About Guide Numbers:
Note: The GN normally given for a flash unit is for ISO 100 film speed.
The GN for the same flash changes for different ISO film speeds! The GN
data should also tell you whether it is for subject distance in meters or
feet. For instance, the GN for a T-32 running in Manual Mode is 32 in
meters, and 104 in feet. If all you have is the GN for meters, multiply it
by 3.28 to get the GN for feet. If all you have is the GN for feet, divide
by 3.28 to get the GN for meters.
The GN divided by the distance is the aperture you should set. So, if you
are using ISO 100 film, subject distance is 16 meters, and a T-32 with ISO
100 GN 32 (meters), then 32 divided by 16 gives an aperture of f/2. Reduce
subject distance to 8 meters and the aperture is f/4.
For those of use who deal in feet, the ISO 100 GN of a T-32 is 104. If
subject distance is 26 feet, dividing 104 by 26 gives an aperture of f/4.
Reduce subject distance to 13 feet and the aperture is f/8
Now, say you are using ISO 200 film, what do you do now? The GN for the
T-32 is *not* 32 (meters) or 104 (feet) any more as the film is faster.
You can find the new guide number (OK, dig out your calculators and make
sure the batteries are good) by dividing the new film speed by 100 and
taking the square root of the result. Then multiply the ISO 100 Guide
Number by the answer you got after taking the square root.
Examples for computing the GN for a T-32:
Simple example for ISO 200:
200/100 = 2
SQRT(2) ~= 1.414
meters:
32 * 1.414 = 45.248
Round off and use 45 for the GN
feet:
104 * 1.414 = 147.056
Round off and use 147 (heck, 145 is close enough)
Another simple example for ISO 400:
400/100 = 4
SQRT(4) = 2
meters:
32 * 2 = 64
Use 64 for the GN
feet:
104 * 2 = 208
Use 208 for the GN (using 200 or 210 is probably close enough)
Harder example for ISO 160:
160/100 = 1.6
SQRT(1.6) ~= 1.265
meters:
32 * 1.265 = 40.48
Round off and use 40 for the GN
feet:
104 * 1.265 = 131.56
Round off and use 132 (130 is close enough)
Another harder example for ISO 125:
125/100 = 1.25
SQRT(1.25) ~= 1.118
meters:
32 * 1.118 = 35.776
Round off and use 36 for the GN (35 would work just as well)
feet:
104 * 1.118 = 116.272
Round off and use 116 (115 or 120 would work too)
Hope this helps,
-- John
At 18:27 6/22/00 , Gregg wrote:
>So, how does that guide number thingie work again; something divided by
>something gives your something?
>
>Gregg
>Who only used a guide number except to compare flash output before 1975!
>
>(in standard American English) Lex wrote:
>
>> Finally, regarding flash, if your young charge is really interested in
>> pursuing roots-level 35mm SLR photography, learning guide numbers is a
>> necessity. And really not all that difficult. So the OM-1 with any old
>> flash will do. Then he'll appreciate the advantages of auto-flash and,
>> finally, the subtleties of TTL flash.
>>
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