment you use.
Q6: How can I find out if I really have to buy a circular polarizer?
A6: It depends on the camera you're using if you need a circular polarizer or 
    not. Most autofocus cameras have a semi-silvered mirror, and this can 
    cause a significant difference in the amount of light reaching the
    photocell when using a linear or circular polarizer. Most of the manual-
    only cameras have their photocell in the prism, and they will not see the
    difference between linear and circular polarizers. So, in case of doubt,
    try a linear filter and -looking through the viewfinder- see if the reading
    of the light meter changes when rotating the polarizer. If it changes more
    than 1/2 stop, use a circular polarizer. This test has the most validity
    if you do it in daylight, looking at a grey wall.
Q7: Why do I keep getting reflections, even if I use a polarizer?
A7: Light which reflects off any surface is polarised to some extent.  The
    degree of polarisation is related to the angle of incidence of light and
    the refractive indices of the two materials.  At a certain angle known as
    "Brewsters angle", light is 100% polarised.  At other angles of incidence
    the light is partly polarised.
    Brewsters angle is given by
            Brewsters angle = arctan( n'/n)
            n' is the refractive index of the material giving off the
               reflection (eg  glass, water).
            n  is the refractive index of the material through which the
               light is incident (eg air).
    Refractive indices of common materials
    Material                 refractive index          Brewsters angle
    air                         n=1                         45
    water                       n=1.333                     53
    glass                       n approx 1.5                56
                             (depends on the glass)
    So enough theory,
    All the surfaces a photographer wants to control lie in the 50 degree
    range.  Say you want to take a picture through a glass window.  If you have
    no filter on you will see a reflection.  If you put on a polarizer and take
    your picture looking straight through the window the reflection will still
    show up.  But if you move around so you are looking through the window at
    an angle of 50 degrees, the reflected light will be 100% polarized.  You
    then rotate the polarizing filter on your camera lens until the reflected
    image disappears.  This is because the direction of polarization can vary
    with respect to the camera depending on the angle of incidence of the
    - UV/Skylight Filters -
Q1: What does an UV or skylight filter do?
A1: Both of the filters filter out the UV light that can cause a blueish haze
    on your pictures, since normal film is not only sensitive to visible light,
    but also to UV.
    A skylight filter is also slightly coloured (pink or yellow), to give your
    pictures a 'warm' appearance (not so much blue).
Q2: What is the difference between a normal UV and a skylight filter?
A2: As mentioned above, the skylight filter is slightly coloured to give your
    pictures a 'warm' appearance.
Q3: In what situation do I use an UV filter?
A3: UV and skylight filters are useful when you are taking pictures in the 
    mountains or at sea or any other place where there is a lot of UV light.
    It will filter out the blue haze that normally blurres the background of
    your picture. It is also very useful when taking pictures in the snow,
    since snow is a very good UV reflector.
Q4: Can I always leave the UV filter on the lens?
A4: About half of all the photographers keep an UV filter on their lens perma-
    nently, for it prevents your lens against dust, scratches and perhaps
    damage due to accidentally dropping the lens. A filter is much cheaper than
    a lens, so ruining your filter will not be as bad as ruining the front part
    of your lens.
    On the other hand, some photograpers (the other half) think it unneccesary
    to keep the filter on the lens, since everything between the original
    picture and your film, including filters, can cause blurrs or errors in the
    image, and that's one of the things we don't want to happen.
    It is just a matter of personal preference. I must admit I always have a
    filter on my lens, and I take it off only for cleaning.
Q5: What effect does an UV or skylight filter have on taking pictures?
A5: First of all, it will filter out most of the UV light. A skylight filter
    will also colour your pictures a little. Because the UV is filtered out,
    you may notice the sky in your pictures is not as deep blue as it used to
    be. This is because the UV component is now missing, resulting in another
    kind of blue on your pictures. Just try a few pictures with and without
    filter and see what you like best.
    An UV or Skylight filter doesn't have any effect on the amount of visible
    light falling through the lens, so you won't have to correct for it.
Permission to include the FAQ, as long as you don't change it without 
noticing me. I'll send you the FAQ again every time I've changed it, so 
you'll be up to date.
