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    FAQ or Answers to Frequently Asked Questions                 Section 17
    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. 
                      These files are available in SECTIONS. 
              This is Section 17 and its contents are listed below.
    17.01  -< Infrared - basic information >-
    17.02  -< Stereo Photography Discussion >-
    17.03  -< Compensation for Enlarger Magnification in "C" >-
    17.04  -< Pinhole Photography Primer >-
    17.05  -< Processing Infrared Film - How To >-
    17.06  -< How to use Tech Pan in Medium Format >-
    17.07  -< How to determine intermediate f stops >-
    17.08  -< Making a home-made light slave trigger for flashes >-
    17.09  -< How to get Rec.Photo FAQ material by FTP>-

Note 17.01             -< Infrared - basic information >-

From: andpph@itvax.isc.rit.edu "andrew davidhazy"

There are several filters to choose from. 87 is but one of a series. Others are
88A, 87C, 89B, and 70 are others. They differ in the cut-off wavelength. The
Kodak High Speed Infrared film is itself only sensitive to about 900 nm. ISO
ratings are not necessarily useful becasue the subject one may be photographing
may have a different ratio of IR to visible reflectance than a "normal"

>This filter allows only infrared light thru.  However, you cannot see through
>this filter, so you must check all your readings, etc, then place the filter 
>over the lens and snap the picture.  

In the case of a static subject this is of course not a problem. With a
rangefinder or TLR camera it is not a problem either since you are not looking
through a lens. On the other hand, with an SLR you could cut a piece of IR
filter down to the size of your film aperture gate (about 24 x 36 mm) and then
install this just in front of the film plane or just in fron of the shutter
frame, behind the mirror of the SLR. With some SLR's that have hanging metering
devices behind the main mirror this needs to be done VERY carefully. It is
better to just place it directly in front of the film.

>A flash is probably not going to do you much good in this case, time exposures
>would probably do better.

I think a flash could be very useful especially if AC power is not readily
available at the location that IR photography will take place in, such as in
caves, dungeons, etc. :-)

>Be sure you have the owners manual for the camera lens as well, since the
>focusing for infrared film is different.  This is usually explained in the
>owners manual.


>P.S.  Konica has a very nice infrared film that I've had quite good luck

While it is infrared it's sensitivity extends only out to 750 nm or so. I would
like to suggest that at the inexpensive end simple IR image converters such as
used for some surveilance work and for darkroom inspection in photo labs or
more sophisticated IR imaging video cameras at the more expensive end may also
be suitable for archeological applications. 


Note 17.02             -< Stereo Photography Discussion >-
                         STEREO MATERIAL

        There is a company somewhere inthe midwest, I believe, that uses a 
computer program to produce large format rear-lit transparencies that are used
for autostereo displays.  You might have seen them in various airport lobbies.
 They don't used a lenticular grid but rather black-bar barrier strips -- very
easy and cheap to produce -- ideal for large format displays.

David Hutchison, STARLOG Magazine
INTERNET: 71036.1477@compuserve.com


  Enjoyed reading the variuous descriptions, and will definitely
try some of the refinements which have been described.
  My method is quite similar to that described by Ken Luker, but I
cut all my pairs and lay them out before mounting any of them.
  Instead of relying on the heatseal mount's tabs for alignment,
I use a steel ruler, its top edge pressed firmly against the
mount in alignment with the top edges of the bottom horizontal
tabs, and I then gently push the bottom edge of each film chip 
against the edge of the ruler. I use a very small piece of
scotch tape to hold the chips temporarily while checking the
alignment in a hand viewer , then slide the mount-with-chips
out fo the viewer and onto my ironing surface. If done right, this 
virtually guarantees perfect alignment (i.e., the bottom edges
of the chips are along one straight horizontal line). However,
I seem to mess it up about every 10th slide, and can't figure
out why.

Larry Glazer


Date: Wed, 25 Nov 92 9:59:02 WST
From: jcross@ecel.uwa.edu.au (Jennifer Cross)
Subject: Re:  homebrew autostereo

  Hi All!

> > I also suspect the average laser printer can't print small enough dots
> I can think of two possible solutions:
> 	Print at whatever resolution the printer will allow and then reduce
> it in a photocopier.
  This might help, but you will start getting problems with the photocopier
  toner grain size (often pretty corse).  The new HPO laserjet 4 claims
  600 dpi + dot size control + superfine toner which should make a pretty
  clear print but I haven't seen one yet.
> 	Try using it directly on a screen.  (Assuming the screen has the
> right resolution, can always output to a smaller screen if not)

  my Sun (mono) monitor is 72 dpi, square pixel... I think the Macintosh
  screens feature a similar dot pitch.  Most good color monitors
  vary in dot pitch between the horiz and vert direction (horz is the
  quoted figure and works out to about 70-90 dpi) but this is for a single
  color, so the "colored pixel" spacing is considerably more (say 40 dpi)
> Unfortnatly, it's been about 8 years since I did any programming and that
> was two computers ago.  I wouldn;t have a prayer of trying out any of the
> above ideas.

