AF Sensor Nerdvana Achieved
Prepared 2006-01-04 (150/79706) by Bill Claff
The result is 7 images (from -3 to +3) centered about the AF point.
I would compute sharpness for the 7 images and plot the result.
The desired result is a U-shaped curve with 0 as the sharpest point.
Lines that slope up are front-focus; lines that slope down are back-focus.
Turn the peg CCW to fix front-focus; turn it CW to fix back-focus.
I get access to the peg the same way as I would to do a sensor cleaning.
I lock the mirror up and I take the battery out.
After a number of attempts (10!) I have reached AF Sensor nerdvana.
Here is the chart of my attempts:
My first adjustment was in the wrong direction and then I moved slowly toward my goal.
This adjustment is quite sensitive and I think would be much more challenging without an objective way to measure sharpness.
Note that the final set of 7 images are visually identical when viewed at 100%
Here's a sharpness chart for the final cam position from -5mm to 5mm:
To state the obvious, I adjusted for the center AF Sensor; and the others are just along for the ride.
If the AF Sensors are not all in the same plane then the off-center sensors will be out of adjustment.
In that case I would imagine the camera would need Nikon service rather than self‑adjustment.
However, in my case things look pretty good.
Here are sharpness profiles for all 5 AF sensors:
And considering that my LCD monitor might not be perfectly square to the camera I think this results is excellent.
(Looks to me to be more square left/right than up/down.)