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    FAQ or Answers to Frequently Asked Questions                  Section 28
          Please check "root" (faq$txt) file for acknowledgements. 

    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 28 and its contents are listed below.
      28.1   -< Lost Film Leader Retriever - Making Improvised One >-
      28.2   -< Using Camera Meter to determine Foot Candles >-
      28.3   -< Reloading Unreloadable Cassettes with Bulk Film >-
      28.4   -< Circular vs. Linear Polarizers - more scoop >-
      28.5   -< Depth of Field - a formula approach >-
      28.6   -< pointer on better photographs of nudes >-
      28.7   -< Exposure Correction in Enlarging >-
      28.8   -< Pinholes, f#s and proper exposure Determination >-
      28.9   -< Optimum Pinhole Diameter - Further Suggestions >-
      28.10  -< Painting with Light basics >-
      28.11  -< IR _BLOCKING_ filters - what/where/why? >-
      28.12  -< Underwater Dome Ports - a mathematical approach >-
      28.13  -< Catadioptric Lenses - brief description >-
      28.14  -< Tintype Parlor - tintype materials suppliers >-
      28.15  -< Adhering Liquid Light to Glass >-
      28.16  -< Pro School Photographers Association info >-
      28.17  -< ISO, DIN and ASA speed relationships >-

Note 28.1    -< Lost Film Leader Retriever - Making Improvised One >-
       Making a film retriever to use when you need to retrieve film 
           that you have rewound completely back into its casette
Basically the concept is that you cut the sprocket hole areas in such a manner
that you generate a "sharkskin"-like edke to the film.  The idea is that these
"barbs" slide easily into the casette but which  engage  in the sprocket holes
of the film inside the casette when pulled out. Below is a basic illustration:
|  __     __     __     __     __     __     __     __     __     __     __  
| |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  | 
| |__|   |_ |   |__|   |__|   |__|   |__|   |__|   |__|   |__|   |__|   |__| 
|                                this is a piece of regular scrap film!
|  __     __     __     __     __     __     __     __     __     __     __  
| |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |  | 
| |__|   |_ |   |__|   |__|   |__|   |__|   |__|   |__|   |__|   |__|   |__| 
               this is what you make out of the film above!
/  __     __     __     __     __     __     __     __     __     __     __  
| |   \. |  |   |   \. |  |   |   \. |  |   |   \. |  |   |   \. |  |   |  | 
| |_____\|__|   |_____\|__|   |_____\|__|   |_____\|__|   |_____\|__|   |__| 
|                                  ^
|                                  |
|                                bend "barbs" _down_  towards emulsion side   
|             insert             notice then they will  slide  into casette  
| <---  this end into casette    easily but on way back will catch sprocket  
|             first              holes 
|                                  |
|                                  V
| _________     _________     _________     _________     ______ __     __  
| |    ./|  |   |    ./|  |   |    ./|  |   |    ./|  |   |    ./|  |   |  | 
| |__ /  |_ |   |__ /  |__|   |__ /  |__|   |__ /  |__|   |__ /  |__|   |__| 
You can usually just use one piece of this sharksin film and stick it as much
as will go into the casette. Then wind the core until you feel resistance. 
Then pull it out and the film may come out with it.
Or, use two pieces of film, one plain with no sharskin pattern cut into it and
the other which is modified as shown above.  Insert a length of  plain film as
far as it will go into the casette ( usually it will only make one turn inside
the casetten and get stopped by the felt. Then insert the modified film "under"
the plain one  and proceed as above.  The function of the _plain_  piece is to
smooth out the inside of the casette so the  leader  will not have a chance to
get caught on the felt light trap's edge.
I hope this works for you! 
andy, andpph@rit.edu 
Note 28.2      -< Using Camera Meter to determine Foot Candles >-
>Can somebody tell me how I can measure footcandles using my camera's light
Here is a rough way to do this ...
Get an 18% grey card. Set your film speed to ISO 125. Set your shutter speed
to 1/125 second. Make a meter reading using standard care to avoid glare and 
casting shadows.
