- Ed Dozier
Nikon Auto AF Fine-Tune Calibration Analysis
Nikon provides an automatic focus-calibrating feature on some of its newer cameras, which uses Live-View autofocus feedback to perform the calibration. Live-View focus uses the more accurate (and very slow) "contrast detect" focus. This article analyzes just how good (or how bad) this feature is.
The camera expects an ‘easy’ target in good light to get a reliable focus result. The tests in this article were shot at EV 7.6, which should be well inside the realm of “easy” focusing for this camera. I am using the D850 for all of these tests, along with my Nikkor 85mm f/1.4 AF-S lens shot with a wide-open aperture. This fast lens exhibits spherical aberration, so the optimum focus fine-tune calibration value is different for other lens apertures, but that’s a whole other story.
You’ll typically want to calibrate with your lens aperture wide-open, since stopping down will just hide small focus calibration errors. I’m using my 85mm f/1.4 for this evaluation, because it has an exceptionally narrow focus depth when shot wide open at short distances. It desperately needs accurate focus calibration.
Automatic Focus Fine-Tune feature
To perform the automatic focus calibration, you begin by focusing your lens on the target in Live View mode (with AF-S), and then press the “AF mode” and the “Movie Record” buttons simultaneously for about 2 seconds. After you give the camera permission to continue (as shown above), it then assigns the focus fine-tune value to your lens. The “AF mode” button is on the front of the camera near its base.
Plot of a perfectly focus-calibrated lens
The holy grail of focus calibration is shown in the ‘profile’ plot above, generated by the MTFMapper program. The blue line shows where the expected focus should be, and the green plot shows where sharp focus actually occurred. Truth be told, taking multiple photos with the same focus calibration multiple times results in perfect focus only approximately 70% of the time with this lens. The other shots would exhibit small shifts both in front of and behind the desired focus plane. The “correct answer” starts to involve statistics, due to the inevitable small amount of slop in the imperfect camera/lens focusing system.
I got the results shown above my manually performing focus fine-tune calibration. Next, I used AF-C continuous phase-detect autofocus while taking the shot.
The focus target
I used the target shown above to perform the automatic calibration. This shot actually shows what the MTFMapper program calls an “annotated plot”, which includes little resolution measurements added to each edge that it can measure. This target makes it really easy for a camera to focus; the camera focus point is aimed at the right-hand edge of the large black central rectangle. The target itself is rotated 45 degrees relative to the camera image sensor, so that the right-hand side is closer to the camera.
The MTFMapper web site offers free files of focus targets and resolution targets that you can then print and mount.. It's of course not mandatory to use a target like this for automatic focus fine-tune calibration, but you'll find that it's superior to almost anything else you could use.
Target measurements detail, fine-tune +2
The close-up above shows my manually-calibrated lens (85mm f/1.4) using the focus fine-tune value of +2. The plot shown earlier in this article was generated using all of the measurements it makes using this chart. This photograph of the focus target used phase-detect focus, with AF-C continuous focus. The highest MTF50 edge measurements pretty much align with the vertical right-hand edge of the large rectangle, just as they should.
I made a series of attempts to have the camera focus-calibrate itself, just to see how these measurements compared to my manually-determined focus fine-tune value of +2. All of my attempts were done at f/1.4, and the camera was on a tripod.
My first automatic focus calibration attempt got an answer of -8. I then took a shot of the target using this new focus fine-tune value using AF-C and phase-detect.
Focus results calibrated with fine-tune -8
The results, just as I had assumed, showed the lens focus was now much too near to the camera. The automatic calibration feature got it very,very wrong. Maybe that was just a fluke. This focus miss would result in a resolution loss of about 18% at the desired focus plane.
Target measurements detail, fine-tune -8
The measurement details shown above indicate that focus got shifted to the right, nearer to the camera and away from the large vertical edge of the target where I had aimed the focus sensor.
Not wanting to give up just yet, I tried the whole automatic focus-fine tune calibration procedure again. This time, the calibration fine-tune value result was -5. Slightly closer to the correct answer, but still really poor.
Focus results calibrated with fine-tune -5
The plot shows that the camera is still near-focusing and not very close to the target edge that I had aimed at.
Time to try calibration again. The next fine-tune answer was -3. Close, but no cigar as they say.
Focus results calibrated with fine-tune -3
Focus fine-tune of -3 shown above is fairly close to the desired edge, but not as good as what it should be.
I tried auto-calibration yet again, and the next answer was -5. The plot expectedly looks almost exactly like the “-5” plot I showed earlier.
At this point, I’d seen enough. The results would have been closer to what was really needed for correct focus if I had instead never even bothered to calibrate the lens at all.
I tried the same auto-calibration experiments on my D500 quite a while ago, and I got similarly disappointing results with that camera. At the time, I thought that maybe my camera was an exception and other cameras would do a better job than mine. I had tried a few different lenses, but none were properly auto-calibrated.
Sadly, you will probably get much better results by manually calibrating lens focus yourself. I’ll bet everyone thought that Nikon had finally solved the focus calibration issue. The MTFMapper program feedback (with the focus target files at the same web site) makes manual focus calibration fairly simple, and the results are very accurate. Nikon’s engineers need to take their auto-calibrate idea back to the drawing board.
By the way, Nikon’s mirrorless cameras also provide a focus fine-tune calibration feature. Even those cameras can use a little help getting focus just right, because lenses don’t always do exactly what they’re told to do. They’re still better than DSLRs for focus in most situations, but even mirrorless cameras aren’t flawless focusers.
You can get the free MTFMapper program here.