### Guide numbers

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Can someone explain guide numbers?

Thanks, mary

-- Anonymous, February 28, 1997

The flash guide number for a given ISO is the *flash* distance multiplied by the f-stop. It can be converted from one ISO to another by the formula GN2=GN1(sqrt(ISO2/ISO1)). The guide number is more accurate for handle and shoe mount flashes. Studio flashes have trouble with it because of the variety of reflectors that are too different from a point source. When using guide numbers for photomacrography calculations, measure distance from the back of the hot shoe.

Amilcar

-- Anonymous, March 09, 1997

Mary,

Guide Number is an indication of the "light output" of a flash :The bigger the number, more "light output" the flash will have (and usually the more expensive it will be). Practically it means that the bigger the GN, the farther to object can be for the same f-numnber, or that you can use a smaller f-number for the same flash-object distance.This is only a physical (qualitative) explanation of what it means. The "quantitative explanation" is in Amilcar's answer. It is worth to point out that the formula (shown in Amilcar's answer), involves the term "distance". This distance is the "flash-object-camera" distance divided by two. This means that if you are working with the flash mounted in your camera is just the flash-object distance, but if you are working with the flash detached from the camera you will have to add both distances (flash to object and object to camera) and divide it by two. Have in mind that the GN is specified for "A" film speed (usually 100ASA) and you will need the formula written by Amilcar to find the GN for a different ASA number. If you really want to play with the maths, I recomend you to try to work out the chart that most flashes bring on their back. You will be able to do so with these formulas. Hope this helps.

Regards, Marcelo

-- Anonymous, March 13, 1997