Originally Posted by neuroanatomist
But that's comparing the lenses wide open and uncropped. The result is exactly as expected: the slower uncropped lens wins. This isn't what I meant by "switching the lenses". I meant, instead of making the 50mm faster and cropping it, make the 85mm faster and crop it.
Originally Posted by neuroanatomist
They wont take exactly the same pictures, of course. One will have better IQ. But you can compare two systems, one FF and one APS-C in which each system has the same framing, same working distance, same DOF, same compression and same noise. Yes, the two systems will use different lenses, and this effects the outcome. But if I consistently get far better IQ taking the same picture (modulo IQ) with cheaper lenses using one of the two systems, that's the one I want.
Originally Posted by neuroanatomist
This is the core of our disagreement. I say you not only can change the lens, you *must* change it. You must compare systems with the same *effective* focal length and *effective* f numbers. You seem to feel we must compare systems with the same actual focal length and same actual f number.
Originally Posted by neuroanatomist
I agree. But I think you're using the wrong variables. If we change the point (3,4) in Cartesian coordinates to the point (6,8), we've changed two variables because x and y both changed, right? Maybe. But in polar coordinates, I've kept theta the same, and only changed the variable r.
Keep everything else the same and change only the sensor. Yes, you have to change the lens, too. That's too bad. Try to choose lenses that aren't too different. (Don't compare a cheap zoom to an expensive prime, for example). One system will consistently win, and the lenses you need to perform the test for that system will consistently cost less.
If you insist that it makes sense to compare IQ in systems that take pictures with different DOF, compression, exposure time, etc, then I agree that we must agree to disagree (don't read that ten times fast [])
Anyone else out there want to weigh in? Is anyone else still there? Can you say "hijacked thread"? My apologies, John.