If you were out in broad daylight and no clouds (sunny 16 conditions) you would
be under 6400 foot candles if the camera suggested an aperture of f:16.  If the 
camera suggests f:11, you are under 3200 foot candles, etc. as follows:
f:32   25,600
f:22   12,800
f:16     6400
f:11     3200
f: 8     1600
F:5.6     800
f: 4      400
F:2.8     200
f: 2      100
f:1.4      50
If you change your shutter speed to 1 second then at f:16  you'd find 50 foot
candles and the progression would go like this:
f:32      200
f:22      100
f:16       50
f:11       25
f: 8       12
f:5.6       6.25
F: 4        3.12
f:2.8       1.55
F: 2         .80
f:1.4        .40
All of this is based on the fact that we "know" that the sunny 16 condition is
set up for approximately 6400 foot candles and that then the following
relationship holds true for correct exposure assuming no RLF.
                    25   times  f# squared               25 is a "constant"
Foot candles =   ---------------------------            sometimes 20 is used
                   ISO  times  exposure time           (calibration suggested)
                                         25  x 16 x  16
for example you see you'd get 6400 if   ----------------
                                          125  x 1/125 
corrections most welcome!
Andrew Davidhazy, at RIT's Imaging and Photo Tech Dept, andpph@rit.edu
Note 28.3     -< Reloading Unreloadable Cassettes with Bulk Film >-
>When reloading bulk film currently  I'm using these  cassettes that make me 
>real nervous.  I've noticed that the  caps appear to be always in danger of
>popping or falling off ... yikes! Is there someone that would recommend one 
>cassette over the other for those that roll their own film.?
I almost hesitate to mention this again 'cause it is a pet procedure of mine
but I have found that the best reusable cassettes are the supposedly
unreloadable ones the manufacturers put their film in.
I either use ones I've saved myself or ask for them at a local photofinisher.
When I unload them I pull the film out of the cassette (most photofinishers do
the same) and cut the film away from the cassette leaving a "stub" about 3/4
inches long protruding from the cassette.
To reload the cassette I slip a piece new film under the stub and into the
cassette and affix the two with this 3M translucent tape. This piece only joins
the backs of the two pieces of film but runs the width of the film. The basis
on which I do this is that the shear strength of the tape "connection" is very
Since I reload the kind of film into the cassettes that they held in the first
place the DX code on the cassettes works properly. The film is also secure from
accidental falls on hard floors, etc.
Andy Davidhazy, at RIT's Imaging and Photo Tech Dept, andpph@rit.edu 
Note 28.4        -< Circular vs. Linear Polarizers - more scoop >-
>What is the difference between linear vs circular polarizing filters?
Linear polarizers can be made to "interfere" and thus remove linearly polarized
light such as that caused by glare off non metallic surfaces. At the same time
the linear polarizer imparts "linear" polarization to the light rays that pass
through it.
If you place a second linear polarizer behind the first one you can arrange
their orientation so that most of the light arriving to the first one is 
extinguished by the second one.
Circular polarizers can do the same but the light that leaves them is not
polarized. This is because it consists of two "devices" sandwiched together.
The first, located closest to the subject or source of glare, is a plain linear
polarizer. Behind it, closer to the camera lens, is an optical retarder whose
function is to make the exiting light circularly polarized basically meaning
that the exiting light rays are not polarized.
If you place a linear polarizer behind a circular polarizer you will find you
can not diminish the intensity of the light rays by turning one with respect to
the other (which you can do with two linear polarizers "in series").
OTOH if you place the linear one in front of the circular one then the familiar
"variable density" effect will be exhibited.
It is interesting to note that a linear polarizer can be turned around and
perform its function of eliminating glare regardless of which side is facing
the subject. A circular polarizer _must_ be used in the correct orientation for
it to work.
Andrew Davidhazy, at RIT's Imaging and Photo Tech Dept.
>What is the difference between linear vs circular polarizing filters?
This may be more than you wanted to know, but here it is anyhow:
Light always has a polarization direction, crossways to the direction
it's travelling. You can think of it as being like arrows flying
sideways, where the tip of each arrow is pointing in the polarization
direction. (Actually, for this discussion, the arrows have tips on
both ends.) For most light, the polarization direction is constantly
changing in a random way, and we call it unpolarized light.  When more
of the arrows point one way than another, we call the light polarized.
The _strength_ of polarization can vary - maybe there is only a slight
excess of arrows pointing in some one direction, or maybe all of them
point the same. 