  If the above physical constraints work with the lenticular screens, the
  programming to interleave n images is pretty trival if all the images
  are the same resolution (and the same file format :-)
  Lets have suggestions for resolutions and file formats required and
  I'll see what I can do. (subject to work and holiday celebrations of 
  course :-)  (happy turkeyday to all those americans out there!)

--    ___
     (   >                  /)                        (voice) +61 9 362 6680
      __/_/> ____  ____  o //  _  __            (home)  cjcross@DIALix.oz.au
     / / (__/ / <_/ / <_<_//__                     (work) jcross@ecel.uwa.oz.au

  This is a re-editted and expanded version of a previous posting. I recieved
mail requesting clarification. The original posting pertained to enlarger
exposure calculation.

  I hope this stimulates some interesting discussions.

  Included are three sections:

    1) The first is a broader brush view of the methodology I use.

    2) The second is a copy of the original posting that sparked the
	   request for clarification.

    3) The third is documented 'C' program that I used
	   to generate the scale on my enlarger.

  Even though I am distributing this information without charge I retain
the copyrights to it. Please do not reuse or redistribute it without
permission. (C) 1992 Charles J. Luciano

  If I still haven't made this clear enough, maybe we need to talk. Anyway
I am happy to help out in any way I can.

Chuck Luciano

Part 1:

  Let me start by saying that I try to work towards minimum effort in my
darkroom work, and that this begins when buying film, continues behind the
camera, it continues in film developing, and culminates in printing. There
are a number of "tricks of the trade" that I employ to acheive this and I'd
like to address the whole process.

  I have a tendency to use one type of film as much as possible. Personally
I prefer Vericolor III type S. I find that when printed on the middle contrast
RA-4 paper (supra) I get very good results. I do use other types of films, but,
since I mainly shoot people anyway, VPS works for me.

  I try to buy as much film of the same emulsion number as I can afford. With
VPS I buy bulk and roll my own.

  I try as often as possible to use incident light readings for available
light and guide number calculations for flash. The goal here is to make every
negative "perfect", or at least the same as every other.

  To I develop negatives I buy gallon sizes of C-41 chemicals. I mix the whole
gallon of developer, but I mix Bleach, Fix, and Stab in pints. When I store
developer I put a shot of dust off in the bottle which displaces the oxygen
and makes a gallon of chemistry last a long time.  I reuse Bleach,
Fix, and Stabilizer about 5 times before tossing, but I __NEVER__ reuse
developer. I work at making sure that the process ___DOES__NOT__CHANGE__ from
one time to the next.

  I tend to buy paper in largish quantitys. I recently got a deal on 600 feet
of 11 inch paper in rolls (6). Again all the same emulsion number.

  The result of all of this is that if I take a picture in open sunlight,
when I go to print it, the same filter pack I used for another open sunlight
shot 20 rolls ago still applys.

  Now I could go into a lot of detail about the methods above, but, I was
asked to clarify how I calculate exposure on my enlarger.

  I have a calibrated scale on the column of my enlarger that lists a time
for every height. Initially I accomplished this by using an analyser. If I
move the head of the enlarger I read the new exposure time off of the column.
The aperature always stays the same.

  So now I've shot a roll of film and I put it in my contact printer. I put the
head a height that covers all of the negatives and focus (important!). I set
the aperature to f9.5 (this seems to be average for the times I use), and make
a contact sheet. Based on all I did to get the exposure right, I get most of
my shots within 1/2 stop. If there are some shots that are too light or dark
I may make another contact sheet or two a stop under or over. Maybe even
2 stops under or over if I really blew shots I care about.

  Now as I'm chosing shots I want to print. I can look at the contact sheet
knowing the aperature that was used for the sheet. Of course I'll judge the
shot on the contact sheet that was closest. I may decide to make a small
compensation from the aperature used to make the sheet.

  And here is the rub! Once I arrive on an exposure in the form of an aperature.
I can set the enlarger head to the magnification I want, read the exposure time
off the column for that magnification and BINGO! I get a print that matches
the exposure in the contact sheet. No screwing around. _DONE_ !

