wrote:
Jeremy Nixon wrote:

No, there was; if the meter indicates one stop underexposure, but you can
increase three stops and end up with a proper exposure,

What is "proper exposure"?

The exposure you want is the proper exposure. In this case, +2 from the
meter reading.

then the meter was actually off by two stops.

No, the meter is dumb, and it puts the mid-key scene at middle grey,
which is not *optimum*, unless your goal is to use a JPEG as-is, or
print directly from the camera. In terms of RAW capture, it is a
relatively poor digitization and/or exposure.

Right. The meter is off by two stops from the exposure you want.

You're comparing "trust the meter" at ISO 200
with "don't trust the meter" at ISO 800, and that's not a useful
comparison, because if you trusted the meter in the first situation
you'd trust it in the second and still underexpose by two stops, but if
you weren't going to trust the meter in the first place (and thus be
willing to go up two more stops from what it said at ISO 800) then you'd
have been willing to do so at ISO 200, too, if you could -- leaving the
choices being either underexpose at 200 or expose properly at 800.

I clearly stated that we were a stop under because of *NEEDED* f-stop
and shutter speed! You're going to blur the image now?

No. Using the needed f-stop and shutter speed, at ISO 200 you are
underexposing, and at 800 you are not. You're suggesting that at
ISO 200 you are blindly trusting the meter, but once you go to 800
you suddenly aren't and are willing to use +2 EC from what it's
telling you to get a good exposure.

In real life, if someone is willing to use +2 EC to get a good exposure
at ISO 800, then they were willing to do so at ISO 200, and the only
reason they didn't is because the shutter speed would end up too low.
So, the conclusion is that there isn't enough light for ISO 200, and
the shot would be underexposed. Since it's better to boost the ISO
than to underexpose, you boost the ISO.

The properly exposed shot at ISO 800 will be of better technical quality
than the underexposed one at 200. However, the properly exposed shot
at 800 will have *more* noise than the properly exposed shot at ISO 200;
it's just that you couldn't get the shot at 200.

The usual thought process here, from the photographer's perspective,
would be: good exposure is at +2 EC. I can't get there with shutter
speed because it'll be too slow to hand-hold; I can't get there with
aperture because I need the depth of field; so I get there with ISO.

Let me try another way. Let's pretend the camera is greyscale (no CFA),
to simplify matters. The camera meters a grey card (or the ~18% grey
scene) as if to wind up with an average RAW value of 400 (fairly
typical) out of about 4000 possible levels. At ISO 100, you fall a stop
short of that with your needed f-stop and shutter speed, so the RAW
output would be 200. Using the same aperture and shutter speed (as
needed), with ISO 800, the average now is a RAW value of 1600. If you
used ISO 1600, the average RAW value would be 3200, and you might have a
few brighter spots clipping past 4095.

Right. At ISO 100, you're drastically underexposing. At 200, you're
still underexposing. At 800, you're not. At 1600, you're overexposing,
unless you intended for those bright spots to clip (spectral reflections,
point light sources, etc).

--
Jeremy |