Feel free to add your own e-mail address to the header, but don't change 
anything other than that without noticing me. If you get questions about 
the filters that are not answered in the FAQ, please let me know, I'll 
see if I can add them to the FAQ, with an answer, to make this FAQ an 
authority on filters. ("All the answers to every question about 
polarizers and uv filters" is the goal...)
| Lt. Frank Drebin: Nice party, Hapsburg... I see a lot of familiar  |
|                   facelifts.                                       |
|                                                                    |
|                      from: 'Naked Gun 2 1/2: The Smell of Fear'.   |
| Richard Gutteling      |  E-mail: RAGutteling@tudedv.et.TUDelft.nl |
| Lavendelhof 9          |     or : RAG@octopus.et.TUDelft.nl        |
| 2991 HH  BARENDRECHT   +-------------------------------------------+
| THE NETHERLANDS        |  Delft University of Technology           |
| tel. (+)31-1806-14035  |  Dept. of Electrical Engineering          |
Note 26.03      -< Diffraction, Depth of Field and Common Sense >-
         Musings on Diffraction,  Depth of field, and Color Saturation
                                 by John Peele  
         Reference Librarian, Pepperdine Univ.,  School of Law Library         
                       Internet: jpeele@Pepperdine.edu
           "Some would rather be fishin', but I'd rather be FOTOIN'"
Although I located math which YOU can use to correlate diffraction with f/stop
and focal length, I question the value of such data since most cameras using
35mm film do not permit the aperture to be closed to such a small diameter
that diffraction becomes a 'real' concern.  Also, except that the problem may
become 'real' for large film cameras, such as view cameras, extremely rarely
do I close my lenses to where diffraction may occur.  Incidentally, a neutral
density filter can be used to cut down light, while maintaining a 'reasonable'
aperture and/or shutter speed.
Recently, I read in a magazine (paraphrasing) > Good color is worthless
without good subject matter and good composition.< For the majority of my
problem photographs, diffraction and other optical conditions are not as great
a problem as 'subject failure'.
Today, I doubt if you can buy a new camera or lens for that camera in which
diffraction becomes a factor.  Likewise, the other optical aberrations,
spherical aberration, coma, astigmatism, curvature of field, distortion and
chromatic aberration, have been significantly minimized or eliminated from
most lenses of contemporary design.  That is, the majority of lenses today are
designed with the aid of a computer, enabling optical engineers to reduce
aberrations much more effectively than in pre-computer days.  Also new glass
compositions permit these engineers to bend light in ways not dreamed of 'in
the old' days.
Continuing this semi-technical, but I hope informative, discussion. ...
Diffraction affects how light rays are bent as they pass through the lens and
the aperture controlling stop or iris.  Diffraction or bending of light rays
around the edge (interior) of the lens diaphragm, tends beyond a certain point
to limit the improvement in image sharpness that can be produced by making the
aperture smaller.  This occurs because normally the quantity of light through
an aperture opening overwhelms the neglible amount of light which is
diffracted.  Usually, loss of some image sharpness through the effect of
diffraction is compensated for by greater image detail.
Depth of Field is the distance from the nearest point of sharp focus to the
farthest point of sharp focus, and depends on (1) the focal length of the
lens, (2) the lens stop, (3) the distance from the lens to the point focused
on; or camera to subject distance, and (4) the size of the 'circle of
confusion' [See below].  'Hyperfocal distance' is the nearest distance in
focus when the lens is precisely focused for an infinitely distant object.
The 'circle of confusion' describes this whole discussion.  [Oops, wrong
definition!] The 'c of c' is the destruction of a dot such as a period when
light rays image that dot as a tiny circle when focus begins to degrade from
sharp focus.  Such dots make up the total image of the remote subject; and
light rays should register on the film as a faithful representation of those
dots, and thus a small version of the remote image.  When light rays through
different parts of a lens are focused on the film plane, the dot is formed. 
But when focused in front of or behind that film plane, tiny circles are
formed as the rays no longer come to a point.  Visualize the light ray pattern
as an 'X'.  Note how the legs/rays spread out before and after crossing the
film plane.
Image size depends on the film format.  Design criteria for 'c of c' vary
according to film format and, of course, the crispness of the lens. Although
an acceptable limit of the 'c of c' is stated in one source as "1/1000 of the
focal length" of the lens, other sources report up to 1/2000 of the focal
length.  These numbers seem to convert to approximately 1/200 to 1/500 of an
inch.  The eye is understood to see an image with each dot as a point; and the
image is seen as sharp when the 'c of c' is smaller than 1/100th of an inch.
