Flash sync speed, first curtain sync, and second curtain sync · Mar 23, 12:29 AM by Simon Mackie
One of the things that confuses many beginning photographers is why, when using a flash, they can’t use a shutter speed of faster than 1/200th or 1/500th of a second (on modern SLRs, at least – on many older cameras you can’t use speeds faster than 1/60th of a second). This maximum speed is known as the flash sync speed or sometimes the X-Sync speed (sometimes marked by an “X” on the shutter speed dial of old manual cameras). On modern cameras, you’ll find that the camera simply doesn’t let you use a higher speed than the sync speed.
To understand why this is the case, you first have to learn a little bit about how the shutter on your camera works (technically, this type of shutter is called a focal plane shutter). Your camera’s shutter actually consists of two shutter curtains. At slow shutter speeds, when you press the shutter release button, the first curtain moves aside (usually down), exposing the film (or digital sensor) to the image. After the appropriate amount of time, the second curtain moves down, ending the exposure. After the image is taken, the curtains move back to their starting position, ready to take another exposure.
At faster shutter speeds, there’s not enough time for the process of the first curtain moving out ofthe way and the second curtain moving down to happen separately. The first curtain moves down and before it has completed its travel, the second curtain starts to move. This results in a slit that moves across the film or sensor – at faster shutter speeds, the slit becomes smaller. As the slit moves across the film or sensor, it is exposed for the correct amount of time, forming a complete image.
This is all fine, until you need to use a flash. When you fire your flash, the amount of time it takes for the flash to illuminate the subject is very short (typically for less than 1/1000th of a second). If you’re using a flash, and the two shutter curtains are moving together forming a slit, when the flash fires, the image formed on the sensor will show dark bands – because only the scene through the slit was illuminated. So for the whole image to be exposed, the first shutter curtain must open fully before the second starts to close.
Some Nikon D-SLRs cameras have an electronic shutter in combination withe the focal plane shutter that enables them to use a much higher sync speed.
Related to this discussion is the concept of first curtain sync and second curtain sync (also sometimes called rear curtain sync). The “default” setting on most cameras is first curtain sync. With this setting, the flash fires as soon as the first curtain has opened fully. This is great because usually you want to capture the image as soon as possible after you press the shutter release.
However, with this setting, you can get unnatural light trails when shooting moving objects with a slow shutter speed at night. Say for a typical example you’re shooting moving cars at night. You press the shutter, the flash illuminates the car, and then for the rest of the exposure the car is moving. This creates an unnatural-looking image where the light trails come out in front of the moving object – not what you want! By using second curtain flash, the flash fires as the second (or rear) curtain starts to close, meaning that you capture the light trails first, and then the flash illuminates the car, so the trails appear behind the moving subject – much better. Second curtain flash can also be used for some interesting creative effects.
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