Shelley,
First, pinhole photography's technical aspects are not different from those
of glass lens photography, and that includes f/stops. So you can get/make
pinhole apertures that would give you f/stops that "CLOSE DOWN" in the same
sequence of full stops you are use to with glass lenses, i.e., closing down
from f/16 to f/22 to f/32, f/45...,f/128, f/180, etc. that
unfortunately would require to make pinholes to the proper sizes that would
give you the above aperture numbers. More often than not we end up with
f/stops that aren't full stops, like f/160.
As you can see, f/160 falls between f/128 and f/180, one could make the math
calculations and find that f/160 is just about equal to f/128 +2/3 stops.
Since pinhole is not an exact practice and since pinhole exposures tend to
be more underexposed than overexposed and since for practical reason is best
to use full stops rather than fractional, the wise thing to do is to
approximate f/160 to the next smallest full stop, f/180 in this case.
Guillermo
