Comparing FF vs. APS-C

[Jon Ruyle]

n theory, a decrease in aperture and a corresponding increase in available light, holding shutter speed constant, means the same amount of light hitting the sensor, right?

When you say "decrease in aperture" I think you mean "increase in f number". In your tests, the apertures were the same, as was appropriate (f numbers were different because the focal lengths are different). If the apertures (not f numbers) are the same, effective focal lengths are the same, lighting is the same, and the shutter speeds are the same, each sensor is getting the same amount of light.

I now just take for granted that the 7D is noisy by comparison.

That depends on how you do the comparison. At the same ISO, the of course the 7D has far more noise. But if you take the same picture with both cameras, I think the 7D will have less noise.

Honestly, I wasn't thinking much of noise when I set this up

I wasn't either when I first looked at it... it wasn't until you said the 7D was noisier that I thought... "wait, something is wrong...." (Not wrong necessarily, but not geared to compare noise.)

Jon, Rick, how would you suggest I set that up? I'm thinking of shooting the ColorChecker, using the same lens for equal transmittance, but...the 100 L Macro and change distance, or the 70-200/2.8 II and zoom to compensate for crop?). Manual exposure, same aperture for both cameras (since sensor size doesn't affect exposure), and a flat target obviates DoF. What else?

I think you're making it too complicated. Set your cameras so they take the same picture (as you have done with different lenses and the same aperture). Give them both the same amount of light (ie, same lighting, same shutter speed). Then you have a fair comparison. I would suggest using manual mode under the same lighting and no flash, and setting the ISO so the pictures look similarly bright or so the histograms look similar (ISO does not affect photon noise).


Transmission will be different because the lenses are different, but I don't think that is a biggie, and I don't see a way around it (using the same lens on different cameras with different f/ settings will not give exactly the same transmission percentages either)