how does it work: Exposure time

This scale has essentially no theoretical limits. It can be as long as days and be fractions of a millionth of a second. For really long exposures we simply hold the shutter open. For extremely brief exposures – shorter than can be given by shutters – we have to rely on brief flashes of light from flash-units: even in small cameras, the exposure time can be as short as 1/10,000 sec or shorter. In normal photography we generally work between about 1/30sec and 1/2000sec.

Ah! it’s that pesky forward slash again: here we’re dividing our shutter setting into 1: which is why photography professors call this measure “reciprocal seconds”. One second is 1/1 – one divided by one equals one, giving us a pretty long exposure. 1/10 is one second divided by ten parts, so each is a tenth of a second long. 1/1000 is one second divided into a thousand parts, so each is a very brief one-thousandth of a second slice of time. Of course in stills photography, we only take one slice at a time. And our cameras display just the lower (divisor) number, such as 1000, or 250.

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