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jcolwell wrote:
Also, if most AFMA testers are competent and use relevant methods, then I believe that statistically significant trends will emerge, as the number of test results grows.
It certainly can be done, but I think it's a pretty huge task. We want to compare different camera types. But we have a big variation within each camera type. For example, let's just take a dozen copies of Camera A. There are at least 6 calibrations (mirror angle, submirror angle, AF sensor angle ((3 of these actually)), electronic for each AF sensor spot on the AF sensor, electronic for distance) within each copy, and if they're calibrated, then they vary. Then we've got lenses we use, and those have calibration points (AF motor speed, lens distance sensor, electronic recognition of infinity focus point).
The reason I only tested center point focus is that lateral focusing points are affected by lens variation. Few of us want to know this, but on optical bench testing about 25% of prime lenses and nearly half of zooms have a detectible tilt. It's not severe enough to affect our photographs, but it probably is severe enough to affect off-axis focus (since phase detection is comparing the light path from two sides of the lens).
All that being said, enough data points can overcome multiple variables. We gave up before doing off-axis testing, though, because we figured we needed at least 500 data points from different copies of cameras and lenses for each camera-lens-lighting configuration (so 500 off-axis data points for 5DIII with 24-70 f/2.8 at 24mm under tungsten light of XXX lumens, for example).
I'd suggest limiting to prime lenses (less variance) and a few bodies. Then if maybe a dozen people each did a series of 50 shots under reasonably controlled conditions, we'd have a reasonably accurate database. But I doubt it's going to happen. In the meantime I trust comments made by numerous people who have two or three cameras and notice a difference. I think those comments are pretty consistent.
For me, personally, the more I understand phase detection AF, the less I complain about how well it works and the more amazed I am that it works at all. It may be the most amazing engineering triumph in optics.
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