Originally posted by RioRico The formua for effective aperture: EA= NA*(M+1) where EA is effective aperture, NA is nominal aperture (the lens setting), and M is magnification. ...
I now remember reading about this somewhere, but it obviously didn't really register.
So, last week, I put a roll of film past a Macro lens I was using (Elicar 2.5/90 Macro), but accidentally forgot to reset the ISO on the camera (an OM2n) and had the ISO set to 200 for a 400 ISO film.
I didn't realize I had done this until about half-way through the film, when I reset it to 400.
I wondered how my shots would look overexposed by 1 stop (compared to my usual practice)
Well, actually, they came out very well.
Now, I see that -using the above formula- for a magnification of .25x (which is about what I was using, photographing flowers), I
should be allowing a +1 stop exposure.
This is very interesting, and I will try to adopt this in my future technique.
Thanks