Because the 'c of c' becomes larger than design criteria and will overlap
adjacent circles for image portions out of focus, critical focus should be a
primary concern when seeking well saturated imagery.  Consider that diluted
black becomes grey, as does contaminated white.  Thus, an out-of-focus image
of a black line on a white background, for example, will be recorded with less
edge separation than a comparable sharply focused image.
I believe color saturation to be determined primarily by the film you select,
exposure of that film, and the subject matter being exposed. Images with
primarily bright pastel colors, and few saturated colors and shadows appear
less saturated than images with a full range of light and shadow.
If your 'slides' are continuously over-exposed, expose the film at 1/3 to 1/2
stop less than metered.  I expose my ASA 64 film at ASA 80, but have learned
to expose certain ASA 25 film at ASA 25.  Because of the compensations made in
prints from machine printed negative films, such compensations do not always
provide me 'better' prints.
Use of a sun-shade and a polarizing filter may increase the apparent
saturation in your slides.  [These are subjects for another time.] But be
aware of atmospheric haze, heat distortion as over a desert or the hot
concrete of a highway, light bouncing off of dust or finger prints on external
lens surfaces or bouncing between internal lens element surfaces, and other
diffusion causing factors, which may degrade an image and effectively reduce
color saturation.
In addition to ideas and experience from several years of enjoying
photography, the sources include: 'Focal Encyclopedia of Photography'; and
'Photography - Navy Training Courses/U.S. Government', both from the '60s.
These or similar sources are possibly available at your local library, your
favorite bookstore, or your 'full-service' photography store.
Specifically, in answer to your question, YOU can find a quite thorough
discussion of the math for diffraction in 'Encyclopedia of Physics' edited by
Rita G. Lerner et al.  ISBN 3-527-26954-1; Library of Congress Classification
No. 530.03, and the Dewey No. QC5.E545 1990.
...and this is a follow up by Jim Michael, jmichael@VNET.IBM.COM
                        ...on Diffraction
Well I really didn't want to reinvent the wheel if someone else had already
cranked the numbers through, but...
Just for fun lets look at the resolution of a 1/4" branch of an oak tree at
100 meters. I like pictures of naked trees in winter, especially when you can
see superfine detail. Let's use Rayleigh's criteria to determine the limiting
resolution of a circular aperture and determine the minimum aperture required
to resolve the branch. According to Rayleigh's criteria the limiting angle of
resolution of a circular aperture is
For the discussion,let lambda=555 nanometers which is about the peak spectral
response of the human eye, and theta is 6.35*10**-5 radians. Solving for D
gives a required diameter of 11 mm! Let's assume we are shooting with a 4x5
with a 150mm lens. The f-stop corresponding to the 11mm diameter aperture is
about f-14. Surprising result. Maybe 1/4" is too fine to try to resolve, so
let's see how fine we can resolve at f-32 at the same distance. The required
diameter for f-32 on a 150mm lens is 4.688mm. This implies a value for theta
of 1.444*10**-4 radians. At 100 meters, the smallest thing to be resolved
would be 14mm or a little over 1/2". We haven't looked at any effects due to
optical defects, but only the limiting effect on resolution of size of a
circular aperture.
Note 26.04     -< Guide for Forte Films used in Kodak Developers >-
         Processing instructions for FORTE films in Kodak developers
               all times at 68 degrees F,  all times in minutes
                       D-76      D-76   1:1   Microdol-X 1:1  HC110 Dil B
35mm films
Fortepan 100           5.25         6.75           12            4.6
Fortepan 200           5.25         6.75           12            4.6
Fortepan 400           7.5          9.25           16            7
120 films
Portrait Pan 100       5.75         7.6            14            5.25
Fortepan 100           5.3          7              12.5          5
Fortepan 200           6            8              14.3          5.5
Fortepan 400           7.60         9.5            16.5          7.3
* Small Tank processing is based on 8 to 16 oz of developer per roll.
* Agitation should be continuous for first minute and then 5 to 7 inversion
  cycles in a  5 second time span every 30 seconds for the remainder of the
  development time.
Note 26.05          -< Making Duplicate Slides with Enlarger >- 
    > How do I make duplicates of my slides?