Most of the things you take pictures of are emitting (or reflecting,
or scattering) _unpolarized_ light towards the camera. But some things
give polarized light.  Reflections from water, glass, and many other
surfaces are partially polarized. Skylight is polarized, with varying
strengths depending on angle from the Sun. 
A linear polarizer allows light that's polarized in it's
characteristic direction to pass through. Light that's polarized
crossways to the polarizer's direction is absorbed. And light that's
polarized in between is partially passed and partially absorbed. The
part that passes through is weaker, but is now polarized in the
polarizer's direction.
Linear polarizers are used to eliminate the parts of the light that
are polarized in some direction.  So you can, for example, eliminate
the reflections from a pool of water, or darken the sky, by using a
So where do circular polarizers come in? Modern SLRs, by and large,
use components that are sensitive to polarization in both the light
metering system and the autofocus system. So if you're using a
polarizer to modify the photograph, you may wind up confusing the
camera. The solution is to use a circular polarizer. This is a
sandwich consisting of first, a linear polarizer, and second, a
quarter-wave plate. The linear polarizer does all the good stuff you
want a polarizer for in the first place, but the light coming out of
it is polarized. This doesn't matter to the _film_, but will confuse
the camera body. The 1/4 wave plate solves this problem. It is able to
take those arrows that are coming out of the linear polarizer, and
start them spinning, so their tips move in circles. That way, the
camera's internal systems get light that, _on average_ is polarized in
every direction equally, and for the camera's purpose this is
equivalent to unpolarized light.
Circular polarizers are more expensive than linear ones because they
have to include both the linear polarizer and the quarter-wave plate.
William Tyler
Note 28.5          -< Depth of Field - a formula approach >-
>Does any one have a formula for depth of field based on Focal length of lens, 
>film format and f-stop?  I was out shooting this weekend in a stand of pines 
>and the combination of deep shade and overcast sky made it impossible to see 
>much on the ground glass when the lens was stopped down. With the above info
>I could make a spread sheet and carry a print while shooting.
A method of calculating the nearest and farthest point in focus is:
        Nearest point=UxFsquared/[(Uxcxf)+Fsquared]
        Farthest point=UxFsquared/[(Uxcxf)-Fsquared]
where   U = subject distance in mm
        F = focal length of lens (mm)
        c = circle of confusion (assumed to be 0.036mm for a 35mm neg
            enlarged to 8x10)
        f = aperture
This gives you an answer in mm - if you subtract the nearest point in 
focus from the farthest point in focus you will get the depth of field.
I hope that you can understand the way I have typed the above formulae.
"x" = multiply and "Fsquared" = FxF.
I haven't used this method myself but it comes from a great little pocket 
handbook called "The Professional Guide to Photo Data" by Richard Platt 
(Mitchell Beazley 1991)
from: Guy Newcomb, Brisbane, Australia, (G.Newcomb@mailbox.uq.oz.au)
Note 28.6      -< pointer on better photographs of nudes >-
                        Tips on Making Better Nudes   
                               Frank Wallis
    I have several suggestions for better images, or at least some things
    to think about. Before I do that, I recommend devouring as  many books
    on the subject as possible, and not "how to" books, but monographs and
    anthologies of nudes.
    Here are some things to try:
    1) try torso shots in closer, with the model's arms raised
    2) move around the model's torso, or have her/him turn 1/4 turns,
        in order to find the most pleasing view. Finding pleasing
        views is one of my favorite activities.
    3) move in closer, or use a longer lens, to fill the frame with
        fascinating, interesting, pleasing forms
    4) for 3/4 or full body shots, move the model at least six feet
        from the backdrop. We want to see the model, not the backdrop.
    5) try props, which can be anything really (flowers, whips, balls)
    6) try articles of clothing; dressing or undressing
    7) try "drapery": gauze, fishnets, sheets, etc. (although I rarely
        see effective use of this, and "drapery" usually comes off
        shopworn and even silly).
    8) try a theme and attempt to make a portfolio based on this theme:
        such themes can sound absurdly simple sometimes, but photogs
        such as Christian Vogt have used sticks, bathrooms, posing
        blocks, hats, masks, etc.
    9) try two models; woman/woman, man/woman, etc.
    10) tie models up with rope, chains, scarves (with their permission!)
