Flash Sync Speed

Flash sync speed is a camera feature that can - under certain conditions - cause a lot of headaches for photogaphers new to flash.

The problem that you run into depends on the type of flash you're using: the camera will either prevent you from changing settings or will introduce dark bands in your photos.

Neither one of these options is especially great, but before you feel like you just wasted a lot of money on a camera with limitations know this: flash sync speed is a limitation of EVERY digital SLR camera.

A deeper understanding of how flash sync speed works in conjunction with shutter speed should help clarify what this feature is all about and what you can do to work with it.

Shutter Speed

The only camera setting that the flash sync speed impacts is shutter speed.

A shutter is a curtain that sits in front of your camera's sensor and blocks out all the light. When you press the button to take a photo, the shutter opens for a very brief period of time: this is called the shutter speed.

Shutter speeds are typically measured in fractions of a second (although you can use shutter speeds longer than a second) and there's a common scale for them (with the numerator removed):

400020001000500250 12560 30 15  8  4  2  1  

In this case, 4000 is really 1/4000th of a second.

This is the high-level introduction to a shutter - a deeper understanding of the shutter mechanism is required to clarify why flash sync speed limits the shutter speed.

Shutters are actually made up of two separate curtains, and you can think of them exactly like the ones that hang in front of windows.

When you take a photo with a slow shutter speed, the first curtain snaps open to expose the sensor to light, then the second curtain snaps closed to block the light.

But when you use a very fast shutter speed, the second curtain starts to close before the first one opens all the way.

If the curtains travel together like this, they won't play nice with your flash: this is the flash sync (shutter) speed.

Effects of Flash Sync Speed

Now that you know that the flash sync speed is related to shutter speed, we can talk about it in more concrete detail.

The most common flash sync speed for digital SLR cameras is 1/250th of a second, regardless of manufacturer and regardless of camera model.

Some budget digital SLRs will have slower flash sync speeds, but there are very few that have a faster sync speed (the most notable digital SLRs with a faster sync speed are the Nikon D50 and the Nikon D70s).

The flash sync speed is a limit - and you'll see it manifest in one of two ways:

Using Built-in Flash or Flash Attached to the Hot Shoe

In this case, the camera prevents you from selecting ANY shutter speed faster than the sync speed. You can use shutter speeds faster than the flash sync speed, but only if the flash is turned off.

Using External Flash With Remote Triggers

Since your camera isn't immediately aware that a flash is active, it will allow you to select shutter speeds that are faster than the flash sync speed. Unfortunately, the end result is that the second closing shutter blocks some of the light from reaching the sensor, and you wind up with an image that has a dark band on one side.

No matter how you work with flash, the flash sync speed limit simply means that you can't use blazing-fast shutter speeds when you're also trying to use flash.

The most you can do when photographing fast-moving subjects is to crank the shutter speed up as fast as it will go and wait to photograph your subject when the action peaks or pauses instead of when it's in full swing.

It's because of the flash sync speed that I often suggest that low-light action photographers dispense with flash completely and rely instead on a high ISO setting to obtain a faster shutter speed.

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