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What is the flash sync speed?

The way the camera actually operates when doing an exposure is by pulling two curtains across the sensor. This is how an exposure is done in modern SLRs cameras. When the shutter opens the first curtain is pulled across, followed by the second curtain at the speed of the “Shutter”. At “sync” speed, the second curtain follows the first curtain exactly when the entire sensor is open. If your shutter is faster than your cameras sync speed, let’s say 1/500^th. The second curtain has started to close when the flash goes off. This usually results in some portion of the image being darker than it should be.

A quick example, let’s assume that the camera’s sync speed is 1/250^th. If you set your shutter speed to 1/60^th, the first curtain fully opens, 1/60^th of a second later the second curtain follows so that the entire sensor is fully exposed for 1/60^th of a second. Now let’s assume that you set your shutter speed to 1/8000^th. The first curtain opens, as soon as it does the second curtain starts to close as well since it to be able to keep the exposure on the sensor to 1/8000^th of a second. That means only a small sliver of the sensor is exposed at any point in time. If there was a “fast” bright source (like a flash) that was introduced only several small slivers portions of the sensor would record that light.

To summarize, the sync speed of any SLR is the fastest speed at which the entire sensor is exposed before the second curtain starts to close.

Most consumer DSLRs have a sync speed of 1/200^th, most professional cameras have a sync speed of 1/250^th or higher. Some of the really good exceptions are the original Canon 1D bodies which had a 1/500^th sync speed which is unheard of now days.