    11) explore the graphic possibilities of fetish gear, e.g., latex,
        rubber, leather, high heels
    12) cast patterned shadows upon the model by employing screens of
        various shapes and designs, placed between the light source and
        model. This can be done with natural or artificial light.
    13) use natural light; place the model near a window, or pose them
        outside if conditions permit. I've had models pose nude on the
        streets of Manhattan, but you must not assume the police will
        look the other way if they observe such an event.
    14) try different lenses, filters
    15) if you find a model you like working with then work with her/him
        a dozen times if you wish. Who said you need to have a hundred
        different models? Let Playboy Studios operate the factory.
    16) your model doesn't have to smile, but if you really want a smile
        I hope you have a quick wit. 
    17) no grinning allowed. It looks like your model is holding back. 
        Tell her/him to relax the mouth, jaw, lips, etc. Of course, if
        you are concentrating on a torso or body fragment, then the 
        facial expression is irrelevant.
    18) try different printing techniques on the same negative
    19) have the model sign a release and try to get your work published.
        What good will your creativity do if civilization is denied the
        opportunity to learn from it and appreciate it?
    Hope this has been of some use.
    Frank Wallis, 
Note 28.7            -< Exposure Correction in Enlarging >-
>i would like to know if there is any formula for darkroom enlargement. for 
>instance, 8x10 b&w printing required f8 and 15sec. what would it be for 11x14?
Yes there is. The correction you need to make assuming you already know what
the correct time is at a given sized enlargement is as follows:
Take the new dimension and divide it by the old dimension and then square that
number. This gives you factor by which the exposure needs to be changed.
You can then either change the exposure time or the aperture to match the
desired factor. However, keep in mind that papers are susceptible to
failing to follow the reciprocity law and you may need to fine tune any changes
you make in exposure time to allow for this.
For example:
to go from 8x10 made at f8 and 15 sec. to 11x14 you do this:
11 divided by 8 equals about 1.4   1.4 squared is equal to 2  therefore you
need to open up the lens to 5.6 and keep the exposure time at 15 seconds or
keep the aperture at f8 and change the time to 30 seconds. But probably you'd
change to something like 35 seconds to allow for RLF.
to go to a 4x5 from the same 8x10 you'd do this:
4 divided by 8 is equal to .5   .5 times .5 equals  .25  therefore you need
1/4 of the original exposure which you get by stopping the lens down to f:16 or
cutting the time down to about 4 seconds.
Andy Davidhazy
Note 28.8   -< Pinholes, f#s and proper exposure Determination >-
>I am making a pinhole camera and I have calculated my f#'s to be these with
>various distances between my pinhole and the film ... making a zoom pinhole.
>Pinhole dia. (mm)      Film-hole dist (mm)     f/
>0.15                   10                      67
>0.15                   15                      100
>0.15                   20                      133
>0.15                   30                      200
>0.4                    60                      150
>0.4                    80                      200
>0.4                    100                     250
>0.4                    120                     300
>0.4                    200                     500
>How would I use the reciprocal rule to calculate the exposure time on polaroid
>film if my meter said f/16 at 1/250 second??
It is not clear what you mean by the reciprocal rule. If you simply want to
find out what the exposure time would be at a "new" (pinhole) f# given the
exposure time at f:16 then you do this.
 new f# divided by old f# squared multiplied by old exposure time = new time
for example:
     32                16                   1/250                     1/60
     64                16                   1/250           16/250 or 1/15
    128                16                   1/250           64/250 or 1/4
    256                16                   1/250          256/250 or 1 
now this is likely to lead to underexposure due to the fact that Polaroid
materials exhibit significant failure in responding appropriately to the 
Reciprocity Law. This states that "exposure" remains constant if an increase 
in exposure time is matched by a corresponding decrease in aperture. Well,
photographic materials do not produce the same _density_ as one changes the
components that make up exposure.
So, for materials that exhibit long exposure time "reciprocity failure" one
must add _extra_ exposure to achieve a particular density. This can be done
most effectively by increasing the aperture (impractical in your case) or
increasing the time. The factor by which the time component needs to be
increased given a particular starting time varies from emulsion to emulsion.
Actually for Polaroid Type 55 film it seems that the adjustment with either
aperture or time given a starting exposure time of 10 seconds is +1/3 stop or
+3 seconds. At a 1 second starting time the film exhibits no reciprocity
failure according to a Polaroid Data sheet I am reading.
i am not sure if any of this is "on target" ...
