DotTune: AF tune technique without taking photos

Horshack wrote:

I have a proposal for a new AF tune technique that doesn't require taking photographs. At least I think it's new because I haven't read it anywhere else yet. If anyone knows of a reference that describes this already please let me know.

Nikon DSLRs have an electronic rangefinder in the viewfinder that shows you when:

• Subject is in focus (green dot)
• Focus is behind subject - green arrow pointing left, indicating focus ring should be turned to the left, away from infinity
• Focus is in front of subject - green arrow pointing right, indicating focus ring should be turned to the right, toward infinity

The rangefinder's determination of the above conditions is based on the phase-detect AF system's evaluation of focus. This system includes a configurable AF fine tune value which can be adjusted on a per-lens basis or globally for all lenses that don't have a per-lens value configured. Most would assume that this AF tune value is an "output" bias that the camera adds to all lens movement commands, so that instead of telling the lens "focus to X" it instead tells the lens "focus to X + tune value". In actuality the AF tune value is an "input" bias that feeds into the PDAF's sensing logic, altering when the camera believes it has acquired focus. You can read more about this here.

Since AF tune affects the camera's PDAF evaluation of what's in focus, it also affects the electronic rangefinder indication of focus. You can demonstrate this by establishing critical focus on a subject (green dot), changing the AF tune value, then without refocusing, observe that the rangefinder now shows a left or right arrow indicating the camera thinks the subject is no longer in focus, even though you didn't adjust focus.

Proposed AF Fine Tune Technique

1. Enter Live View and establish critical focus on a high-contrast subject. For cameras with poor LV clarity (D800/E), you may want to take a photograph and evaluate it on the computer to confirm critical focus
2. Set the body to MF. You could set the lens to MF instead but you may jostle the focus ring accidentally if you do so.
3. Look through the viewfinder to see if the camera thinks the subject is in focus. You'll need to half-press the shutter while doing this to keep the metering/rangefinder from going to sleep. If you see the green arrow pointing to the left, then the camera+lens is back-focusing, so you need to decrease the value of AF tune (negative adjustment). If you see the green arrow pointing to the right, then the camera+lens is front-focusing, so you need to increase the value of AF tune (positive adjustment).
4. Repeat step 3 until you get a green-dot from the rangefinder.
5. Depending on focal length and subject distance there will be a range of AF tune values which produce green-dot confirmation. For example on my D800 w/50G f/1.4 @ 5' subject distance, I get the green dot from -6 to -14. Conceptually the optimal AF tune value should be the value that's the middle of the range, which in my case is -10 (I independently verified this using a LensAlign). So you'll want to establish the range of green-dot values by progressively increasing the AF tune values until the camera stops showing the green-dot (it'll alternate between the green dot and an arrow when it's at the margin of the range)...and repeat that procedure in the opposite direction to find the other end of the range.

Some notes:

• When establishing the precise range of green-dot AF tune values you may need to examine the rangefinder for several seconds. I've found that for the extreme margins of the AF tune range the camera may take a few seconds to flicker the arrow/green-dot.

Questions and suggestions welcomed!