# Category : A Collection of Games for DOS and Windows

Archive : PHOTASST.ZIP

Filename : PHOTAST.DOC

Copyright 1988

Brian Allen Kuehn

This is a SHAREWARE product. You are invited to copy and distribute this

program provided that it is not modified in any way. If you find it to be of

value to you, I expect you to pay for it. It will have different values to

different people. It is worth a maximum of $5.00 and a minimum of $1.00. Send

me whatever it is worth to you within this range. Your comments/suggestions

are hereby solicited. Send cash and comments to:

BRIAN ALLEN KUEHN

5815 CORD COURT

PEORIA HEIGHTS, ILLINOIS 61614

-2-

ELECTRONIC FLASH CALCULATIONS

It would be nice if we all had access to studio strobes. Most of us

don't. When we want more light output, we generally start hooking up whatever

pieces of equipment that we have available in order to add up to the amount of

light desired. With most of us, that means using unrelated pieces of equipment

that have different guide numbers. Once you have assembled the various

strobes, how can you determine the amount of light that they will be putting

out when fired together? This is not a simple question.

An easy way to measure the light output would be to use a flash meter to

measure the combined light output or use a polaroid back on the camera to make

some test exposures. These items are not always available.

Any single flash unit used on manual (full power) produces a specific

amount of light. How can that light output be described as a specific, finite

number? I looked at guide numbers, B.C.P.S., and other conventional means of

describing light output. My Metz flash has an A.S.A. 25 guide number of 93.

My Sunpack's A.S.A. 25 guide number is 50. Does this mean that I can add 93

and 50 to reach an A.S.A. guide number of 143 for the two flashes combined? No

way! Attempting to do so is like comparing apples and oranges.

In studying a chart of various A.S.A. values and their corresponding guide

numbers, a pattern emerges:

METZ 402: A.S.A. VALUES WITH

CORRESPONDING GUIDE NUMBERS

ASA GN

____________

25 < 93

32 < 105.22

40 < 117.54

50 < 131.52

64 < 148.8

80 < 166.36

100 < 186

125 < 207.95

160 < 235.27

200 < 263.04

320 < 332.72

400 > 372

800 > 526.08

100 > 588.18

1600 > 744

The chart shows A.S.A. values with corresponding guide numbers for my Metz

402 flash used on manual (full power). For A.S.A. values below 320, you will

notice that the A.S.A. values are smaller than their corresponding guide

numbers. For A.S.A. values above 400, you will notice that the A.S.A. values

are larger than their corresponding guide numbers. You have probably noticed

that the A.S.A. values increase at a proportionately greater rate then do their

corresponding guide numbers values. This is because light falls off with the

square of the distance. If you vary A.S.A. and guide number values in small

enough increments, you will eventually come to a point at which A.S.A. will

equal its corresponding guide number. In the chart above, that point falls

somewhere between A.S.A. 320 and A.S.A. 400.

-3-

The point at which A.S.A. value equals guide number provides the means to

compare apples and oranges. These numbers, which I have arbitrarily named

"ABSOLUTE LIGHT VALUES" are additive. To determine "ALV" for a particular

strobe you need any correct A.S.A./GN combination for the flash in question.

At A.S.A. 25, my Metz 402 flash has a guide number of 93. Apply the following

formula:

(GN * GN)/A.S.A. = "ALV"

93 squared is 8,649. 8,649 divided by 25 equals 345.96. Thus 345.96 is

the "ALV" or ABSOLUTE LIGHT VALUE" for my Metz 402 flash. If you plug any

other A.S.A./GN combination from the chart above into the formula above, you

will arrive at the same "ALV". Try it!

Once you have determined "ALV" for a specific flash unit, it can be used

to calculate the guide number that corresponds to any A.S.A. which you might

wish to use. The following formula applies: Take the square root of ("ALV"

times "DESIRED A.S.A.") = "GUIDE NUMBER".

Assume that you have acquired a small quantity of a rare Russian film that

has an A.S.A. value of 112. Assume further that you are using my Metz 402

flash and that you need to know the correct guide number to use with this rare

Russian film. You will recall that the "ALV" for my Metz 402 is 345.96. Use

the formula set forth above. 345.96 ("ALV") times 112 (A.S.A. DESIRED) equals

38,747.52. The square root of 38,747.52 is 196.84. The correct guide number

to use is 196.84.

You can handle any combination of electronic flash units by adding their

individual "ABSOLUE LIGHT VALUES" together. The total of the individual "ALV"s

is the "ALV" for the flashes used together. These formulas are used in

portions of the .EXE file that you have.

DEPTH OF FIELD CALCULATIONS

Many cameras have depth of field markings engraved on their lens barrels.

Some of these are fairly accurate and some are not. Most larger format cameras

do not have depth of field markings anywhere. It can be very useful to know

exactly what your depth of field at any given aperture will be. There are two

rorutines in the .EXE program supplied that rely on depth of field

calculations. One routine will produce a depth of filed chart for all

apertures at a given focused distance. The other routine will let you provide

the depth of field limits that you desire. It will then tell you the point

that you need to focus on and the aperture that you need to use in order to

achieve the desired near and far depth of field limits.

The formulas used are derived from the following well know depth of field

formulas:

F = focal length of lens

f = f/stop of relative aperture

H = hyperfocal distance

u = distance focused upon

d = diameter of circle of confusion

O = angular size of circle of confusion. For critical

definition, O is 2 minutes of arc and the linear size of the

circle of confusion is approximately f/1720 (tangent of 2

minutes equals .00058).

L = effective diameter of lens = F/f

-4-

METHOD #1:

Near limit of depth of field (measured from camera lens):

(H * u)/(H + (u - F))

Far limit of depth of field (measured from camera lens):

(H * u)/(H - (u - F))

METHOD #2 (used in the .EXE file):

Near limit of depth of field (measured from plane focused upon):

(u * u * tan O)/(L + u * tan O)

Far limit of depth of field (measured from plane focused upon):

(u * u * tan O)/(L - u * tan O)

The results will change depending upon the angular size of the circle of

confusion used. Since I assume that most people using this program are fairly

serious about photography and therefor rather fussy about the results achieved,

I have used 2 minutes of arc as the circle of confusion value in the .EXE

file.

Hyperfocal distance is the minimun distance that you can focus at and

still be in focus clear to infinity. This is useful if you want to be in focus

from infinity back to the closest possible distance. This will give you the

widest total depth of field at any given aperture. Hyperfocal distance varies

with the aperture selected. Hyperfocal distances are given for each aperture

in the depth of field charting routine.

I would appreciate receiving any comments or suggestions for future

additions to this program. It is written in Microsoft QuickBasic and is my

first attempt at programing in this language. Comments can be left for me on

CompuServe although that is haphazard at best as I can go for months without

signing on to CompuServe. It is probably better to communicate the old

fashioned way, either by mail or by phone. My address is: Brian Allen Kuehn

(pronounced "Keen"), 5815 Cord Court, Peoria Heights, Illinois 61614. Phone 1-

(309)-688-6208.

Very nice! Thank you for this wonderful archive. I wonder why I found it only now. Long live the BBS file archives!

This is so awesome! 😀 I’d be cool if you could download an entire archive of this at once, though.

But one thing that puzzles me is the “mtswslnkmcjklsdlsbdmMICROSOFT” string. There is an article about it here. It is definitely worth a read: http://www.os2museum.com/wp/mtswslnk/