Flash Compendium

Flash Basics

Guide Number

There are a number of scientific units for light intensity, but they are too specific to be applied easily to photography. So in photography a “helper unit” is used, the Guide Number (GN). The guide number specifies the distance of an object to be exposed correctly with a flash unit, using ISO 100 film and an aperture of f/1.0. The GN is expressed in meters or feet, because it's a distance unit (I will use meters here). It basically describes the intensity of a flash pulse required to correctly expose the object.

The simplified formula for flash exposure at ISO 100 is:

GN = aperture × distance

distance = GN / aperture

When you buy a flash unit you will find the maximum GN of the flash in the tech data of the flash. This is the maximum intensity of a flash pulse that this unit can produce.

Manual Flash

A flash unit is a near-point light source. The light that the flash unit emits illuminates a larger area the farther you're away from the flash. Specifically, the intensity of the light decreases with the square of the distance from the flash. This is because the flash illuminates the surface area of a (virtual) sphere with the flash at its center, and, as we've learned in school, the surface area of a sphere increases with the square of the radius. So when you increase the distance from the light source by a factor of 1.4, the light intensity will be only 1/2 of the original. If you double the distance, light intensity decreases to 1/4, and so on.

When we use a fully manual flash system, we can calculate the required flash intensity when we know the film's sensitivity, the distance between flash and subject and the effective aperture. For example, when the subject is 10 meters away from the flash, we use ISO 100 film and we shoot at f/2.8, the required GN for correct exposure would be 30 m (f/2.8 is 3 stops slower than f/1.0, so we need three times the light intensity for the same exposure).

With manual flash you adjust flash output exactly in this way. You first calculate the required guide number. Then you set up the flash unit to emit such a burst. Most flash units can not be adjusted in such fine steps, so you choose the closest setting and fine-tune with either flash distance or aperture. After that's done, you can shoot, and the object will be exposed correctly.

The nice thing about manual flash is what is not relevant:

Shutter speed (that's the case with all flash systems, as long as you stay below the x-sync speed of the camera)
Brightness or reflectivity of the object

Flash output is not varied with the reflectivity of the subject. When an object is brighter than mid-toned, it will appear brighter in the image. When it's darker, it will appear darker. That's exactly what you want, and manual flash gets it right automatically.

However, using manual flash is also very tedious. You have to measure the distance, factor in the film speed and aperture, factor in everything else that influences either flash output (diffusors, concentrators) or effective aperture (filters, extension tubes), adjust the flash unit and probably do some fine-tuning. This is not only very complicated, but also quite slow. With quick-changing situations you will not want to use manual flash.

Many Minolta flash units offer a manual flash mode, but I bet it is rarely used.

Correct Flash Exposure

When I said “correct exposure” above, I meant the technical definition of “correct”. This means that a mid-toned object (eg. a grey card) will also appear mid-toned in the image. Sometimes you intentionally increase this exposure to get more details in the shadows of the image, or you decrease it to get details in the highlights. This is a different aspect of exposure, and I will concentrate on the technical aspects here.

Readers' comments

#1: Comment posted by Thinh Nguyen on January 1st, 2009 - 04:50:55 PM:
Hi Michael, great guide for beginners like me. If I were to integrate the ISO sensitivity into the equation. I think it would look like: Distance = [sqrt(ISO)/10] * GN/Aperture i.e. distance (flash range) are 2x @ ISO400 and 4x @ ISO1600. This is consistent with the rest of your reasoning with the light sphere.

Michael Hohner answers:
You're correct. As a simplification you can also change aperture by a full stop for every full stop of change in ISO, thus avoiding calculating square roots. For example, when using ISO 200 with a f/2.0 aperture, you can make the calculation for ISO 100 with a f/1.4 aperture and get the same result.

#2: Comment posted by Martin Evans on February 22nd, 2010 - 11:59:01 PM:
Is there a rule of thumb for estimating scientific units of illumination, such as lux.seconds, from Guide Number? Thank you.