manual focusing vagaries

Well, just one vagary to start.

 

I just got my D810 back from Nikon (it was sent in to remove an obnoxiously large piece of debris from the bottom of the pentaprism—no idea how that go there). In testing the manual focus calibration to make sure they hadn't messed up the focusing screen standoff adjustment like they did with an earlier camera, it seemed that the calibration was pretty good for most of my lenses.

 

But with one lens, the 55mm f/2.8 AI-S micro, something is amiss. The focusing screen seems miscalibrated just with this lens.

 

The way I test manual focusing calibration is straightforward, if not scientifically infallible. After adjusting the eyepiece diopter as well as possible, I focus on a suitably clear, high-contrast target as well as I can and take a picture. If the sharpness of this picture matches that of another photo taken with live-view focusing, then I pronounce the calibration good. These tests are conducted always with the aperture wide open to avoid focus shift issues.

 

How is it that this test can pass with most lenses but not with the one 55mm? With that one, focusing through the viewfinder is resulting consistently in front-focus.

 

I will test again after work tonight with conditions as controlled as I can manage.

Did you test with this lens before the repair? Is the error the same at all distances? I'm wondering if the macro lens hits the AF sensor wrong in some odd way.

 

I've had some issues with the focus dot on lenses in the 50 mm range - so for example the "dot tune" method of calibration does not work on my 50/1.4D because the dot never goes off in the calibration range - but it sounds as if you're focusing with the image itself rather than the dot. But I've also found that on some lenses the visual confirmation is different from the dot, and the dot is more often right, even when the diopter seems right. I've never found just what makes some lenses much more difficult to focus visually than others, as they're all over the map, but some definitely seem to do better when I ignore my perception and use the dot.

Yes, I'm using "VFR" instead of "IFR" (by analogy with flying airplanes), and I agree that different lenses give different MF experiences.

 

I did test with the 55mm before the repair, and it didn't seem unusual then.

I'm happy to report that I was wrong. The 55mm is focusing well, in fact as well with the viewfinder as with live view.

 

I blame my eyes and the short throw of the focus ring in the non-macro range for the earlier failures.

Glad to hear it. My micro 55/3.5 is the sharpest tool in the shed, and I'd hate to think something had gone awry.