Vivitar 13MM-C, Instruction Manual

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Brightness Control
You can set the amount of brightness you want by turning the aperture adjustment ring when
using your lens.
5.6 and 8 are 1 stop apart (1EV). Set a 0.5 stop (0.5EV) between 5.6 and 8
When you turn the aperture adjustment ring between 22, 16, 11, 8, 5.6, 4, 2.8, 2 and 1.4 you are
making 1 stop each turn. Brightness can be set more precisely by making half stops as well (see
below).
[Aperture adjustment ring markings]