Andrew Davidhazy, at RIT's Imaging and Photo Tech Dept., andpph@rit.edu
Note 28.9   -< Optimum Pinhole Diameter - Further Suggestions >-
I have done some research into correct size of pinhole for a given image 
plane distance.
The optimum pinhole diameter is a tradeoff between 'geometric' resolution
and 'diffractive' resolution.  The former is the fact that the minimum
'pixel' size is equal to the pinhole diameter (thus smaller being better)
and the latter is the fact that the resolution of any optical system is
proportional to its aperture (thus larger is better).
For those that don't want to read further, the formula is
  d = squareroot( 1.22 L w )
where d is the pinhole diameter, L is the distance from pinhole to
image plane, and w is the wavelength of light.  The units don't matter
so long as you are consistent.  E.g., in millimetres, w = 0.0005mm.
If L = 250mm, then d = 0.38mm.  The f/ratio is
  f = L / d
which is f/645 in this example.
Now for the details...  The above formula is based on the summation 
of formulas for the angular resolution for geometric and diffractive
considerations.  This is bound to be on the conservative side.  In
practise, the image should have better resolution than this, but 
don't count on it.
The formula for angular resolution (geometric) is
  phi = d / L
which is based on to point sources being resolvable if their image
circles do not overlap.  phi is the minimum resolvable angular
separation of two point sources (in radians).
For diffractive resolution,
  phi = 1.22 w / d
which is based on the Raleigh criterion.
Summing these formulae gives
  phi = 1.22 w / d  +  d / L
which gives an optimum (minimum) phi when d = sqrt( 1.22 L w ).  This
information was largely obtained from 'Optics' by Hecht and Zajac,
Addison Wesley press.
Stephen J Hardy,
Canberra, Australia
Note 28.10              -< Painting with Light basics >-
> I do not have a problem determining the exposure for night time pictures,
> but I am a little confused on how to determine the exposure time or rather
> flash output when playing around with adding light bursts to different
> parts of a night time scene. This is an activity that I am doing wiht my
> high school students for a fun photoclub activity. I can't even remember
> what I have done in the past, but I know that the results were not
> consistent. Does anybody have any suggestions? 
Two simple suggestions to determine the correct flash exposure.
1) Use the guide number of the flash and divide it by the f-stop you are 
exposing the night shot by and that will give you the distance in feet 
(as long as the guide number rating is for feet also). If you then want 
to make it brighter or dimmer you can either move closer or further or 
use power ratio. 
2) Simply take a flash meter reading and change the distance until you 
get the f-stop to match the f-stop used for the night exposure. Shutter 
speed will not play a part as long as you are slower than the synch speed.
I assume in night photography that will always be the case.
David Litschel
Brooks Institute
Note 28.11        -< IR _BLOCKING_ filters - what/where/why? >-
>>I'm doing a camera project and I need some help with an IR filter.
>>I need a filter that blocks IR and passes visible light. Ideally,
>>it would fit on a C-mount camera lens (often used on closed circuit
>>TV cameras). Hoya makes a material called CM-500, which is billed
>>as a Cyan color compensating filter, that would work fine. But,
>>I can't find anyone who sells this stuff. Apparently it is not part
>>of their normal photographic filter line. Does anyone know of such
>>an IR filter, or of a supplier for the Hoya CM-500 material?
>Depending on the quality of the filter and wavelengths to be eliminated you
>could try a heat absorbing glass from a slide projector. Some of these are
>pretty good short wave IR blockers (and some are not so good).
>Otherwise you could try making a "liquid filter" with a solution of Copper
>Sulfate (the liquid will have a cyanish-greenish look to it).
>Or, you could contact Corning Corporation or Pittsburgh Glass (I think) and see
>what they have available as either rough stock or polished material. This is
>the expensive solution.
>from: andrew davidhazy, andpph@rit.edu
I am not disputing any of the above just adding to it.  I think what you may 
need is what is known as a HOT MIRROR, or infrared cuttoff filter.   Heat 
absorbing glass removes some infrared by absorbtion but the transition from 
visible to IR is  gradual not a sharp cut so you will still get some near IR 
transmitted.  A hot mirror is an interference type filter which reflects the 
IR (in the region 700 to 1000 nm) but transmits the visible to about 650nm 
with a fairly sharp transition.  My 1978 Kodak Filters Catalog lists the type 
301A  filter which is of this type.  The Oriel Optics Catalog also lists 
filters of this type as do other suppliers of laboratory optical components.
