Canon 40mm STM misfocus issues

Started Oct 11, 2012 | Discussions thread
WilbaW Forum Pro • Posts: 11,423
Re: 40mm STM misfocus / Phase & Contrast detection difference

Nightly wrote:

I will do another test, with long distance soon. But does it really make a difference?

It does with a lens that doesn't focus consistently from minimum focus distance (MFD) to infinity, in which case the system needs to be calibrated for the current subject distance, rather than at one arbitrary distance, for optimal focus. Fortunately, not many lenses are like that. The rest focus the same near and far.

Where it does matter is that the easiest way to see a focus error is when the depth of field (DOF) is narrowest, which is with the widest aperture, near the MFD, and using the longest focal length on a zoom. If you want to test at longer distances (e.g. 50 times the focal length), you need a much larger scale at a low angle, which means you need a parallel target, and you have to zoom into the image to see the error. I suspect one of reasons why Canon specifies a test method like that is it makes it easy for them to meet their slack standard of autofocus accuracy. If they made it easy for you to spot and document errors they'd have a lot more work to do.

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