Flash Photography Tips

DRicherby (Critique Team)

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28 Jan 2011 - 2:19 PM

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The point that underlies much of this video isn't explicitly made, and it's a point about how camera shutters work.

The shutter actually consists of two moving plates, called the 'first curtain' and 'second curtain'. For a relatively slow exposure (depends on the camera but let's say 1/250s or faster), the first curtain slides open to expose the sensor to the incoming light, then the camera waits for a little while with the shutter fully open and then the second curtain slides up to block off the sensor again. This technique only works when the exposure time is relatively large compared to the time it takes the shutter curtains to move across the sensor.

For faster exposures than that, a different technique is used. The first curtain starts to slide open as before but now, before the sensor is fully exposed, the second curtain starts to move up at the same speed as the first. The effect is of a slit moving across the sensor and, the sooner the second curtain starts to move, the narrower the slit becomes. It still takes 1/250s for the slit to cross the sensor but, now, each individual pixel of the sensor is only exposed while the slit is above it. So, for example, if the slit is only half the size of the sensor, each pixel is exposed for half the time that the shutter mechanism is moving, i.e., roughly half of 1/250s = 1/500s.

And this is why the flash doesn't work for fast shutter speeds. The flash fires when the first curtain has fully opened. For slow shutter speeds, the sensor is completely exposed at that point so the flash lights the whole frame. However, for faster shutter speeds, the second curtain has already obscured the lower parts of the frame by the time the first curtain reaches the top and the flash fires. This means that only the top part of the frame is lit by the flash, as you can see in the examples in the video from 4:50 to 6:30.