# Do flash guide numbers assume some amount of ambient light built-in?

My flash shows its potential range in meters, but is that indoors or outdoors?

The question came up the other day when I was trying to work out whether fitting a diffusion dome would be suitable for some outdoor portraits at a given range but if it says 2.8m as a maximum distance, how does that apply outside or in heavy sunlight?

I know what a guide number is, how to calculate it, and what my guide numbers are. The question is more specifically based on the range calculation of the flash and how the calculation factors in the different environments. Is the max range the maximum achievable in worst case scenario, or was it worked out in a dark room?

The range, guide number, whatever you want to call it, is totally irrelevant if the conditions are too bright, so the flash's calculation must be based on some kind of light level right?

• As an aside to my answer below, you don't want to use a diffusion dome for outdoor portraits. See When and how to use a push-on flash diffuser? — that's asking about the smaller type, but it applies for larger ones as well. They just aren't big enough to give real diffusion like a softbox or umbrella — instead, they rely on scattering light in every direction so it bounces off of walls and ceiling (which are usually scarce outdoors). Jan 5, 2015 at 16:58
• The dome diffuser thing often comes up in a post-Joe-McNally-video context. The thing to keep in mind there is that he's usually trying to fill a reflector or diffuser of some sort as well as whatever direct light he's throwing, or just to maximize environmental fill (walls, floor, ceiling). And there's a bit of "it won't hurt" at closer distances (for fill, f'rinstance) — you're rarely starving for light in that case, and five square inches of source is marginally better than 2. (It helps that he's got 17 more in the bag when he needs them, too.) Jan 5, 2015 at 22:51

The guide number represents the light output of the flash alone, with no ambient light factored in. Unless you are using slow sync flash, the ambient light is just assumed to have no meaningful impact. And, when you do want it to be a factor, the simple isolated number is much easier to actually use to figure out your light ratios.

Why doesn't the ambient light matter? I made this diagram to compare the relative brightness of different light sources. Since the flash is instantaneous rather than continuous, this is an approximation, but it's fairly close. (I can go into details in the comments if you like.)

You note "The range, guide number, whatever you want to call it is totally irrelevant if the conditions are too bright". That's actually not quite true. When the conditions are too bright, the flash brightness is still relevant theoretically, but not noticeable practically, because since stops are exponential, each stop is very big when the light is bright, and in that environment, the flash is only able to contribute a tiny fraction of a stop. You can kind of see that above, but as a further visualization, here's what it would take to go from that full sunlight to one stop brighter still:

You can see that the flash only adds a 16th of that next stop — barely noticeable at all. And if the flash were less powerful or further away, it'd be even more negligible.

The inverse situation happens when you are shooting with a flash in a normally-lit interior; your home lights will contribute theoretically to the exposure, but in most circumstances, so little compared to the flash exposure that we just ignore it.

And, going back to the question, the nominal guide number does not include any amount factored in. As I was making the diagram at the top, a very important reason became obvious to me: the guide number is used to compute light at a certain distance, but the ambient light is independent of flash-subject distance. Of course that light follows the same inverse-square law, but it's separate from the flash calculation, so if some base amount were included, the relative impact of that base would change with flash-subject distance — which would be very confusing!

It's much easier to just have the "by itself" number, which you can use in combination with awareness of the ambient light to work out the right total result. When using flash with a non-negligible other light source, you effectively have a double exposure, the flash exposure (as calculated with the guide number and aperture) and the ambient exposure (shutter speed and aperture).

The easiest way to do this to start with assuming that each exposure is going to be half:

Assume you have GN 54 flash, and the subject distance suggests f/5.6. To adapt a visualization from my answer about the exposure rectangle, that looks something like this:

Then, set it down one stop, to f/8. Each stop is half the light — you can see that the rectangle is half the area in this visualization. Of course, that will mean that this is badly underexposed:

Then, meter for the ambient — put your camera in aperture priority mode and see what it says at f/8. Say it says 1/30th of a second; that'd be like this:

But, switch to full manual mode and keep the aperture the same but cut the shutter speed in half too, to 1/60th:

But if you use the flash with these settings, the two half exposures will combine into a correctly exposed whole:

And you can work out more complicated balances from that as a starting point. Typically, full-integer ratios (in either direction — like 1:2 or 3:1) are good, and I wouldn't really bother pre-computing beyond that — in practice with digital, even with manual flash, it's easier to just experiment for the fine differences.

