Flash ⚡️ Exposure Calculator

Flash exposures can be difficult to guess on a manual or a toy/disposable film camera. This tool offers an easy way to get it right.

Your camera or flash manufacturer will often list the Guide Number in a manual or a spec sheet. Look it up and find a matching or close value in the calculator along with your film speed (ISO). Most disposable/toy cameras and plastic compacts like the Kodak Snapic A1 use GN 10.

Set your distance and the Flash Exposure Calculator will give you a correct aperture for your camera. That’s it!

Your shutter speed should be your camera’s ⚡️ sync speed (typically 1/60s) or slower. Cameras with leaf shutters, including disposables, do not need to worry about shutter speeds unless you want to try the shutter drag technique.

For help estimating distance to your subject, see this section of a guide on zone focusing (zone focusing = guessing distance to set manual lens focus).

 ☝︎ Further reading: “A Simple Guide to Using Flash.”

Shutter drag.

When you set your shutter speed slower than the sync speed, you are essentially operating your camera in shutter drag mode. This term is easy to remember if you imagine that the flash quickly makes the main exposure as the open shutter keeps dragging slowly to close, letting in more light.

First-curtain sync is when your camera fires the flash immediately when the shutter opens, leaving shutter drag to take place after.

If you’re photographing a moving object, like someone on a bicycle, at a slow shutter with the flash on and your camera’s shutter is synchronized to the first curtain, the biker’s earlier position will appear crisp while their movement after that moment will appear smudged.

 ☝︎ Further reading: “How to Make Sharper Photos on Film.”

Second-curtain sync is when your camera fires the flash immediately after the shutter closes. The shutter drag will happen before the flash.

If you’re photographing a moving object, like someone on a bicycle, at a slow shutter with the flash on and your camera’s shutter is synchronized to the second curtain, the biker’s movement before the flash will appear smudged while their last position will look crisp.

Second curtain sync works well with scenarios like the one of the biker I described above. It can make more sense when someone fast has an appearance of a comet: crisp exposure at the front with a tail going back. But it may also feel strange and more difficult to time action, as when you press the shutter button there will be no flash until the shutter closes (which could take longer than a second in dark settings) — which is why most cameras are synchronized with the first curtain. But some cameras may let you choose.

Leaf shutters work with flash at any setting because they expose the entire film plane — not just a part of it — at high speeds. But the principle is the same: if you set your shutter to a slow speed, the flash will fire just when it opens (first-curtain sync) or just before it closes (second-curtain sync).

Of course, if there isn’t much movement in your scene and your camera is on a tripod, there won’t be noticeable blurring. Instead, parts that aren’t lit by your flash will look brighter (or as bright as your shutter/ISO/aperture settings allow).

Fill flash.

You may not need flash in daylight or under strong lights. But if you’re shooting against the light and want to illuminate the shadows or prevent silhouettes, you can add flash using the same calculations.

Filter factor.

If you’re shooting with flash and a filter, you’ll need to compensate your aperture value by the amount FF requires. Filter factors aren’t stops; instead, they represent the amount by which the light is diminished. So a FF of 2 means that half the light enters the lens and thus the exposure should be increased by one stop. A FF of 4 means that exposure needs to be increased by two stops.

Filters can cover flash units themselves, but their action and the calculations are the same.