  Initially I calibrated this whole approach using an analyser and by making
test prints. Later on I acquired mathematical formulas that related the height
of the enlarger head to exposure. I wrote a program that calculated the
exposure time and magnification on 1/6 inch increments and printed it on a
6 line per inch printer and voila! a direct reading scale that I attached
to the column of my enlarger.

  There are a couple of boundary conditions that I can also handle using this

  I don't like to use my enlarger lens either all the way open or all the
way closed. All the way open can bring out distortions in the lens. All the
way closed can bring out diffraction characteristics in the lens. My lens goes
from f4 to f22. I try to use it between f5.6 and f16.

  The other boundary condition is when exposure times get to be under 3 seconds
or over a minute. Under 3 seconds is a problem because when the enlarger bulb
is turned on is doesn't come on instantly, it starts out red, then yellow then
white. Turning off it does the reverse. As it's cooling the red emissions may
continue for 1/2 second after the blue is gone. I find very short exposures
will throw color balance out of whack. Likewise over a minute reciprocity
failure takes its toll in both exposure time and color balance.

  The solution is simple. I consider the exposure time that I read from the
column to be a starting place. In my note taking I refer to this time as "C".
If I have a print that prints well using "C" and f9.5 at magnification 10
where C is 9.53 seconds, that I'd like to print at 27X, where C is 69.32
seconds, I'll cut the time in half and open the lens up a stop, thus avoiding
the need to compensate for reciprocity. When I record the exposure used I
simply record C/2 at f6.7.

Part 2:

  I have devised an interesting method of calculating exposures on my
enlarger.  I started with a successful print made from an average density
negative using f8 on the enlarger. What I figured is that the exposure is
related to magnification, and the magnification is related to the height of
the enlarger head.

  I obtained formulas that take the negative to subject distance and convert
it to a magnification. I then put a negative in the enlarger and measured
the neg to paper distance and the magnification. By using the formula I found
that my 50mm (nominal) enlarger lens is actually 53.something millimeters.

  Once I had the exact focal length, I wrote a computer program to print
magnifications, neg to paper distance, and a time value. The time value at the
height where I made the test print matched the time I actually used at that
height. The program printed out the data in 1/6 inch increments. I printed it
out on a printer that prints 6 lines per inch. I now have a direct reading
scale attached to the column of the enlarger.

  Now when I print a negative I always set the time to the scale on the column.
Sometimes (like when doing B&W) I can't get the lens opening to match the time,
no problem! When this happens I simply use the time on the column *2, or *4, or
/2, or /4 or whatever. When I make notes on the exposures used to print a
picture I record the time as "c" or "C * 2" or whatever. "C" is the "Column"

  So now anytime I print a picture I have a direct reading means of compensating
for changes in magnification. It speeds things up very nicely.

  Now why do I bring this up now? Because of the f-stops discussion. When I
record f-stops I do so in 1/4 stop increments. Here are the numbers I use:

2.8 3.0 3.3 3.6
4.0 4.4 4.8 5.2
5.6 6.1 6.7 7.3
8.0 8.7 9.5 10
11  12  13  14
16  17  18  20
22  24  26  29

  Note that all of these numbers do not conform to the same decimal accuracy,
they are simply accurate enough to distinguish them from one another.

  No I do not have them memorized! I have a little post-it note on my enlarger
with them written down.

  Interestingly enough a I made the acquaintence of a fellow who works for
ESECO, a company in OK that make real high end custom lab type enlargers. He
told me that their enlargers automatically calulate exposure in this way by
sensing the height of the head. Their enlargers are microprocessor based.

All For Now,
Chuck Luciano


Part 3:
	Print a time scale for enlarger column. Note that this program works
	for enlargers with a head that moves vertically. If your column is
	angled, a change is needed.

	Don't forget to link with math library.

	The mag function takes three arguments. The first is a focal length
	the second is a constant multiplier.

	These were determined experimentally. I first set the constant multiplier
	to 1. I made several measurements of my enlargers actual magnification
	at various heights. The two commented out arrays contain the measured
	heights and magnifications. I then printed the predicted magnification
	and the measured magnification at their ratios at the measured heights
	and "Tuned" the focal length until the ratios were constant. Then I
	"Tuned" the constant multipliers until the ratios were all around 1.

	The reason for the 5.23cm focal length is that the 50mm stamped on the
	lens is a nominal dimension. The reason for the constant multiplier is
	that the formula used actually ignores the distance between the front
	and read nodal points of the lens.

	The reference time is the time value I wished to be at the reference
	height. In other words I started by making a test print at f9.5 with
	the head at ref_ht and the time used was ref_time.