In the Kodak carousel slide projector there is such an 
interference type filter with a sharp transition which appears to cut off at 
about 700nm.  I expect that cold mirrors would only be available 
as square glass types.
 As for the hoya material check the IR transmission curves first as all dyed 
filters I have come across transmitt freely in the near IR,  even ND filters.
from: djmcmill@trl.oz.au (DESMOND J MCMILLAN)
Note 28.12    -< Underwater Dome Ports - a mathematical approach >-
        Mathematical Characterization of Underwater Dome Ports
You can get more insight into dome optics by doing a little quantitative
analysis. Most books on optics contain at least one chapter on thick
lenses. In particular, chapter 5 in "Fundamentals of Optics" by Jenkins 
and White has some general formulas which are directly applicable to
a concentric spherical dome lens. In the following, I assume a left to
right arrangement of water/dome/air with respective indices of refraction 
n/np/npp. The dome's external surface (on the left) has radius r1, the 
internal surface has radius r2, and the thickness of the dome is d=r1-r2. 
For an object to the left of the dome, the following 15 lines of Fortran 
code will compute the position and size of the image.
(all variables declared real) 
 f1=n*(r1/(np-n))     !primary focal length of dome's external surface
 f1p=f1*np/n          !secondary focal length of dome's external surface
 f2p=np*(r2/(npp-np)) !primary focal length of dome's internal surface
 f2pp=f2p*npp/np      !secondary focal length of dome's internal surface 
 enovrf=np/f1p + npp/f2pp - (d/f1p)*(npp/f2pp)
 f=n/enovrf           !primary focal length of the dome
 fpp=f*npp/n          !secondary focal length of the dome
 a1f=-f*(1.-d/f2p)    !position of the dome's primary focal plane relative
                      !to the dome's external vertex (on the dome axis) 
 a2fpp=fpp*(1.-d/f1p) !position of the dome's secondary focal plane relative 
                      !to the dome's internal vertex (on the dome axis) 
 a1h=f*d/f2p          !position of the dome's primary principal plane relative
                      !to the dome's external vertex (on the dome axis) 
 a2hpp=-fpp*d/f1p     !position of the dome's primary principal plane relative
                      !to the dome's internal vertex (on the dome axis) 
 s=p+a1h              !p=object distance relative to external vertex
                      !s=object distance relative to primary principal plane 
 spp=npp/(n/f - n/s)  !image distance relative to secondary principal plane
 ppp=spp+a2hpp+d      !image distance relative to external vertex 
 mag=abs(spp/s)       !size of image relative to size of object
(These equations assume paraxial rays, i.e. rays making small angles with the 
dome axis). If you substitute some reasonable values for the indices of 
refraction and dome radii, you will find:
     - the dome focal lengths are both negative, indicating a diverging lens
     - the image distance "ppp" is negative, indicating that the image is to 
       the LEFT of the dome, and therefore the image is VIRTUAL
     - the virtual image is always upright and always smaller the the object
Consider an example: 
          n=1.33 (fresh water), np=1.5 (glass), npp=1.0 (air) 
          r1=3.0 inches (a 3-inch dome), r2=2.5 inches (half-inch thick)
The secondary focal plane, where objects at infinity have their images
(apparently) focussed is 6.53 inches to the left of the dome's external
vertex. An object 6 inches from the dome is imaged at 2.3 inches from
the dome with a magnification ratio of 0.44. And so on.
If the primary principal plane of the camera lens is positioned at the center
of curvature of the dome, which I think is approximately the case by design,
then the distance of the virtual image from "the lens" is (for this example)
6.53+r1=9.53 inches. "Twice the diameter" is 4*r1=12 inches, which is in the
ballpark. The rule of thumb obviously depends on camera placement and lens
By playing around with the various input variables you will also discover 
- increasing the index of refraction of the water (e.g. going from less saline 
  to more saline) brings the secondary focal plane closer to the dome. You expec
  this, since horizontal rays striking the front surface of the dome follow 
  shallower entry paths into the dome, and end up diverging more at the dome/air
- increasing the index of refraction of the dome also brings the secondary
  focal plane closer to the dome, because of the enhanced bending at the
  dome's rear surface
Consider all of the above as just a theoretical exercise on my part. I have
NO practical experience with dome ports, so if anything I've said is incorrect, 
please let me (and everyone else !) know.
