Nikon D300 AF Fine Tune

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I've been looking into the AF Fine Tune function on the Nikon D300 recently. Googling for information seems to raise conflicting ideas about what it is, how to use it, and how it works. So I'm not much wiser.

My own experiments have resulted in the following observations:

  • most of my lenses don't seem to benefit significantly from it
  • different tuning techniques produce different results
There seems to be two schools of thought on testing: use a 45 degree target and adjust for centre sharpness; or use a parallel target and pick the sharpest image. I even found a Canon test image to be used with Live View on a computer monitor!

On top of this, lenses allegedly produce different settings at different distances. And I won't even get started on different zoom settings.

Eventually, I decided that I wouldn't create settings for most of my lenses, as there seemed to be no significant benefit — although it pains me to think I might just be able to get them a little sharper, if only I knew how.

My only prime (a Nikkor 50mm f/1.8 D) was the only lens which definitely improved with tuning (+10 at last attempt). I don't know if the fact that this is the only AF screw lens I own is a factor in this.

I still don't understand what AF Fine Tuning actually does on a technical level. I'd guess that it's an AF positioning offset; and I don't fully understand how AF works in general, and how this affects both automatic and manual focussing accuracy. Being an engineer, this bothers me ;-)

Has anyone got any insights or experience into this whole subject which might explain things more, or dispell any popular myths?
 
I still don't understand what AF Fine Tuning actually does on a technical level.

In laymans terms it shifts the center of depth of field either nearer or further, depening on whether you use a + or - adjustment. Typically you get about 1/3rd DoF in front of the target, and 2/3rd behind at a zero setting.

Many fast zooms have focus shifts caused by spherical abberation which varies according to focal length. The current implementation only allows adjustment at one focal length, and one distance, which is why its generally useless for zooms, but its OK for primes in a pinch. This is a bandaid, NOT a fix.

Personally if anything was "out of spec" enough to need this feature it would go back to Nikon to be fixed...
 
Well, yeah, I know what what AF Fine Tune does, what I want to know is how it does it, i.e. what the setting actually controls.

It would also be useful/interesting to know if a value of 0 is the same as No tuning; and if there's any problems with leaving AF Fine Tune switched on for lenses with no saved settings.
 
I don't bother messing around with the fine tune, probably because I'd mess it up :p. But I'm quite happy with the focus from what I've seen so far so if it ain't broken... ;)
 
Mine ain't broken neither, it's just cheaper to adjust a setting for a potential gain than buy pro glass.
 
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