D5 auto AF fine-tune: User notes

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Marianne Oelund Veteran Member • Posts: 6,375
D5 auto AF fine-tune: User notes
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After experimenting with this new feature for a while, I've found some issues and characteristics that I thought would be worth sharing.  Here is a guide for use of the feature, with some basics about how it operates.

Before starting, you want to make sure you have the center AF point selected if using a phase-detect AF mode which takes a user-selected point; the feature requires this, and will only allow auto-calibration of that one point.  Set the camera up on a tripod, aimed at a suitable AF target with good lighting.  The auto AF fine tune feature will not operate under dim lighting (under LV 5 or so) even though AF is rated down to LV -4.

Engage Live View and make sure the LV focus mode is set to AF-S and Wide or Normal single-point.  You also need to make sure the LV focus position is at center.  Now for the critical part:  Carefully focus the lens so it is precisely focused on the subject at the center AF point.  If you have a good target, using LV AF may be sufficient, but you should still check at high magnification, preferably with a slant-ruler in the image to help judge the precise focus plane.  Auto AF fine-tune will use this lens focus position as its reference, so if the focus is slightly off, so will the automatic fine tune adjustment be!

Hold down the AF-mode select button (on the AF-M switch next to the lens mount) and the video-record button simultaneously for several seconds.  This should bring up the dialog box "Before proceeding, fix the camera in place and check that it is focused.  Proceed?" with Yes and No options.

[Note:  If any of the AF settings listed above are not correct, you will see an error message "Auto AF fine tune is not available at current focus settings."  However, there is a potential alignment issue, and sometimes you will get this error message even when everything appears set correctly.  To proceed in this case, exit LV, move the AF point away from center, then back to center, and return to LV.  This realigns the LV AF point as needed.]

Press OK to initiate the process.  The camera will exit LV, operate the phase-detect AF system to measure any focus error that it believes is present, then calculate the required offset (fine tune value) to correct that error.  Finally, the fine tune value is entered into the internal table for that lens, and a completion dialog box is displayed.  Press OK to confirm; you may then look at the AF fine tune menu item to see the new value that the camera calculated.

If there was not sufficient light, or if the lens requires a fine tune value beyond the -20 to +20 range, the error message "Auto AF fine tune failed." is displayed.  Note that this could be caused by the lens focus being accidentally disturbed, so you may want to try repeating the process.

It is important to understand that due to the fickle nature of phase-detect AF, the calculated fine-tune value may not be optimal.  I recommend repeating the procedure at least half a dozen times (if not 10 or more) to establish the range of values that can result.  Then, manually select a value in the center of the range.  This means, of course, that the complete process is not so quick, and you may prefer to use manual AF fine tune methods instead.

AF target quality

By repeating the process a number of times, you will obtain a range of values which will give some guidance regarding the suitability of your AF target.  A good target can produce a range of fine-tune values which differ by only 5 to 6 from lowest to highest.  Less suitable targets may produce a much larger range of values, and may even produce distributions of values that are end-weighted rather than center-weighted as you would normally expect.

Some good targets I've found are bar-code strips, or printed block letters on a highly contrasting background, sized so just a few characters will fit within the AF point.

I will continue to do some work evaluating AF targets, and post again if I make any noteworthy discoveries.

Interestingly, I have not found a correlation between light level and the consistency of auto AF fine tune.  Provided the lighting is sufficient (about LV 5 or higher) to allow the feature to work, increasing the illumination does not appear to help.

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Source credit: Prov 2:6
- Marianne

Nikon D5
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