Flash Guide Number - std units??

photobug · July 17, 2006

I'm going a little nuts, trying to compare light output of flash units. I know that watt-seconds is a useless measure, because it is a measure of the electrical energy stored in the flash's capacitor, not a measure of light output. A flash guide number is useful, if it specifiies to which film speed (ISO) rating it applies and to which distance unit it applies (feet or meters). When I see guide numbers quoted with no units whatsoever (on this Adorama page, for example) it gives me a major episode of head-scratching.

If a guide number is quoted without a film speed (ISO) rating, can I pretty safely assume that it's for ISO 100? (that seems to be pretty standard)

But if neither "feet" nor "meters" are citied, what good is the guide#? Is there an unwritten "standard" that I'm not aware of? (I'm sure it's "meters" everywhere but the U.S., but what do I make of "bare" guide numbers cited by a U.S. company, like in the web link mentioned above?) Feet, meters, or does one just flip a coin?

Thanks for any light you can shed on this! (...pun intended)

= Dave

photobug · July 17, 2006

But if neither "feet" nor "meters" are citied, what good is the guide#? Is there an unwritten "standard" that I'm not aware of? (I'm sure it's "meters" everywhere but the U.S., but what do I make of "bare" guide numbers cited by a U.S. company, like in the web link mentioned above?) Feet, meters, or does one just flip a coin?

I got a quick (but, it turns out) partial answer by contacting Adorama directly. Its guide numbers for studio flashes are quoted in feet, at ISO 100 The 300 watt-second flash example I indicated has a guide number of 115, in feet, at ISO 100. So that part of the mystery is solved.

However, now how do I compare a studio flash with guide number of 115 against, say, my Canon 580EX which has a guide number (in feet) of 190 (at 105mm) and guide number of 30 (at 28mm)? The problem is, the studio flash doesn't specify an angle of coverage when quoting its guide number, so I still can't compare light output with my existing portable flash or, for that matter, any other studio flash.

Surely this isn't as difficult as it sounds -- can any pros with studio flashes help dig me out of this quandry?

(if more context helps: I'm trying to decide whether to buy a studio flash or two + stands/umbrellas, or just get a couple of stands/umbrellas to use with my 580EX and 420EX for [1] an outdoor family portrait coming up in 2 weeks, and [2] for some father-daughter dance portraits I might do next spring.)

Flash Guide Number - std units??

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