Originally posted by jpzk How do you calculate the EV from the shutter speed, aperture and ISO?
JP,
I see your question has been answered.
Just an additional remark. EV can be regarded in two possible ways: EV of the light source and EV of the subject.
Normally, EV is given for the light source. Like with the Sunny 16 rule.
But here, we really care about the EV of the subject as this probably is what determines AF performance.
EV of the light source and subject can be considered the same if the subject reflects all light (bright white paper) and is exposed to saturate the image (100% gray).
However, the camera would expose said sheet of bright white paper to ISO norm (18% gray) which is 2.5 stops less.
Therefore, if you compute a light sources' EV value from the automatic exposure value for a sheet of bright white paper (w/o EV compensation dialed in), subtract 2.5 from your computed EV value. If Pentax talks of EV -1 as low AF limit, then they must mean light source EV.