Quote:2. Some lenses can do inaccurate, or "crude" AF steps when the camera is going through its AF algorithms. They basically mess up what the camera accurately says about where focus should be reached. Your Tamron 17-50mm is an example of such a lens. The Tamron service center has tried to lessen the inaccuracy of the steps the lens makes, to make it step more in line with what the camera body tells it to do. The Canon 35mm f2 and 70-200mm f4 L do not make such inaccurate steps, and hence produced better focus results without the need of "calibration".
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Hope that sheds some light on how PD AF works, and why you have had trouble with that Tamron.
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Your Tamron will not behave the same on the M5 as it does on your 1000D. There are some lenses which will not focus well or at all with live view, because their AF implementation is just that horrible, but probably that Tamron is not one of those. But you just have to try that out and see. The work your Tamron has seen being done to it in service centers will not have a negative bearing on how it functions on an EOS M5, or another Canon APS-C DSLR, for that matter. Your Canon lenses will focus pretty accurately on an EOS M5 (or even a EOS M (the 1st EOS M camera).
Now that makes a heck lotta sense to me now after re-reading your comments again. After Tamron Service fixed a very obvious front-focus issue (they did confirm that to me verbally back then over the phone), they adjusted the lens a couple of times until the front-focus was basically gone. However, I still observe(d) (to this day) that sometimes the AF focal plane is just not where I expect it, sometimes slightly behind, sometimes slightly before. I always attributed this to some manufacturing/adjustment "tolerance ranges" between Canon, Tamron, EF lens mount specs, etc. - and eventually started to doubt my own photography skills and thought I'm getting older and can't keep the camera still and micro movements messed up my focus - until not so long ago I took a series of pictures using a tripod (I don't use a tripod often) where I observed that the AF was not spot on a lot of times.
The crude AF steps could explain this - it's sometimes just slight off - a big before or behind the focal plane. And I never observed this with my Canon lenses. By any chance, do you know if crude AF stepping is a general issue with Tamron? Or just certain lenses? And do you know if this is the same with Sigma, e.g. the Sigma 17-50 f/2.8? And could we safely assume Canon is generally not doing such crude things?
Quote:Thanks for the suggestion! The problem exists at f/2.8.
I can basically confirm that as well - on my Tamron 17-50mm (VC version) it appears to randomly happen at any aperture.
(However, in general, I have to say the few times I used live view for focusing, my impression was the focus typically was more "spot on" than focusing through the mirror. Not sure how that fits into all of the above .... I could be wrong here though. )