Kodak makes a wonderful dup film in sheet sizes, numbered 6121.  It is used in
the enlarger with tungsten illumination, and the image is projected on the 
easel, just like paper. Of course, since the image is normally down in the
range of 1:1, learning to focus and size is critical. Using shorter focal
lengths than normal is typical, as is sizing with the focus knob and focusing
by moving the enlarger up and down. If you have an omega autofocus, you will
learn to appreciate it, if you don't already.
Also, if you put the original in the carrier emulsion side up instead of down
as usual, the resulting dupe will read right through the back side. Color
balance is handled with whatever filtering system you use for making prints.
While this may seem like more trouble, it offers a lot of flexibility.   You
can make 4x5 dupes from 35mm, 35mm dupes from 4x5, 11x14 dupes if you need
them. You can combine images, etc., too, but I suppose the Mac has made history
out of that. I expect that was how Pete Turner, among others, might have
accomplished some of his images. I loved to do this; I did a poster for the
opera _Faust_ once, combining an old man and a young man into the same person.
It was creepy.  Fun.
Here in Seattle, we have a specialty lab (Duck Island Ltd.) that makes dupes to
precise sizes, strips them together so they can be scanned just once. It
results in huge overall savings for the client.  I suppose the Mac will make
history out of that, too. 
    Larry Bullis
    Shoreline Community College
    From: Larry Bullis 

Note 26.06        -< Duplicating Slides - Procedures and Films >-
> I want to make duplicates of my slides. How would I go about doing this, what
equipment is required and what films to use?
Here is a summary on direct slide dups, plus better detail on the film issues:
Seems there are four options:
1) Slide Dupe Attachment.  A tube w/built-in lens which mounts on the camera
body, and has a slide holder on the end followed by a diffuser. Aim at light
source of choice.  If flash, I guess you'd have to run tests for exposure.
I saw one made by Cambron at ~$80. (Anyone know of others?  Recommendations?)
I assume this is similar to Richard's "Nikon PB-4 w/PS-4," although I'd be
interested in the differences.
2) Extension.  Place between lens and body to give proper magnification; amount
of extension needed is determined by the focal length of the lens used. Mount
camera on copystand or tripod.  Backlight and mask off slide.
3) Enlarger.  Put slide in the enlarger and the camera body, sans lens, on the
platen.  Image slide onto the film plane.  
Focusing for this method is achieved by one of three methods:  1) a nice finder
(e.g., right angle, magnifying, etc.); 2) a custom jig which would  allow rapid
and dependable switching between the camera body and a white  focusing block at
the same height as the film plane; or 3) a custom jig which holds a camera body
w/standard finder and incorporates a mirror to allow you to see into the finder
(this seems easier than the other jig).
4) Duping Device.  I once saw a unit which had a mount for a camera over a
small box having a slide holder, a diffuser, a filtration unit, and  both a
flash and a focusing lamp in the box.  It was nice, but more expensive I'm
sure.  No idea who made it.  (Anyone know of such devices?)
Ok, those are the better methods.  (Reshooting a projected slide is also
possible, but not recommended -- it'll trash the image quality.)  Seems that
option 2 -- extension -- is the cheapest do-it-yourself if you  have a camera.
For films, Kodak has three options:
1) Ektachrome Slide Dupe Film 5071, for originals on E-6, optimized for 1 sec
of 3200k tungsten exposure.  Come in 35mm magazines.  Also  available as 7071
-- same film, different formats.
2) Ektachrome SE Duplicating Film SO-366, for originals on E-6, optimized for
flash.  Also available in 35mm magazines.
3) Kodak 6121, sheet film, balanced for 3200k.
There was also a special dupe film for Kodachrome originals -- Ektachrome Slide
Dupe Film, Type K, 8071 -- but I am told that this is being discontinued.
From: Chris Dobbs 
Another possible set up is one my wife used while working at the photolab at
Brown University, before they sprung for an expensive duper:
use your enlarger's color head, dismounted and upside down (condenser/diffuser
pointing up). a diffuse light source is best, so any color correct diffusion
material can be used if the head doesn't have a diffusion source to begin with.
make a mask that fits the head with an opening large enough illuminate the
original (this prevents any possible flare from the surrounding area). mount
your camera, with the necessary extention to achieve a 1:1 ratio, above the head
on a copy stand or whatever, level the set up, and you're off and duping with a
color correctable light source.
