Aperture sizes are measured in f-numbers, which are essentially the denominator of a fractional diameter. In photography settings are measured with "stops", where any adjustment by one stop means letting in half as much light or twice as much light. Adjusting the aperture by a "full stop" means changing its area by a factor of 2. In terms of the diameter, this means chaning by a factor of sqrt(2) = 1.4, so the f-numbers grow as powers of 1.4. Larger numbers mean smaller apertures.
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Dotted focus line indicates looking through the mirror