Manual Flash Question

[brian steinberger]

My question is, when figuring out Guide numbers, using GN= shooting distance x aperture / ISO factor, what factor should I use for ISO 160? If there is no ISO factor for 100 and 1.4x for 200, should 160 fall at about 1.2x? Also, if the ISO factor for 400 is 2x, what is it if you rate 400 speed film at EI320? Or would it just stay the same? Thanks alot.

Brian

[phil sweeney]

Hi Brian,

Since most guide numbers are for ISO = 100, calculate your aperature with: f stop = Guide number/distance. Then adjust for your film speed.

[Michael Gudzinowicz]

[pre]
Brian,

I've pasted in a table of calculated fractional f-stops which
agree with those published in table 3.7 of the SPSE Handbook.

Fractional Stops
f# 0.25 0.33 0.50 0.67 0.75
------ -----------------------------------------
1.00 1.12 1.16 1.23 1.29 1.32
1.41 1.58 1.63 1.73 1.83 1.87
2.00 2.24 2.31 2.45 2.58 2.65
2.83 3.16 3.27 3.46 3.65 3.74
4.00 4.47 4.62 4.90 5.16 5.29
5.66 6.33 6.53 6.93 7.30 7.48
8.00 8.94 9.24 9.80 10.33 10.58
11.31 12.65 13.06 13.86 14.61 14.97
16.00 17.89 18.48 19.60 20.66 21.17
22.63 25.30 26.13 27.71 29.21 29.93
32.00 35.78 36.95 39.19 41.31 42.33
45.26 50.60 52.26 55.43 58.42 59.87
64.00 71.55 73.90 78.38 82.62 84.66
90.51 101.19 104.51 110.85 116.85 119.73
128.00 143.11 147.80 156.77 165.25 169.33

In going from EI 100 to 160 film, the speed is 2/3 stop faster.

Use the factor from the first row corresponding to a 2/3 stop
difference from f/1... 1.29. Multiply the EI 100 GN by 1.29 to
get the EI 160 GN. For instance an EI 100 GN of 100 would be
a GN of 129 (round to 130) with EI 160. If you choose f/22 as
the aperture, the flash would be placed at approx 5.9'. If
you choose to place the flash at 4.5', the f-number would be
f/28.8 - just a bit past the 1/2 stop between f/22 and f/32.
(See the table.)

EI 400 film is two stops faster than EI 100, so the factor
to multiply the GN by is 2. EI 320 film is 1 2/3 stop
faster, and the factor is 1.83 (from the table).

For slower films, determine the speed difference in f-stops
from EI 100, and divide the GN by the factor. For instance,
EI 40 film is 1 1/3 stops slower than EI 100. Divide the
EI 100 GN by 1.63 to convert it to the EI 40 GN.

For field work, you might want to print out the table,
and make up another one in Excel for the f-stop/distance
settings for different films and flashes. For flashbulbs
used at high shutter speeds (M sync), another correction
is required but should be included with the bulbs.
[/pre]