From: Hank Randall <70317.3131@compuserve.com>
>How difficult is it to do contact dupes? I would think this is especially easy
>if the original slide film or negative was uncut. Then just use an enlarger to
>expose 6 frames at a time using direct contact. Will that work?
Yes - you can contact print, but in my experience the dupe image is not as
sharp as with an optical path, and to get reasonable sharpness you must
contact emulsion to emulsion so the dupe is a mirror image.  One thing
further, the original must be emulsion side up in the enlarger for the dupe
to be correct.  Mirror image dupes can give focusing problems when
projected, especially when mixed with other slides.
From: Ross.Wylde-Browne@anu.edu.au (Ross Wylde-Browne)
>RE: unsharp dupes made by contact printing 
> Well, if a normal large area light source is used then, yes, the contact 
> dupe might not be perfectly clear if not done emulsion to emulsion. 
> However, if a very sharp point source of light were used, then I think I 
> can do contact print with the emulsion facing away?
If I were doing this, I would live with the wrong-reading image and note 
the fact on the mount.  In fact, a lot of dupes made in the past were 
made wrong reading.  I don't think there was a convention to supply them 
that way, but there was always a question when working with dupes whether 
they were coming or going.  Often, dupes are encountered that are of 
unknown generations -- and they look like it.  Just how many times has it 
been flopped?
> Do you know what film (brand and model) is low contrast enough to do 
> slide to negative dupes. I want to take negative pictures of my slides.
You will need an internegative film.  Kodak's Vericolor Internegative  film is
available in long roll 35mm and in sheets.  VIN663 is 35mm x 100  feet, 4112 is
the sheet film code.  Vericolor Internegative type 2 is  4114, and comes only
in sheets.  
The catalog I am using is a couple years old.  If anybody has better  data,
maybe you'd send it up?
It used to be that Interneg films were very tricky, requiring testing and 
plotting results -- which most people just weren't prepared to do.  It  was
possible to do very fine work through internegs, but in fact most of  it looked
like garbage because the color crossover was so serious.  I  hope the materials
that are available now are better -- or at least more  forgiving.  
The contrast curve is shaped so that the contrast increases as the  density. 
So you control the contrast by placing the exposure on the curve.
I have not had to use this stuff now for the past almost 20 years.  I have not
missed it at all.  I made good internegatives, but it was somewhat  lacking in
fun as a daily activity.
RE: Small copying unit that holds camera atop a light source ....
Got one sitting on my desk. Bowens Illumitran. Heiland also made a  Repronar.
From: Larry Bullis 
One system I have had  excellent results from is with a Nikon N90 (or any Nikon
that has TTL capability) and a 105mm macro lens that will focus 1:1 such as the
AF unit. Hook a Nikon Slide Copying Adapter (Es-1) on to the end of the 105mm
lens and adjust focus. Use a TTL capable strobe (Nikon SB24 or 24, Sunpak with
NE adapters, etc.) from about 18". Using So366 Slide Dup film I got outstanding
results after the smallest amount of trial and error.
Buy the film in 100 foot rolls for consistent results. Follow Kodak's 
filtration advice and make a ring-around for exposure and filtration for a
typical slide. I've used CP filters in front of the light path with good
results however Kodak will probably recommend the more expensive CC filters. 
Once you zero in, duplication is easy as long as you are copying from the same
film. If you have Kodachromes and Exktachromes to copy you may need to make the
ring-around of each type.
From: edwardk9@umcc.umich.edu
Re; Determining exposure with Slide Dupe Attachment used with flash....
One method is do tests and control exposure by keeping flash at a  measured
distance from slide. Problem: How to adjust for different  effective f-stops at
different magnification?
Because of this, I prefer TTL flash control.
Re: Nikon PB-4 w/PS-4 ...
PB-4 is a bellows. PS-4 is a nice slide holder with milk glass diffuser, 
attaches to PB-4. The "4" stuff is old and discontinued (it's mid 70's era) 
and the current item in Nikon line is PB-6.
> 4) Duping Device.  I once saw a unit which had a mount for a camera over
> a small box having a slide holder, a diffuser, a filtration unit, and 
> both a flash and a focusing lamp in the box.  It was nice, but more expensive
> I'm sure.  No idea who made it.  (Anyone know of such devices?)
