How to Calculate Flash Exposure

It sounds pretty scary but it’s actually not that complicated. What are the ingredients? It’s a simple division, lets get started.

Guide Number

The Guide Number of your flash unit or build in camera flash denotes its power at a given ISO. Let’s take a built-in camera flash as an example. Common (D)SLR in camera flash on average have a Guide Number of 12 at ISO 100. Grab your manual or start Googling to find your Guide Number. <10 minutes pass> “Hey!! I have a DSLR with a Guide Number of 39!!! I’m soooooooo cool”, ah yes undoubtedly you’re a stud however you have to continue reading: Guide Numbers come in two flavors, the lower number is for people that like to use the metric system to calculate distance and the high number is for people that like to use their feet. Just like the distance scale on your lens, hey how handy is that! Which brings me to…


Because light likes to fall-off or lose power as it travels, we need to take the distance between camera and subject into account. A guide could be the distance scale on your lens when your subject is in focus, but my preferred method is guessing the distance. You’d be surprised what you know you can do when you realize you can do it… So with your distance guesstimated, and your guide number looked up, let’s go to.

The Division

We’re ready to calculate our flash exposure! What we need is the f-stop. Shutter speed is only for continues lighting like the sun etc. Flash duration to power ratio is too short for the shutter to have any effect (this is a whole different subject, so just forget about it now) so we calculate the amount of light hitting the film or sensor surface (f-stop) and not the duration (shutter speed). We do that by a simple division, let’s take the Guide Number 12 as an example and speculate that we’re 5 meters from our subject:

GN12 / 5 Meters = F2.4

For feet:

GN36 / 16 Feet = F2.2

Not too difficult, I would say, setting your camera to F2.8 should be fine.


What if I like to use a portable flash like my sb-800 and bounce it of the ceiling?

No problem, just add that distance. So if the ceiling is 1.5 meters from your flash-head and the subject a similar amount from the ceiling, you end op with a distance of 3 meters.

If shutter speed doesn’t matter, why is my camera manual talking about flash sync speed?

Okay, there’s that issue. The flash sync speed of your camera is the maximum shutter speed with which your camera can keep the shutter in sync with the flash. Just don’t set your shutter any faster than that (often around 200/s) or you will start to see exposure differences between the top of your photo and the bottom. The dark part is the shutter going down in the middle of your flash pop. You can play a little with your shutter for creative effect, the longer you leave the shutter open the more available light will come in, this way you can combine your flash exposure with available light. Just play, you’ll see what I mean.

My Guide Number is calculated at ISO 100, but I want to shoot at ISO 800, now what?

That’s going to add another calculation on your Guide Number. From ISO 100 to ISO 800 is in total 3 stops more light sensitivity. This means that your flash unit becomes 3 times more effective. So the only thing you need to do is multiply your Guide Number by the amount of stops you increase your ISO. In this case, GN12 x 3 stops = GN36. Be careful with this, though, as there’s also a minimum to the light a flash unit can output.

I really want to use my flash manually, but I can’t calculate fast enough, now what?

If you use a digital camera the answer is simple, set your camera to manual and play with the power division scale (1/1, 1/2, 1/4 power) on the flash. Shoot and review. Pretty soon you’ll develop a sense for it, and you’ll get it right on the first or second try. If you’re like me and love to shoot film, the previous is not an option. When shooting black and white film you can really do a rough calculation, just pick an easy round number. I know the world parties about the ability to shoot raw and fiddle with the exposure in a raw editor, but I made 5 stop exposure errors on Kodak Tri-x 400 film and got away with it just fine.

Do you know of an iPhone or Android app that can calculate for me?

No I don’t. You can search for it, but most camera flash units come with one build in, you probably just never knew it was there. I sometimes use my Nikon SB-800 with my Rolleicord or one of my Range Finder bodies. I switch the flash unit to manual and set the film ISO in the advanced menu. When you regulate the power level, there’s a small distance scale in the top right corner which shows distance in, you guessed it, meters and feet. How is that for easy! Just guess the distance and set it, done.

Closing words

It’s actually a pretty long explanation for a very simple calculation. And if you look at the text without reading it, you might think it is a waste of time as your modern camera can TTL it all for you. But even if you do use your camera on full auto, it’s still valuable to know why it behaves like it does, and how to make it do a bit more of what you want even in auto mode. I hear often people complaining about their entry level D-SLR, saying it takes bad quality photographs, even asking me which one they should buy instead. The fact is the quality of D-SLRs these days it is wonderful even if you get a really cheap one. But if you let the camera decide everything, you get what it meters for, an average picture (18% gray average to be precise).

This also goes for TTL flash exposure…

You’re walking around on an average sunny day and see a pretty flower you want to take a picture. Being already one step-up, the cool leader you shoot in A mode to get that nice depth of field. You realize you have some light coming in from the background, “fill flash!”, you think, and you’d be right. You pop up the flash and let the TTL do its magic. Not only that, but you see some blinking in the view finder, but you don’t know what it means, so you quickly fully press the shutter. The result is a white background with a washed out flower in the foreground. You try to take the shot again, maybe the camera made a mistake, same result. In complete desperation, you decide to gamble on taking the shot without the fill flash, quickly deciding that flashes are ugly anyway. The result is horrible, still parts of the background are blown-out and the flower now looks under exposed and lost all its vibrancy. Being beaten by your camera, you decide to take the photo home and “correct” it in your wonderful raw editor. You pump up the “fill light” and play with the magic “recovery” slider to save some of the highlights, then you increase the blacks to get some contrast. Because you picked-up that book from Scott Kelby, you decide to start up Photoshop and play with adjustment layers. Finally giving up you’re left with a feeling of disappointment, you blame yourself for buying the wrong camera. You decide to go read your mail, maybe someone fav’d a photograph you uploaded on Flickr (I’m old, I mean Instagram) yesterday….

What went wrong here?! The first step was the correct one, fill flash would have done the trick. But the camera’s default setting to sync at 1/60th of a second when doing flash exposures was not right for this particular case! By blinking the exposure calculation in the view finder, the camera’s light meter even tried to warn you that it would horribly over expose the available light at 1/60th. But this wasn’t a high-noon scene, and at 1/250th of a second the background would have been perfect. Sadly, you had no idea, flash was still scary magic. You could have changed the default sync speed in the advanced menu, probably not the best idea. Or you could have switched your camera to manual and decide about the shutter speed yourself. You would have done that because you know what makes a flash exposure and an ambient exposure and how those two relate to each other.