Flash Fundamentals Synchronizing Shutter Speed
Electronic Flash and Limitations on Shutter Speed in DSLRs
Nov 16, 2009
It is essential for the understanding of basic and advanced flash photography techniques to know the operating principles of focal plane shutters and their peculiarities.
Focal Plane and Curtains
There are two curtains in focal plane shutter. The first curtain moves across the front of the sensor to allow light from the lens to reach the digital sensor, and then the second curtain moves to close the shutter preventing the light from reaching the sensor. Focal plane shutters came from film SLR cameras where film instead of a digital sensor to record the image.
The moving curtain is the key to the focal plane shutter’s interaction with electronic flash. The first, sometimes called the front, curtain takes a certain amount of time to go from fully closed to fully open. Then the trailing or second curtain starts to close to end the image exposure, and this takes a certain amount of time.
There are times when the sensor is recording only part of the image and one of the two curtains hides part of the image. In normal photography with steady ambient light this is not an important factor as both curtains start from the same point, and operate at the same speed, so each part of the image receives the same amount of partial exposure.
It used to be common for the curtains to move horizontally across the image plane, now most DSLRs have vertically operating focal plane shutters. This reduces the distanced the curtains have travel, making for a quicker transition to fully open or closed.
Electronic Flash
These provide a very short duration burst of intense light to illuminate the scene. It is so quick that if the focal plane shutter is not completely open one of the moving curtains will obscure part of the image making it underexposed.
The camera’s program accounts for this and sends the signal to trigger the electronic flash some time after the shutter begins to open so the shutter is fully open for the very brief flash of light, and before the second curtain starts to close.
Maximum Shutter Speed
At some point, as speeds increase, the shutter is never completely open at any point in time during the exposure. The flash synchronization speed quoted in the camera’s specifications is the fastest practical shutter speed where the shutter is fully open. Commonly in DSLRS when the flash is on, the camera will not allow photographers to select a shutter speed faster than its flash synchronization speed.
Focal Plane Flash Mode
This mode is often found on high end DSLRs and compatible flash units. It overcomes the limitations of their focal plane shutter by firing the flash multiple times during the exposure so all of the image area is illuminated.
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