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D7100: AF focuses on background a lot?


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doppler_fto

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"RE: D7100: AF focuses on background a lot?"

doppler_fto Registered since 23rd Mar 2013
Sun 12-May-13 11:24 PM

A Breakthrough


Okay, after another frustrating start to a shoot yesterday, I finally figured out a large part of what's going on.

The breakthrough was that I noticed that I get out-of-focus images when manual focusing as well as when auto-focusing. Since the numbers in my viewfinder are sharp (demonstrating that my dioptre is set properly), this almost definitely means that my mirror angles are mis-calibrated.

See here for how Phase-Detect AF works:
http://photographylife.com/how-phase-detection-autofocus-works

Note that the secondary mirror is hinged off the back of the main mirror (and if you carefully open your DSLR while it's off and flip the main mirror up, you can see the secondary mirror flip up behind it). So if the main mirror angle is off, the secondary mirror angle may also be off.

Assuming the secondary mirror is off, that could easily cause the image to be misaligned with respect to the AF sensor. This could explain why the camera appeared to focus on the background so frequently — the part of the image falling on the selected AF sensor wasn't the part of the image that should have been hitting it.

But wait...


That said, an interesting aspect of this problem in my case is that it appears that the AF system and the focus screen are calibrated the same. That is, when I AF on a subject, that subject appears to be in focus when looking through the viewfinder. If I manual-focus on a subject, it appears to be equally well in-focus.

It's unclear what exactly that means. One extremely unlikely possibility is that the image sensor is the wrong distance from the lens mount. Another extremely unlikely possibility is that the AF is calibrated via some mechanism that attaches through the viewfinder (or, perhaps more likely, they assemble the mirror box, stick it into the calibration rig, and then stick it into the camera frame?). I really don't know, but it certainly feels like more than coincidence.

So, what now?


At this point, I have a definitive testcase which clearly demonstrates that one of my cameras is seriously miscalibrated (and the other is probably more subtly miscalibrated). I'll probably do some small tests (like swapping lenses) to see what I can see, but really, I'd love to have some confidence in the AF system when shooting, so it'll be off to Nikon pretty soon. I'll probably send one-at-a-time just so I'm not without a low-light option in case anything comes up while it's gone.

If my current hypothesis is correct, then I believe I should see the accuracy of manual-focus differ at different heights in the incoming image, so I'll double-check that. I'll also try using all of the AF sensors to see if there are any strong correlations there.

If you have any other tests you'd like me to do, or alternate hypotheses, please let me know. I haven't yet, but I'm still planning on running nrothschild's stop sign test as well.

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This is a hot topic  It has been locked by a team member D7100: AF focuses on background a lot? [View all] , doppler_fto , Fri 03-May-13 05:19 AM
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