Canon 7D focusing issue revisted

Started Apr 8, 2012 | Discussions thread
kimvette
Senior MemberPosts: 1,226
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Re: Sigh...
In reply to Jerry-astro, Apr 10, 2012

In addition some DSLR bodies are reputedly designed to intentionally bias to front-focusing ever so slightly (so portraits will be well-defined in the nose and mouth areas when you focus on the eyes) but when shooting close subjects where the FOV is practically razor-thin (a few mm at f/1.4) it can be a bit of a problem. You need to take this into account.

ALSO: the focus point markings in the viewfinder and postprocessing software almost never perfectly match the focus points' actual size. The focus points are slightly larger than indicated - and this may seem like a gaff but it is intentional; it is intended to enable the camera to focus even if your aim is slightly off. Again, when shooting subjects at close range with a wide aperture your DOF is only a few mm wide, so you need to take this into account. Either MA your wide lenses to push the focus back a little to be perfectly neutral (in this case the OP and ssteven will still find reason to complain) or consider manual focus instead. Remember that you're focusing on 3D objects and the AF point isn't a single point, but an area slightly larger than the indicator on the VF, so it may not focus exactly on the point you think it is. Your test with a curved object is inaccurate; consider shooting a flat object parallel to the sensor for your test, with another object equidistant from the lens off to the side. Even then, it won't be a perfect test because the focal plane isn't perfectly flat; like the lens, it is curved slightly but it is still a better test for what you are trying to achieve.

Or, search the forums for "lensalign" and follow that method of testing, with a flat target and a slanted rule off to one side - close to the focus point but far enough so it is not within the AF point.

And, if your lenses do require MA, don't sweat it; the tolerance is literally on the order of microns (tens of microns really). If you want tighter manufacturing tolerances, you would have to pay an order of magnitude more (or even higher) for bodies and lenses.

Sometimes the gear is faulty, but in this case it appears to be working as designed.

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