Re: Copying devices with built in light, bellows, etc:
It gets even more complicated. For $1000 plus you get a light source,  slide
holder, mount to hold camera and macro lens, and the unit *blends*  x% light
coming through your slide and y% additional light to "flash"  (i.e. fog) your
film to reduce the contrast. These rigs facilitate making  dupes on *normal*
slide film. I've seen these, but never used. (Bowens Illumitran, ed.)
Re: 2) Ektachrome SE Duplicating Film SO-366, for originals on E-6, optimized
for flash.  Also available in 35mm magazines.
This is the one I like. It was very easy to use. One test run and I was
producing dupes that were excellent. An advantage of these special of these
films is that they have less contrast than normal slide films so  your dupes
look like your originals; if you use normal slide film, the  dupes will be
quite a bit more contrasty. You can use this intentionally  to add contrast to
flat slide (at least theoretically...).
You'll need some "color printing filters"; I bought a set of Unicolor  brand.
You can also use gel filters, but they are expensive. Put color  printing
filter between flash and diffuser glass, not between slide and lens.
From: Richard Karash 
Re: making slide-to-negative duplicates on Kodak's Internegative film...
The current film is Kodak Commercial Internegative Film (5325 in 35mm 100 
foot rolls; not available in 36 exp). Kodak tells me it is an improvement 
over Vericolor Internegative Film (still avail??).
The current film still has the very laborious balancing procedure.  Requires a
densitometer. And a test exposure through a step wedge (available from Kodak).
One new wrinkle: Kodak will take your densitometer readings, run them through a
computer program and tell you the recommended exposure (no charge). They also
sell the program.
I took the film, ignored the warnings and procedure, did a couple of tests and
was able to get nice results. A pro in Canada tells me he used Fuji's
internegative film (ITN) with good results ignoring the balancing process. The
resulting prints were sharp and no noticable added grain. Colors were quite
printable. But, I suspect I was lucky and after reading up on all this, stopped
using the film. Read the Kodak publicationbelow if you are going to try to use
the film -- crossed curves apparently can really mess things up, even though I
haven't seen it personally.
Ref: Copying and Duplicating, Kodak publication M-1.
Call Kodak Pro Imaging 1-800-242-2424 ext 19 (US) or 1-800-465-6325 (Canada).
Sorry I don't know rest-of-world phone #'s.
From: Richard Karash 
To Add contrast to low contrast scenes .... duplicate!
It really works. If you photograph astronomical objects under light-polluted (I
don't mean "not-heavy", I mean outdoor lighting) skies, it's one method of
getting a black (or blue or grey, whichever you prefer) sky _and_ a conspicious
object. To achieve this some people even do second generation copies on normal
slide film. This must sound terrible to any terrestial photographer ...
From: kbagschi@astro.uni-bonn.de (Klaus Bagschik)
Re: TTL metering every slide and achieving proper exposure when using TTL 
flash control using a duplicator.
Since different slides have different densities, I prefer to  sort of set the
flash for a "grey card" type of slide, then use the same  exposure for all
other slides. Otherwise, if I TTL every slide I dupe,  then some slides will
have the highlights washed out if the slides were  overall dark, or else lose
dark details if the slides were overall  bright.
From: "Francis T LIM." 
I use Fujichrome CDU/CR56 duplicating film balanced for tungsten light with a
Durst Chromapro. I buy the film in blocks of five 100ft rolls all the same 
batch so that once I,ve adjusted for the new filtration I can keep the trans
pretty close to the new filtration settings. The Fuji duping film is extremely 
red sensitive so I generally give Kodachrome originals between ten and fifteen
more units of cyan filtration than Ektachromes. I dupe a wide range of
materials, and have found that emulsions vary tremendously over the years of
their production. You just develop an eye for variations in emulsion
filtration over time. I might add that the Fuji duping film is rated as the
longest lived duping film when being projected regularly.
Ted Bundy
Arizona State Museum
Note 26.07            -< 18% Gray Card Reference Articles >-
>Information on the 18% Gray Card
Found several articles in my files on the gray card that might be of interest.
All by William Hyzer. 
"Photometering Devices & Techniques." Photomethods 33:8. Aug. 1989
"Reflections on the 18 Gray Card." Photomethods 32:4. April 1990.
