BetterPhoto Q&A
Category: New Questions

In the dark about photography...

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December 29, 2001

Get the guide number of the flash - I beleive it's about 17 on the Eos 5. Double that number for every stop above ISO 100. Let's take the example of ISO 400 speed film - 2 stops. Now, halve, for every stop below f/1.0 for the aperture - let's say f/5.6, or 5 stops.

= 17 * (2^2) / (2^5)

So the flash is effective to 2.125m. About the distance you would stand from someone talking to them. Anything beyond that is going to be under exposed. If you were trying to take photos of a crowd, or a building 100m+ away, you'd be lucky to see faint outlines.

To fix it? Get a tripod, turn off the flash and shoot with very long exposures.

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January 06, 2002

D = GN / F-Number

D = maximum distance
GN = Guide Number
F-Number = Aperture setting

To adjust the Guide Number for film speeds other than ISO 100, the factor is 1.4 per stop (2 for 2 stops), not 2.

So for the EOS 3000's built in flash with GN = 12m, 400 speed film and f/5.6, the maximum usable flash distance is:

4.28m = 12m x 2 / 5.6

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January 09, 2002

(1) To convert a GN for one film speed to the equivalent GN for another film speed:

GN2 = GN1 * SQRT(Speed2/Speed1)
GN1 = known guide number rating at film speed #1
GN2 = desired guide number for film speed #2
Speed1 = film speed for which guide number is known (GN1)
Speed2 = film speed for which guide number is desired

Nearly all flash units are rated using guide numbers for ISO 100. The GN may be given for use with subject distances in feet or in meters. Typically the specification will read something like:
GN: 105(ISO 100, feet) 32(ISO 100, meters)

The EOS 3000 built-in flash is rated with a GN = 12 (ISO 100, meters). The maximum working distance is dependent on your lens speed using the equation Jon gave [D=GN/f].

(2) If you need to plan what film speed is needed for the distance and widest aperture you can use (or want to use), that can also be calculated:
GNR = D * f
GNR = required guide number
D = farthest subject distance
f = widest aperture

SpeedR = Speed * (GNR/GN)^2
SpeedR = required film speed
Speed = film speed at which flash GN is rated
GNR = required GN (from GNR=D*f)
GN = flash GN rating at "Speed" film speed

Example:
Maximum subject distance: 5 meters
widest aperture: f/5.6
GNR = 5 * 5.6 = 28
GN [EOS 3000]: 12 (ISO 100, meters)
SpeedR = 100 * (28 / 12)^2
SpeedR = 544
This is the minimum film speed to cover this distance at maximum flash power and widest aperture. In available film speeds, the next higher one is ISO 800.

-- John

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January 12, 2002