How camera flash works: free photography cheat sheet

Flash photography cheat sheet: what every photographer must know

There is a time and a place for flash photography. It will often kill the atmosphere at a party, but there are other times when your camera flash is the essential ingredient for a successful shot.

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The secret to success is to use the built-in camera flash on most DSLRs with caution. The key to getting good results is often a matter of exposure, ensuring you use settings that make the flash look as natural as possible.

Using camera flash complicates the usual problems of exposure. You not only have to choose the best shutter speed, aperture and ISO to suit the scene, you have to add the flash power into the exposure equation too.

In the latest installment of our long-running photography cheat sheet series we’ve taken a look inside your flashgun to see how it works and illustrated in a series of infographics how your camera flash is timed to match the movement of your shutter curtains. We’ve also compiled a cheat sheet to understanding flash Guide Numbers.

Simply click on each infographic to expand the image, or drag and drop the photography cheat sheet to your desktop to save the larger file.

What’s inside your flashgun


Flash photography cheat sheet: what's inside your flashgun

Read on to learn more about how flash works and to see our new infographics.


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Facts about camera flash

A flashgun provides a brief burst of light – but the duration can be varied to alter how much flash ‘power’ is added to the scene. The amount of power needed will depend on the aperture used (the wider the f/stop setting, the less power required) and how far away they are.

The power of the camera flash falls away with distance. The maximum power varies – the built-in camera flash has much less power than add-on units – but once the subject is more than a few paces away, flash has little effect.

If your subject is within range, you can leave the camera to set the flash exposure automatically, or switch the flash to manual mode (if available) and work it out for yourself using the camera’s Guide Number.

You take the distance to your subject and divide this into the Guide Number to get the lens aperture you need for the correct flash exposure. Below we’ve provided a handy cheat sheet to understanding flash guide numbers.

Flash photography cheat sheet: understanding guide numbers

The ISO setting is also a factor – the higher the sensor sensitivity, the less flash power required. More power means you can shoot at greater distances, too.

The shutter speed is often not a significant factor in the flash exposure calculation, though the shutter speed matters for other reasons.

The ‘focal plane’ shutter of your camera works in a way that means that you can’t use the full shutter speed range – ordinary flash won’t work with speeds faster than the maximum ‘sync speed’ for your camera.

If you’re using the built-in flash or a dedicated external flash, most of the factors that need to be considered when calculating flash exposure are handled by the camera. For example, if you pop up the flash on a Nikon D3100, it caps the shutter speed at 1/200 sec.


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Calculating camera flash exposure

Chris Rutter using a flashgun

Exposure metering for flash is handled in a different way than for non-flash ‘ambient light’ exposures. The flash fires briefly just before the shot is taken, and the light reflected back by the subject is used to work out the exposure and flash settings. This ‘preflash’ is all-but imperceptible to the human eye.

Inevitably, automatic flash will not always give the results that you want. It’s possible to switch the flash to manual mode, then choose the power you need to suit the subject distance or the aperture setting you want to use.

You can choose ½ power, ¼ power and so on – the power is adjusted in the same ‘halving’ and ‘doubling’ steps as regular exposure settings.

Most of the time, though, it’s simplest to leave the flash set to auto and use the flash exposure compensation control to reduce or increase the power as needed.

You can use flash in any of your camera’s exposure modes. Manual (M) will give you the most control over how it balances with the ambient light, but Program (P) can still give great results if you don’t want the hassle of manual adjustments.

Flash timings explained

Flash photography cheat sheet: timing of your flashgun explained


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