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 it’s power at a given ISO. Lets take a build-in camera flash as an example. Common (D)SLR in camera flash on average have a Guide Number of 12 at ISO 100. Grab you 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…

#### Distance

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 to 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, lets 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 to difficult I would say, setting your camera on F2.8 should be fine.

#### A Bit More (tl;dr)

“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 you 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 you shutter any faster then 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 you 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 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 of 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 a wast 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 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 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…