Quote Originally Posted by Nick Zentena
Dan do you have the manual for the III prism?
Or at least what does the switch underneath do?
It's labeled 0,1,2 I think.
That's built in EI compensation; 0 is none, 1 is .3, and
2 is .6, or 1 or 2 1/3 stops. Use or not as you choose.

At 0 the ISO set is the EI used by the meter to arrive
at an exposure recommendation. At 1 or 2 that EI is
reduced by 1 or 2 1/3 stops.

I found my AEIII meter to give high readings even
after making a reduction from the ISO when setting
the film speed so I added .6 or 2/3 more
with that switch.

I'm quite sure that info is correct. I started using
the rotary a couple of years ago. I'm basically a WLF
type and that eye level was a pain in the neck,
literally. I encounter a lot of verticals when
working in the woods. Dan