Les Wilk 
From: leslie.wilk@HYDRO.ON.CA (Leslie Wilk)
... as seen on the Underwater Photography Mail list
Note 28.13        -< Catadioptric Lenses - brief description >-
>Has anyone ever used a reflecting lens. I dont know exactly what they are 
>called but they resemble a Newtonian telescope. They have no refracting
>elements in them and consist of a parabolic reflector with a flat mirror that 
>bounces the image through a hole in the parabolic reflector into the camera 
>body. I have heard that they only have one aperture setting and are single 
>focal length. Also I have heard that they provide some interesting effects 
>(aberrations) outside the focused depth of field. 
These lenses are called catadioptric.  They are not Newtonian (which
deflects the converging cone of light at right angles) and they DO have at
least one refractive element which also acts as a window to prevent dust
getting into the optics.
'Cat' lenses for cameras are mostly based on the Schmidt-Cassegrain or
Maksutov designs.  In both cases, the concave mirror is the primary optical
element which reflects and focuses light towards a smaller convex
secondary, which is placed ahead of the focal point of the primary mirror.
The secondary mirror deflects the light back through a relatively small
hole in the centre of the primary, and hence to the film plane.  At the
same time, the secondary acts to increase the effective focal length, thus
allowing for a compact, long focal length design.
Aperture is usually fixed at about f/8 because of the mechanical difficulty
of fitting an aperture stop at the optically 'correct' point.
Aberrations as such are no worse than normal lenses, and in fact the
chromatic aberration can be made much smaller.  The worst objection to cat
lenses is the doughnut-shaped out-of-focus highlights, which are basically
the shadow cast by the secondary mirror.  This is not an aberration,
strictly speaking, but does annoy most photographers.
(I remember reading somewhere that there is some sort of sea creature
[perhaps a squid] which has a catadioptric eye.  Presumably these squid
would not have such strong objections to the little doughnuts :-)
A Schmidt camera, in its simplest form, is a special telescope which has
practically zero aberrations over a large angle of view, which it achieves
with a simple spherical primary!  Don't expect to find one in your local
camera store because you typically have to use single plates at a time, and
the plate needs to be spherically curved.  Not only that, this is a prime
focus device, meaning that the film actually sits near the top of the
'scope, looking backwards at the mirror.
Stephen James Hardy
Canberra, Australia
hardy@sweng.stortek.com (Steve Hardy)
Note 28.14     -< Tintype Parlor - tintype materials suppliers >-
>I've been trying to find resources, instructions, and chemical lists/mixing
>instructions/cookbooks for making tintype prints. The only resources I've
>found are too general to be of any practical use. Any literature that might
>be of use is out of print or unobtainable. Any one have any instructions for 
>this type of photographic work? And if any step-by-step directions exist, 
>where would one find the chemicals?
I don't know whether this is precisely what you're looking for, but you may
still find this of use.  There is a commercial product called Tintype Parlor
sold by a photographic store in Los Angeles.  This information that I'm
providing is from their September 1993 catalog, so this information may not
be accurate.
     5124 Sunset Blvd.
     Los Angeles, CA  90027
     (213) 660-3460
     Tintype Parlor
     You can make authentic looking tintypes just like the
     old masters.  The kit comes complete with 5-4x5 tin
     plates, coating emulsion, developer, and fixer.
     183-2016                                      $ 24.95
from: John Oshiro, joshiro@aol.com
Note 28.15            -< Adhering Liquid Light to Glass >-
A "subbing" solution is superior to varnish and paint for adhering
Liquid Light emulsion to glass or glazed ceramics. It consists of
a thin coat of hardened gelatin which penetrates the microscopic
pores of these materials and acts as a strong and permanent bond
for the emulsion.
First, the material to be coated (the substrate) must be made
chemically clean. Soak the substrate in hot water containing sodium
carbonate, then scrub with a cloth. Sodium carbonate is also known
as sal soda or washing soda (not baking soda) and is available in
many groceries. Arm and Hammer powdered laundry detergent is sal
soda and can also be used; do not use detergents or soap.
