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Guide Numbers
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Hey guys
Dont know if this is a really stupid question, but can someone either explain flash guide numbers to me or point me at some info.
I have had a bit of a surf but just get more confused
any help would be good
steve
There are numerous explanations on Guide Numbers to be found on the WWW, but as you’ve already found out yourself, not all of them give a clear understanding of what a Guide Number (or GN) is. One of the better explanations, IMHO, is to be found at http://www.naturephotographers.net/articles0703/jm0703-1.html.
I think where confusion often arises is that there can be a few variables involved in GN calculations, so the best thing is to apply a few standard numbers and work from there. A GN is simply a way of indicating the output power of a flash unit, by combining two known values to create a third, unit-less, number, the Guide Number. The two known values are aperture and distance of the flash to the subject.
For example, if you were to take a tape measure and measure the flash to subject distance as 20 feet and if you then measured the output of the flash using a flash meter (such as is used in a studio) set to ISO 100, and this gave you a reading of f/5.6, the GN for that flash would be 112.
There are some important points to note in this calculation – (i) I used feet as the unit of measure and (ii) I specified a film speed or ISO value of 100. Why are these important? Well, let’s deal with film speed or ISO setting first. As you know, this is an indication as to how sensitive the film emulsion (or imaging sensor) is. A higher ISO means it is more sensitive and therefore requires less light for a proper exposure. So if you were to perform your GN measurements as above but with an ISO of 400, you would get a value of f/11 for your flash measurement as ISO 400 is 2 stops more sensitive than ISO 100. So your GN at an ISO of 400 is (distance x f/no) 20×11 = 220.
And if you were doing your measurements in metric your 20 feet would be 6.09m so the GN for the first calculation would become 34 (at ISO 100) or for the second calculation (at ISO 400) it would be 67.
Guide Numbers are most often used these days to give an indication of the output power of a flash and it is customary for the value to be quoted in terms of a Guide Number obtained at ISO 100 using meters as the unit of measure for a 50mm lens (because a zoom flash head will cause a variation in the GN).
GN’s used to be used a lot in the days before flash automation and it’s still useful to know what they mean, so it’s a good question you’ve raised.
Hope this helps. If all I’ve done is add to your confusion, please ask again, with more specific details as to what may be causing confusion.
Thorsten
Thank you for the time you have spent putting the post together
That actually helps quite a bit,
The reason i was trying to understand is I am looking at buying some studio lighting and trying to understand the strength or suitability of the right product
Steve – No doubt by now you will have found that studio light power is rarely quoted using guide numbers (although a lot will have guide numbers tucked away somewhere in the small print of the specifications). Instead they use Watt-Seconds (WS) or Joules as a more accurate indication of the power. The reason for this is simple, although it may not be immediately apparent. There are such a vast range of light modifiers and attachments available for studio flash, that it simply would not make sense to quote a guide number. Each attachment will have a different affect on the volume of light coming from the studio flash unit. Even if manufacturers were to standardise a guide number using a standard reflector; one manufacturers reflector would have different qualities to another.
One thing to be careful of when buying studio flash units is not to buy something that is too powerful. For example, I have two Elinchrom 600WS units and depending on the scenario, even 600WS can be too powerful. An example where this might be the case is if taking a head-shot using a soft-box very close to the subject. In that case, even after dialing down the power right down to the minimum, you can still end up shooting with a smaller aperture than you might like. So if you really wanted to use f/2.8 for minimum depth of field, you find yourself having to use f/5.6 because the light is too bright.
My advice would be to determine your needs first and buy your lights based around that.
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