Focus consistency and AF-fine-tune

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Bernard Delley Senior Member • Posts: 1,860
Focus consistency and AF-fine-tune
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Achieving sharp images, especially with fast lenses, can be a little challenge. One should be aware of what Nikon says about good and about problematic targets for auto focus. With fast lenses, it may be useful to use AF fine tuning for better results. But, insufficient focus consistency could lead to rejected images due to missed focus.

Gary has provided a great reference image ! Thanks Gary !

is image in full orig size comes at 1142 pixels wide and appears tack sharp on the eyelashes of her left eye. assuming the about the entire FX width of the sensor was used, the projection of the pixels we get to see on the 1142 wide image correspond to about 20micro meter on the sensor. That means that a blur of less than 20micro meters is still OK.

You might set up a test scene for shooting at about 3m distance so that the short side of the sensor gives a view of about 760mm at the focal plane (long side 1140mm) . This corresponds roughly to Gary's imaging situation. You should have a good sized planar target with high contrast and decent illumination for focus. This can be about any box with writing. somewhere else in the image view you install a slanted ruler to monitor the consistency of focus. With this setup a 1mm defocus seen on the ruler translates to 1micro meter at the sensor by the lens action.

You set AF-fine-tune values from -20 to +20 in increments of 4 for example and perform auto-focus via optical viewfinder once coming from the a little defocus on the far and once coming from a little defocus from the close side. And you take record to where the focus fell on the ruler. This setup is sensitive enough that you can really know. From the results you learn the optimal AF fine tune setting. For a blur circle less than 20 micrometers at the sensor, you need to hit focus to better than 28 microns at f/1.4 with f/2.8 it would be to better than 2.8*20 = 56 microns. If Gary had shown the (vertical image) with 2400 pixels on the horizontal short side, then the challenge would be up with half as much blur tolerated and also half as much defocus.

Such measurements can be summarized in a figure like this typical example for an AF-S 105mm lens at f/1.4 on a D850 :

AF-S 105mm f/1.4 E ED at f/1.4 on D850, blue dot measurements are coming in from a little back focus, red dot measurements are coming in from a little front defocus. The scatter away from the regression lines is due minor focus inconsistencies because of residual mechanical friction and photon+electronic noise.   The combo profits from an AF fine tune value of +10. This value is confirmed within +-1 tune units even years later, provided no damage occurred to camera or lens.

Fortunately those pesky little microns defocus translate to accessible mm at the slanted ruler (multiplier is 1000x). [The blur circle diameters only get multiplied by sqrt(1000)=32]

in my sample combo of 105mm and D850 focus consistency is good enough to loose practically no images, even at f/1.4, due to missed focus. At least if the camera operator used a good contrast, well lit focus target for the image.

 Bernard Delley's gear list:Bernard Delley's gear list
Olympus TG-6 Nikon D7200 Nikon D500 Nikon D850 Nikon AF-S Nikkor 14-24mm f/2.8G ED +11 more
Nikon D850
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