• So to come up with the GN initially, do manufacturers test a flash in a giant enclosed area or perfectly dark outdoor environment? Jan 2, 2015 at 17:13
• @dpollitt Just a small (or, normal sized) dark room is sufficient, because once you've measured the flash power in a known environment at any scale, the rest can be extrapolated from there. Jan 2, 2015 at 18:12
• Great visuals! That really helped me too. Jan 4, 2015 at 17:48
• @dpollitt I doubt manufacturers test the flash to come up with the GN initially, they will set out to design a flash with GN of say 43 (according to what market research suggests will sell) and then calculate the power required. I doubt any of the commonly available flash units are actually as powerful as they claim to be. Jan 5, 2015 at 11:26
• It pleases me no end to see that answers as good as this are still being contributed to StackExchange. Bravo, @mattdm. Bravo.
– user456
Jan 5, 2015 at 15:35

The flash will still throw light as far as it says regardless of whether it is inside or outside etc, the effectiveness of that light will vary greatly depending on the ambient light.

Using a flash at its maximum range while in very bright ambient light will probably be pretty ineffective. You would likely need to shoot with the flash off-camera and close to the subject in full sunlight to get it to be effective as fill.

• Yes I know that the light will die off, but my question was more directed at how is the number calculated and if it isn't, is it just based on indoor light of some level. Jan 2, 2015 at 12:34
• en.wikipedia.org/wiki/Guide_number ;) - the actual calculation is Guide Number = distance × f-number Jan 2, 2015 at 12:38
• @connersz meters are the same unit of measurement whether you are inside or outside. The differences might be other factors like the potential for light to be reflected off surfaces when inside. Also the atmosphere would affect the result too (as an extreme of this, think of thick fog when you're outside) I would imagine though that in testing and calculating the guide number, they would have some environment where reflected light doesn't affect the calculation Jan 2, 2015 at 12:49
• @laurencemadill I'm not sure I read the first part of your message correctly but I'm totally away that measurements don't change when you go outside. I didn't have many GCSE's but I know as much to say this. I am asking how the measurement was calculated, those other factors you talked about and I am looking for an actual hard facts rather than suggestions of possible limiting factors. Jan 2, 2015 at 14:39
• @connersz Sorry I didn't mean to be derogatory, I guessed you would know that - I should have put a smiley face after that sentence . That's why I didn't post as an answer, as I haven't a clue how it's done, I thought it may be useful for other people viewing the question. I suppose it highlights the fact that the 'guide' number should only be used as such in real use Jan 2, 2015 at 16:50

According to Wikipedia's Guide Number entry:

The aperture given by the guide number is only correct for certain locations, generally indoors where a reasonable amount of "stray" flash light will be reflected from surrounding walls onto the subject. The effective GN is slightly lower outdoors as any light not falling directly on the subject from the flash will be lost.

There are no references to substantiate this -- it's Wikipedia, after all -- but there may be something to the "small room" suggestion.

Of course, if GN were referring to outdoors, it would have to refer to night. Otherwise, it's not referring to what the flash does.

• The Wikipedia article on this is really poor, unfortunately. Lots of unsorted material, and some based on a poor done experiment written up in a blog post later retracted by its author. Jan 4, 2015 at 21:54

You need to know, which you say you do, the guide number (GN) of your flash.

Example: Canon 580ex has GN of 58.

To calculate if you have enough power, multiply your aperture by distance of subject (in meters). (Also, iso 100 must be used for this calc. A 2 stop increase in iso will double distance possible).

So, for f11 and subject at 5m, you need a flash with a GN of at least 55.