Thursday, 4 October 2007

Guide to Guide Numbers

Guide numbers are the common method of comparing the relative powers of different flashes. The value is found experimentally by firing the flash at maximum power at a subject which is a fixed distance from the flash. With no other source of light, a flash meter is used to obtain an optimum aperture for a "correct exposure", which is of course slightly subjective. Similarly a camera could be used, and photographs compared for the best exposure, although there would be significant complication introduced by having to take additional account of reflectance of the subject being photographed and the distance from the camera to the subject.

One area of common confusion is that Guide numbers can be specified as either feet or metres. Sometimes a flash will be quoted in feet, which makes it seem ever so much more powerful than if quoted in metres, but there is of course no difference.

A second common confusion is use of ISO speeds. Most Guide numbers are calculated for ISO 100, which is traditionally a very common film speed. However, quoting at ISO 200 can make a flash seem more powerful, so it's important to check what ISO speed is being quoted.

So, getting back to the experiment, say we set up our flash 10 metres from our flash meter (and subject). We then fire the flash at full power, and measure the light output. Our flash meter, set for ISO 100, gives a reading of f/8.

To calculate the guide number for our flash, we simply multiply the aperture value with the distance to our subject.

So we get:

Guide Number = Distance x Aperture
Guide Number = 10m x 8
Guide Number = 80m

So our flash has a Guide Number of 80 metres at ISO 100.

For another quick example, lets say we've bought another flash that is rated at a Guide number of 360 feet @ ISO 200. Given this incredible Guide number, it had to be a good deal, and much better than the flash we already had at 80m @ ISO 100, right?

Well let's work it out:

Since quadrupling ISO speed means we double the Guide number due to the Inverse Square law, doubling ISO speed means we multiply the the square root of 2 to get the Guide number. However, since we're wanting to half the ISO speed, we therefore divide by the square root of 2.

Converting to ISO 100, we therefore get

360 feet @ ISO 200 = 254.6 feet @ ISO 100

Converting feet into metres, we get

254.6 feet @ ISO 100 = 77.6 metres @ ISO 100

So despite the very high quoted guide number, it turns out this flash isn't actually as powerful as our first flash.

So what other trick do manufacturers have up their sleeves?

Besides using feet instead of metres, and higher ISO speeds, to make numbers sound more impressive than they are, manufacturers also have a couple of other possibilities for their marketing departments to make use of.

The major one is the hazy description of "correct exposure". In general, a manufacturer will opt for a slightly dark exposure, because this will make the Guide number of their flash higher. You, on the other hand, might feel this is too dark, and would want a brighter photo, so your Guide number wouldn't be nearly as high to make the photo the way you like it.

Another one is only an option for manufacturers of small flashguns. Most of the half decent flashguns on the market now have a zoom capability. As you zoom your lens, your camera sends signals to your flash to tell it to zoom in. What this does is modify the way the light is directed, and make the output of light more efficient for a given power of flash burst, simply because more of the light given out is directed towards the subject, rather than lighting all around the subject.

So why is this important? Well when the flash is zoomed in, it has an effectively greater guide number than when it is zoomed out. So you can guess that the manufacturers will make use of this, and quote the Guide number of the flash at its best zoom position.

The best advice therefore is to do your own tests when you get a flash, if it matters to you to know for sure. That way you know there hasn't been a marketing department involved in the Guide number you arrive at.


Jan Huniche said...

I havejust bought a new Speedlite 580EX II...

It seems that the flashoutput i a little to low.

I set up my Konica Minolta Autometer VF mountet with the optional flat diffuser on a stand at exately 3 meters.

I then mountet the camera on another stand at exately the same hight as the flashmeter.

With the settings ISO 100, 50 mm, the guidenumber is specified ba canon to be 42.

So I was expecting to mesure an output of f:14

But I mesured only f:11 ???

So I took my old EX420 wich has a guidenumber of 34 i the 50 mm range.

And here I mesured f:11 (11x3=33) wich i pretty close to the specified 34.

I contacted the dealed who sold me the 580EX II and he send me a new one...

It does have a different serielnumber (old=252902, new=259432).

But the new 580EX was not performing better...

Infact the output in manual, ISO 100, 50 mm was still f:11 at 3 meters.

That gives (in my calculation) a guidenumber of as low as 33 instead of the specified 42.

And exately the same as my old 420EX ???.

Does anyone have had a similar expirence ?.

Then please mail me at :

Best regards

Jan Huniche

n506 said...

Hi Jan

First of all, it wouldn't surprise me at all to find a large manufacturer overestimating their product's abilities. That said...

A lot of things could be going wrong with the test. Here's a quick list of things which could be wrong:

What batteries are you using? If you're using rechargeables, they are only 1.2V rather than 1.5V. This could mean that the capacitor doesn't get "juiced up" quite as much, and results in a lesser power output when the flash is tripped. You might also have grabbed a set of alkalines to try the 420EX, thereby giving it a little extra juicing up?

If that's not the case, then how sure are you of your flashmeter? How old is it, and therefore how long since it was calibrated? If you're finding your 420EX giving a slightly low value, then you might find that the flashmeter is also giving a non-linear response resulting in a value even further out for the more powerful 580EX. Might be worth grabbing someone who has another flashmeter and doing the test again and comparing results before you blame the 580EX for sure.

Lastly, I believe I've mentioned before in my blog that guide numbers are pretty much just literally a guide. The big bit of hazyness is in the "correct exposure" definition of what the exposure is. What Canon believes is a correct exposure might well be darker than what your flashmeter was set to believe. Not only that, but Canon's marketing dept. could well have changed the corporate view of what is a correct exposure in the generation since the 420EX was built...after all, what's the odd 2/3 of a stop between friends?

Anonymous said...

It's not a good idea to teach things you don't understand. Your claim that "360 feet @ ISO 200 = 180 feet @ ISO 100" is incorrect.

The ISO number is a linear brightness scale, so ISO 200 is twice as light as ISO 100 (and ISO 800 is 4 times as light as ISO 200 (800/200=4), etc). However, light intensity fall-off is exponential. Light intensity is inversely proportional to the square root of the distance.

So, you should multiply your ISO number by the square of the number you multiply your distance by.
E.g., if you multiply your distance by 2 then you should multiply your ISO number by 4, or if you multiply your distance by 4 then you should multiply your ISO number by 16.

Let's say you get F feet @ ISO I:
A) The distance at ISO X = 'the square root of (X/I)' * F feet
B) The ISO number at Y feet = ISO 'the square of (Y/F)' * I.

A) 360 feet @ ISO 200 = 'the square root of (100/200)' * 360 feet @ ISO 100 = 0.707 * 360 feet @ ISO 100
= 255 feet @ ISO 100.
B) 360 feet @ ISO 200 = 180 feet @ ISO 'the square of (180/360)' * 200
= 180 feet @ ISO 50.

n506 said...

Thanks for pointing out that very silly mistake! I've updated the blog post to correct the error.