Light sensitivity of different size sensors?

[normadel]

I was fooling around last night and decided to test how very low room light works.

Took same picture of my barely-illuminated family room...almost could not see at all....with my Rebel T1i, 50D, 60D and recently-acquired 6D. ISO set on Auto, Program exposure mode. To my surprise, the three crops produced good pictures of a warmly- lit room, but the 6D image was very very dark..

The crop-sensor images were set comparably at ISO 1600-3200  at 2.5-4 second exposure. The 6D was at ISO 12800 for 1/6th second exposure.

Why the huge variation between the full-frame  6D and the crops, in selected ISO and overall exposure?

[kvbarkley]

Was it the same focus point? I have never quite understood it, but the focus point influences the exposure. If the 6D was focusing on a white book, say, it might make the rest of the image dark.

[normadel]

Crops set to use all focus points, 6D set for a single point, but the scene was pretty monotone....no bright spots.

Metering mode same on all four....Partial Metering.

[jrhoffman75]

If you are using P try setting Evaluative Metering. 

[jrhoffman75]

Evaluative metering is evaluating the brightness of each of the zones and also takes into account the subject the camera has focused on. The camera is going to expose  a white subject towards middle gray, thus making portions darker than white even darker. 

[rs-eos]

Your testing should have used the same exposure values.  No doubt the 6D needed higher ISO since your shutter was only 1/6s.   Ideally, you'll have a tripod as well.  Use the same aperture, shutter and ISO values.   You will probably see the 6D would have the best low-light performance since it should have the largest sensor sites amongst your gear.    While that's not the only part of the equation, it's a big one.

Note: Depending upon your lenses, you may have differences in field-of-view betweent the full-frame and crop-sensor cameras.

[normadel]

All set in Program Exposure mode, so I would have expected them all to be in the same ballpark as to their automated selections. Isn't that the point of autoexposure? OF COURSE, if I set the exposures manually they would be the same.  The 6D chose to use a 4-times higher ISO with 1/12th to 1/24th of the time. That's a way-different overall exposure.

The field-of-view was the same for all. I made sure of that.

I will try this test again.

[Waddizzle]

Your test seem to prove that your expectations may have been flawed in some way.  Setting each camera to P mode is not a very scientific sensor test.

As noted above, the tests should have been performed with the same exposure setting, which means setting the camera to M mode, so that the camera cannot tweak the exposure setting on its own.

I agree with the advice to use Evaluative Metering.  It is what I use 99% of the time.

[jrhoffman75]

I would disagree with M being an appropriate setting by itself.

Setting Av, Tv and ISO the same will ensure all three cameras are set the same, but it doesn't ensure that all three cameras will give a proper exposure if there is indeed a metering variation.

Setting them all to the same settings and then checking where the exposure meter is indicating would be meaningful.

[Waddizzle]

Are you trying to test the sensor?  Or, are you comparing the exposure program curves in each camera body?  I still say take the automatic programming out of the loop, and go with M mode.