"More Reflections on the 18% Gray Card." Photomethods, p. 14. Aug. 1991.
"The origin of the mystical 18% reflectance is to be found in a psychophysical
 visual scale which was experimentally determined anout 60 years ago by the 
 Munsell Color Company." in August 1993 Photomethods. 
    Gene,  "Eugene R. Prince" 
Note 26.08       -< Cross-processing - what goes on? >-
>       I need help in finding information on cross processing film.  For 
> instance, when a slide film is processed through C41 what exactly is 
> happening in the process?  Also what happens when a negative color film 
> is processed through E6?  I am doing a term paper for school and unable 
> to find adequate information on this topic.  Can someone please help?  
In all of today's color film and paper processes (except Ilfochrome/Ciba-
chrome), here is what happens:  In the film or paper in multiple layers are
silver halide particles surrounded by dye coupler molecules.  These dye
coupler molecules could be thought of as part of a dye molecule, with the
other part supplied by the color developing agent in the color developer.
When the color developer starts to reduce (un-oxidize) the exposed silver
halide particles (to form metallic silver), development by-products in the
vicinity of the developing silver halide particles combine with the dye
coupler molecules to form a visible dye.  Depending on what layer of the
film or paper this is taking place, magenta, yellow or cyan dye would be
produced.  After this step, the silver has done its job and is no longer
needed in the film or paper.  The bleach re-oxidizes it to silver halide,
and the fixer removes it.
The only difference between a negative process (color negative film or
color negative print paper) and a positive process (color slide film or
color positive print paper) happens BEFORE the film or paper gets to the
color developer.  In a positive color process (like E-6), the exposed
silver halides are developed in a black-and-white developer (the FIRST
developer), producing a negative black-and-white image.  The color couplers 
aren't affected. Then, the UNEXPOSED and UNDEVELOPED silver halide particles 
are chemically fogged (or in the old days were exposed to light) and rendered 
developable.  This is the "reversal" step which results in a positive image.  
The film goes to the COLOR developer where the unexposed, undeveloped but
chemically fogged particles are developed, forming the dye molecules in their
vicinity, as described in the preceeding paragraph.  From here on the
process is essentially the same as color negative process.
Now to answer your questions.  When a color slide film is processed in C-41,
a color negative image results because you are "jumping" into the process
without going through the FIRST developer or REVERSAL.  This color negative
image will be high contrast, very color-saturated, and without the orange
mask familiar in color negative films.  I have fiddled around with such
images, usually sandwiching them with the orange mask of color negative film
and made prints.  The prints are wierd, but are fun to play with for
surrealistic results.  Color slide film wasn't designed to be used in this
way, so the results are highly unpredictable.  (BTW, if you want to do this,
underexpose the film by 1 or 2 stops).
When a color negative film is processed in E-6, a positive image WILL result,
but I don't think the results would be very pleasing because color negative
films have that built-in orange mask.
I hope this answers your questions.  Did I go too deep, or not deep enough?
Are you doing your own processing?  Commercial photofinishers get very
confused when you ask them to cross-process film.
If you need more information, send me your snail-mail address and I can send
you some more articles.
Ron Speirs, Evans & Sutherland Computer Corp., Salt Lake City, Utah
rspeirs@velara.sim.es.com       or      rspeirs@es.com
Note 26.09        -< Commercial Silver Recovery Units FYI >-
>Besides just flowing used fixer through wads of steel wool so that silver
>replaces iron and iron washes down the drain, is there another way to recover
>silver from used fixer?
For those in need of reducing the amount of silver exhausted fixer for disposal
and recovery there is a beautufull electrical silver recovery unit for
*smaller* labs. It is Marketed by Byers Industries in Portland  Oregon. The
recovered silver is in the range of 95+% pure silver and may be marketed for
further purification at a higher *rate of return* than the iron or zinc
exchange method. Their address is: Byers Industries, Inc., 6955 SW Sandburg 
Sreet., Portland, OR 97223-8092 ph (503)639-0620 or (800)547-9670 and their 
fax is (503)620-4798
According to the last brochure I received (1992) the make units to recover the
following Capacities (in Troy oz) 25, 50, 60, 120,  180, and 360. The recovery
is 98% flake silver. Unit prices as of my brochure range from $390 to $2500
(i am Not Affiliated with the above organisation)
===========================  end of section 26 ========================== 

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