	This is why my average negative comes out at "C" seconds at f9.5.


float mag(f,o,d)
float f;
float o;
float d;
	float d1p,d1m,mp,mm;

	d1m = (d - sqrt(d*d - 4.0 * d * f))/2.0;

	mm = (d - d1m)/d1m/o;
	/* printf("%f,%f\n",f,d,d1m,mm); */
	return mm;
float hts[] = {

float meas[] = {
	int i;
	float f = 5.23;			/* Focal length in cm */
	float o = 0.99;			/* Magnfication multiplier */
	float ref_ht = 69.1;	/* Reference height */
	float ref_time = 12.0;	/* Reference time */

	float mag_const = pow(mag(f,o,ref_ht),2.0);
	float d,m,e,siz;

	/*for (i=0;hts[i] != 0.0;i++)*/

	/* Loop where d starts at 200cm down to d <= 24cm.
	   increment is 4.2333mm (1/6 inch).
	   If your chassis is on a slanted rail, this is the constant you
	   need to modify. It must equal the change in elevation when the
	   head moves 1/6 inch along the rail.
	for (d=200.0;d>24.0;d-=(1.0/6.0*2.54))
		/* d = hts[i];*/

		siz = m*3.50; 			/* Print this while "TUNING" */

		e = pow(m,2.0) / mag_const * ref_time;
			d,m,e );

From: chuck@DRUHI.ATT.COM
Note 17.04              -< Pinhole Photography Primer >-

From:        bercov@bevsun.bev.lbl.gov (John Bercovitz)
Organization: Lawrence Berkeley Laboratory, California

>When building a pinhole camera, what is the optimum size for the pinhole?
>I thought I read once that it was about 1/200th of the focal length.

There was an article in the "Pinhole Journal" by a fellow named Connors who did
picture quality tests with various pinhole diameters.  Connors found that the
best resolution resulted when the diameter of the pinhole in inches was =
SQRT(.000054 * "focal length" in inches).  As I recall, he did his tests with
the subject at fairly ordinary distances.  As an example, for a "focal length"
of 6", you have a diameter of SQRT(.000054*6) = .018 inches.  If you like your
formulae with bigger constants, you could also say diameter in inches =
.007*SQRT("focal length" in inches). 
The bigger the film format, the better the angular image  resolution.
Here's a table for the 3 focal lengths I use on my 4x5 with B&W Polaroid film:
Focal length   Focal length   hole diameter  drill size    f/no
 in inches        in mm         in inches
  3.375            86             .0135          #80        250
  6.000           152             .0180          #77        333
 10.666           271             .0240          #73        444

I like the 6 inch focal length best - I leave it on an old shutter on a lens
board.  Many others like the shorter focal lengths.  As the astronomer said, an
old felt hat is often the best shutter.  Don't worry too much about the focal
length; just get the pinhole _approximately_ the correct focal distance from
the film.  +/- 25% would't be noticeable, I'm sure. 
There are many methods of making pinholes; here's mine: I sandwich a piece of
.001 inch thick brass shim stock between two 1/16 inch thick pieces of sheet
aluminum and drill through with the drill size from the above table using a
high speed drill motor body such as a Dremel tool or a Moto tool or a Sears'
Lil' Crafty or whatever.  The aluminum backing pieces (the "bread" of the
sandwich) keep the burrs to a minimum.  After the brass is drilled, I take off
the remaining burrs with 600 grit abrasive paper.  Then I ink in the inside
diameter of the hole with a black "Magic Marker" type of thing - whatever's
black and sticks to brass and isn't clumpy so as to leave junk in the hole. 
Just be sure to inspect your pinhole with a magnifier when it's done to make
sure you have a good round hole - irregularities decrease image sharpness
considerably.  Irregularities can be dirt particles inside the hole so I keep
my pinholes in dust-free environs.  You blacken the interior of the pinhole to
cut out reflections off the short cylinder which is the hole's inside diameter.
f/no is always just focal length divided by pinhole diameter, the equivalent of
a lens' diameter of aperture.  So the f/no of the 6" focal length pinhole is
6/.018 = 250. 
I really shouldn't be calling this length the "focal length" of the pinhole as
focal length is a property of a lens, not a pinhole.  Pinholes are afocal,
that's their charm, but they do have a distance from the film or image plane at
which they give their best resolution.  Maybe I should call this "optimum
length" instead of "focal length". 