Rinse well after scrubbing, and observe how the water flows off the
surface of the substrate. It should form a barely visible film, and
not make droplets or bead up. If necessary, continue the scrubbing
until the surface is chemically clean.
Obtain some powdered gelatin such as Knox brand from the grocery
store. Sprinkle 1 level teaspoon of gelatin onto the surface of 1
pint (500 cc's) of cool water. Allow it to stand 15 minutes, then
dissolve the gelatin by heating on the stove.
Before use, add 10 drops of the enclosed chrome alum hardener per
ounce (30 cc's) of the warm gelatin solution. Pour this solution,
still warm, over the clean glass surface, drain thoroughly and
allow it to dry at least 4 hours or overnight. Then coat with
Liquid Light emulsion.
In addition to glass and glazed ceramics, the subbing technique can
be used to improve adhesion with materials that are slightly
porous, such as eggshells (after de-waxing with paint thinner),
rocks, seashells, fired enamels, etc. Do not use on highly-porous
materials like paper and cloth (no preparation needed) or
completely non-porous materials like metal or plastic (use oil-
based varnish or paint).
Rockland Colloid Corp
P.O. Box 376
Piermont, NY  10968
(Rockland sells a small amount of hardener by mail for $1)
Note 28.16       -< Pro School Photographers Association info >-
>I am looking for a contact for what used to be called the Professional School
>Photographers of America. Can you help ?
PSPA is located at  3000 Picture Place,  Jackson, Michigan 49201  517-788-8100
they are comprised of about 300 members and are a section of PMA International
    andy, andpph@rit.edu
Note 28.17         -< ISO, DIN and ASA speed relationships >-
>I am a little confused by all the different film speed ratings I have seen.  
>Does anyone know of any  mathematical relationships between  ASA, ISO, DIN 
>and any others that you may have seen??
ASA and DIN speeds are determined the same way .... except that DIN is a 
logarithmic expression while ASA is an arithmentic one. The ISO speed of 
a material is actually  "ASA/DIN"  (not divided by but ASA and DIN) but 
we in the US tend to use only the arithmetic portion of the pair.
the DIN number is equal to 10 times log ISO + 1
and the ASA number is equal to antilog of (DIN - 1   divided by 10)
eg:   ISO 200   log 200 equals  2.3 times 10 = 23 + 1 = 24   DIN
      DIN 23    24 - 1 = 23 / 10 = 2.3 and antilog 23 = 200  ASA
ASA speed is defined as .8 /log of exposure required to produce a density 
of .1 plus base plus fog while meeting certain other criteria
DIN speed is defined as  log of 1/exposure required to produce a density 
of .1 plus base plus fog while meeting certain other criteria
Because the ASA speed number starts off with .8 rather than 1 an adjustment of
1 in the DIN side needs to be introduced to make the two match in the exposure
(given in mcs) required to deliver .1 above B+F.
In the ASA system a doubling of speed corresponds to a doubling in the
value of the speed number. In the DIN system a doubling in speed is indicated 
by an increase in the speed number of 3. This is associated with the log of 2 
(_double_ the speed) multiplied by 10. 
I am sure that another ways of approaching/explaining this are possible.
From: Andy Davidhazy, andpph@rit.edu
and ......................................................................
ASA and ISO use the same system; each one-stop increase in the film speed 
corresponds to a doubling of the speed rating.  Hence, an ISO 200 film is 
one stop faster than an ISO 100 film, and an ISO 400 film is two stops 
faster than an ISO 100 film.  There are also intermediate ratings; an ISO 
64 film is two-thirds of a stop slower than an ISO 100 film, and 
one-third of a stop faster than an ISO 50 film.
The DIN system was developed in Germany, and has largely been usperseded 
by ISO.  In the DIN system, each one-stop increase in speed adds 3 to the 
speed rating.  An ISO 100 film has a DIN rating of 21 degrees; an ISO 200 
film has a DIN rating of 24 degrees; and so on.
There's also a Soviet/Eastern European system known as GOST, but I'm not 
familiar with its details.  (Perhaps someone who owns a Kiev can help 
fill this in?)
Hope this helps!
From: Richard Hosker, Tennessee Technological University, rph0470@use.usit.net

===========================  end of section 28 ========================== 
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