Using camera as a lightmeter?

[f0m3]

Would it be possible to use a camera as a light meter? I dont know much about that but i somehow think it must be possible...

[Francis]

You already can, no ML required, only a little understanding of photographic exposure.

Set camera to spot metering, choose 2 out of 3 variables (set ISO and shutter in Tv or ISO and aperture in Av) aim the center AF point at your subject and half-press the shutter button to see what the third value is (aperture in Tv and shutter speed in Av). After figuring out the correct exposure for that combination, you can adjust using reciprocals. If you adjust the aperture down a stop (changing from f/5.6 to f/4) than you go up a stop in shutter or ISO (1/125s to 1/250s or ISO 200 to ISO 400), for example.

The only real missing functionality is a flash lightmeter. I don't believe that the metering system built into any camera is capable or designed to be able to meter something with such a short duration, triggered by that short duration change in exposure. Let's say at the long end (based on sync speed) flash duration is 1/200s. So the camera must within 1ms or so detect a flash and then record exposure metering over the remaining duration. Not exactly sure how the electronics in flash maters are designed but it certainly requires a different metering system than cameras use. That would be nice, though.

[a1ex]

See also http://groups.google.com/group/ml-devel/browse_thread/thread/802a299793748d81

[ilguercio]

You already can, no ML required, only a little understanding of photographic exposure.

Set camera to spot metering, choose 2 out of 3 variables (set ISO and shutter in Tv or ISO and aperture in Av) aim the center AF point at your subject and half-press the shutter button to see what the third value is (aperture in Tv and shutter speed in Av). After figuring out the correct exposure for that combination, you can adjust using reciprocals. If you adjust the aperture down a stop (changing from f/5.6 to f/4) than you go up a stop in shutter or ISO (1/125s to 1/250s or ISO 200 to ISO 400), for example.

The only real missing functionality is a flash lightmeter. I don't believe that the metering system built into any camera is capable or designed to be able to meter something with such a short duration, triggered by that short duration change in exposure. Let's say at the long end (based on sync speed) flash duration is 1/200s. So the camera must within 1ms or so detect a flash and then record exposure metering over the remaining duration. Not exactly sure how the electronics in flash maters are designed but it certainly requires a different metering system than cameras use. That would be nice, though.

+1.
About being a flash meter, i always do trial and error even if my Sigma flash is E-TTL compatible. Still, i find manual flash exposure to be much more consistent and it's not a big waste of time to take a test shot of the scene and check the histogram.

[scrax]

Hi, you can use it but only for reflected-light measuring, not for incident-light metering. Also you can't use it, as Francis pointed out, for studio flash metering.
And spot metering on camera is usually bigger (3-5°) than a real spot lightmeter (1°) resulting in a less precise measure.