Rear Curtain Sync
If you have fiddled with the built-in flash on your camera or with an external flash unit, you may have probably heard about the term flash sync. For the uninitiated, flash sync or, as it is sometimes referred to, flash sync speed is nothing more than syncing the speed of the flash with that of the shutter on your camera. There is some advantages in doing it.
A Brief Explanation of the Mechanism of Shutter Curtains
The shutter on your camera is composed of two curtains – the front and the rear. They are sometimes also referred to as the first and the second curtain. When you press the shutter button it activates a mechanism that moves the first curtain. It in effect exposes the sensor to light coming through the barrel of the lens. Once the first curtain has reached the other end, the rear (or second) curtain starts to travel. It covers the sensor and it is no longer sensitized by light.
Rear Curtain Sync & Flash
Ok, now we understand what the curtains do, but how does it relate to flash and what’s rear curtain sync? More importantly, why rear curtain sync is important to learn? Rear curtain sync is basically syncing the flash so that it fires just before the rear curtain starts to move. Ok, so how is that expected to impact your photography? Read on.
You are shooting at your daughter’s birthday party. She is in a playful mood with her friends. The room is lit with home lights and basically something that isn’t good enough for a proper exposure without a flash. So you mount an external flash to your camera. Now, the problem is with your Nikon you are limited to a shutter speed of 1/250 of a second (1/200 of a second for Canon cameras) or less. This is because of the flash-sync speed limitation I discussed in an earlier article. This creates a problem, especially if you use the default shutter mechanism, i.e.; front curtain sync. What happens is the flash fires immediately after the first curtain has finished travelling. So, the flash fires, freezing the subject, but as the rear curtain moves over the course of the next fraction of second, the movement of the subject (remember, she is in a playful mood) is captured as a blur.
You are shooting light trails in low light conditions. But you decide to use the flash to freeze the movement of some of the cars in the shot. If you use the default front curtain sync, the flash will fire first, freezing the cars. But the lens will remain open and continue to make the exposure. This will result in the front lights to appear shooting out but the backlights will appear protruding into the car. This is unacceptable.
To avoid these problems rear-curtain sync is used. In the first scenario the flash will fire at the end of the exposure, freezing the subject and the blur of the initial movements, though recorded, will come after the subject has been frozen. In the second instance the backlights will appear more natural, coming after the car and not shooting into it. Needless to say both are more natural and is thus acceptable.
You would argue that the blur is unavoidable in either of the technique, so why do we have to shoot in rear curtain sync and not in the default front curtain sync. The thing is blur, if any, is acceptable right after the subject and not in front of it. That’s not natural. In 90% of low light situations rear curtain sync thus gives more appropriate result compared to front curtain sync.