Standard camera shutter speeds are marked:
1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second.

But note:
1/15 is not half of 1/8.
1/125 is not half of 1/60.

Why the inaccuracy?

In fact, the marked values are approximations. The true values are:
1/512, 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4, 1/2, 1 second.

This sequence -- the powers of two -- is more familiar from the digital world: it's the binary arithmetic of bits and bytes. Analog film cameras always carried within themselves the template for their inevitable digital extinction.

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