From: andpph@ritvax.isc.rit.edu "andrew davidhazy"
Organization:   Rochester Institute of Technology

A couple other formulas related to determining optimum pinhole diameter:

From Applied Photography, by Arnold, Rolls and Stewart:
            D = V 3.6vL        where v = pinhole to film distance 
                                     L = wavelength of light used
                               all in same units

From Seeing the Light. by Falk, Brill and Stork
            D = 2V vL          where units same as above

From Materials and Processes of Photography by Stroebel, Compton, Zakia, Current
            D = V v  / 141     where D = pinhole diameter in inches
                                     v = pinhole to film distance in inches

From Ilford Manual of Photography
            D =  V v / 125     where units same as in M&P book

From Handbook of Photography by Henney and Dudley (1939)
            D =  V .00007v     where v = pinhole to film distance in inches

Note 17.05           -< Processing Infrared Film - How To >-
                       INFRARED FILM PROCESSING

The information for processing IR film is taken from the Kodak Data Sheet for 
its HS Infrared film:  This assumes you use a small tank and 135 roll film:

Development time in mins

Developer    65F 68F 70F 72F 75F      68F is the recommended temperature
TMAX          -   -   -   -   4
D-76         13  11  10  9.5  8
HC-110 Dil B  7   6   6  5.5  5
D-19*         7   6  5.5   5  4       *D-19 Gives maximum contrast

Film Speeds - these numbers are guides only - use them as starting points:
              Film Speed in ASA rating - assumes development in D-76

Kodak Wratten       Daylight or         
Gelatin Filter      Electronic Flash     Tungsten
No. 25, 29, 70, 89B     50                 125
No. 87, 87A             25                  64
No. 87C                 10                  25
None                    80                 200

Note 17.06         -< How to use Tech Pan in Medium Format >-
                               TECH PAN

   Being another medium format shooter want to be (and in some occusion,
when the moon is just so and the tide is just so, I even dream about
large format), the only b&w film I use is TP and TMAX100.  I use
Rodinal one shot for TP diluted 1:100.  At 68 degree, I soak it for
about 7 minutes with 5 sec agitation every minute.  What you will get
is ultra fine grain and unbelievable tonal range.  Nothing in 35mm
format can touch it.  Unfortunately, at ASA25, it's a damn pain in
the ass to use.  I use 1:3 diluted MicrodolX for TMAX100, and it suprise
me with acceptable result enlarged to 8x10.


From: tse@ra.nrl.navy.mil (Anthony Tse)
Organization: Naval Research Lab, Washington, DC
Note 17.07         -< How to determine intermediate f stops >-
                         INTERMEDIATE F STOPS

To approximate intermediate f/stops, I use

New f/stop = (old f/stop)[sqrt(2)^(f/stop change)]

For example, if you want to stop down 1/2 stop from f/8, take the sqrt(2)
which equals 1.414 and raise it to the power .5 which is decimal equivalent
of 1/2 stop.

    sqrt(2)^.5 = (1.414)^.5  =  1.19


   (8)(1.19) = 9.51

and your new f/stop is appr. 9.5

I guess you need a calculator which has y^x function.

Note 17.08    -< Making a home-made light slave trigger for flashes >-

From: andpph@ritvax.isc.rit.edu "andrew davidhazy"

> I'm looking for a simple schematic for a slave flash control.  I can't see
> spending $20 for a Vivitar when I'm just going to use the insides anyway.

I made something like this and it works ok. If there is a commercially made
device that only costs $5 I would suggest thay you buy an already assembled
and tested synchronizer. :-)

               Low Ohms      High Ohms
       _______       /    |         |            |
      |       |     /  ___|____   C |  _______   |
      |Silicon|    /  |        |    |_|       |  |____>
      |Solar _____/   | Small  |      |  SCR  |          TO FLASH
      |Cell   | -     | Auto   |      |       |_______>
      |       |       | Trans- |      | C106B | A
      |       |-------| former |------|       |
       -------  +      --------     G  -------

Most small silicon solar cells should work. Most small audio transformers with
a resistance of about 8 and 400 ohms should work. The SCR could be something
equivalent but it should be able to handle 200 or preferably 400 volts between
its A and C leads. Since a SCR is used the flash needs to connected to it the
right way, meaning the SCR's output is polarized and it needs to match the input
to the flash. If the flash does not fire with the connections wired one way try
reversing them.
Note 17.09          -< FTP file for material in Rec.Photo >-

                Is there an FTP archive for rec.photo?

	Yes! moink.nmsu.edu ( has a number of possibly
	useful files. Log in as 'ftp' or 'anonymous', with your FTP
	client, please do send your real email address as the password,
	and look around.

        to use this facility you need to learn how to FTP - file transfer
        protocol. With this capability you can transfer files from a 
        host repository. The one above contains photo information files
        as well as picture files.

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