Controlling mechanism for shutter and camera flash
Controlling mechanism for shutter and camera flash | ||
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The burst of light from a flash and the amount of time a shutter is open are both so fast that the two operations must take place with precise timing. They must be in synchronization, or sync. Contributing to the problem is the fact that on most focal plane cameras, a shutter speed faster than 1/250 of a second means the shutter is never completely open at one time so that the entire frame can benefit from the flash. As the speed of the shutter increases, synchronization becomes a greater problem that is countered by increasingly clever scene pro- grams and more intelligent flash attachments. As part of the dance of flash and exposure we just saw in the previous chapters, one of the other things that occurs when the photographer presses the shutter button is that the shutter and the flash begin a nimble pas de deux called synchronization or sync. On most cameras that use focal plane shutters, or leaf shutters, only shutter speeds as slow as 1/60 or 1/125 of a second allow the entire image sensor to be exposed at the same time. These exposures use X-sync. The flash waits until the shutter's curtains have completely exposed the image sensor. Then, in just 1/1,000 of a second, the flash releases its full fury before the shutter can start to close. The top shutter speed for most cameras using X-sync is 1/500 of a second. Faster shutter speeds require the second shutter leaf to start covering up the sensor before the first leaf has finished exposing it. The exposure is made through a slot formed as the shutter's two curtains pass in front of the sensor. An unsync flash used with this type of shutter operation, instead of producing a complete photo, it will capture incomplete picture. What is needed in this situation is a relatively long flash with reduced brightness. But flash units are pretty much full-speed ahead until it's time, according to the thyrister, to turn the light off. Short exposure times call for focal plane sync, or FP sync (also called HSS, for high speed sync). With this setting, the flash executes a series of short bursts at a high frequency, about 50MHz. None of the bursts swell to the flash's full brilliance, but the series lasts long enough to illuminate the full frame evenly. FP sync can work with some cameras at shutter speeds up to 1/8,000 of a second. Photographers use another type of synchronization, slow sync, to capture movement using flash. The flash is timed to go off only when the shutter is fully open. Meanwhile, with most of the shutter open, the image sensor is already recording the moving subject of the photo. Because the subject is moving and because the scene is dark enough to require a flash, this part of the exposure is more suggestion than documentation. The subject is blurred, and even the blur is hard to distinguish except for any lights or bright reflections. Right before the shutter snaps shut, the flash goes off, adding a clear, exposed view of the subject. In this case, the streaks from a car's lights, captured before the flash went off, provide the sense of movement. But notice how the proper synchronization is crucial to the effect. If the flash goes off just as the shutter has opened, as in this photo, the hammer appears to be streaking backward. Slow sync is also called rear curtain sync. | ||