Originally posted by falconeye I think I didn't express myself clear enough.
What you say is correct, but that wasn't my point.
If a vendor says AF in -3EV (like K-5II) or -2EV (like D800), then it usually means incident (not reflected) light (scene illuminance) and a bright black&white target, i.e., optimum conditions. At least, this is my suspicion without any further detail, always being sceptical when it comes to marketing dep. specs. AFAIK, there is no ISO or CIPA standard AF target.
Got you, thank you.
So your suspicions are
manufacturers may use a bright white target (with a focusing mark) to measure their AF lowest light level -
that would indeed inflate their lowest limit by several stops -
since
zone 5 or V (18% gray) to zone 10 or X (white)
is a 5 stops difference......
FWIW -
I do a lot of photography in what I consider ridiculously low light conditions on an almost weekly basis -
with a K-x - the light levels in some areas are below the metering and AF spec limits.
ISO5000, f/3.5, 1/5sec, -0.7 comp; 18mm
metering segment read outs:
Perhaps seeing the original unprocessed (other than re-sizing) might help to visualize the scene a bit better?
EXIF attached
according to the metering segment it is about
-1.3LV at the face
this is about -1.3 stops below the spec'd metering,
and -0.3 stops below the spec'd AF limit of the K-x -
metering is supposed to be 0LV and AF -1LV, but using a f/1.4 lens -
I am using f/3.5 which is -2 2/3 stop slower/dimmer..