Originally posted by starbase218 This is good to know, but I do have a question. If I understand correctly, FocusTune measures the sharpness of the images. The AF adjustment for the sharpest image is then considered the "correct" AF adjustment. However, I often hear people saying they want 1/3 of the in-focus area in front of where they focused, and 2/3 behind where they focused. E.g. if you take a portrait photo and focus on the eyes, you want both the nose and most of the head to be sharp as well, stopping down as needed. But it seems to me that this is not the result you would get with this method. Or is it?
You are correct in that different focal lengths/apertures have differing field position of the target within the DoF, This is a common cause of 'adjusters' scrxwing up their cameras.
But for the test regimes mentioned here this wont happen because though a 30mm@f8 lens has 3:1 ratio @10ft (i.e 25% in focus at the front 75% at the back) the sharpest point will be at that 3:1 ratio position so the test tools will locate the correct focus adjustment.
The problem is those techniques that use finding the DoF edge then adjusting 50% inside, these are only accurate for set circumstances ie 250mm lens @f8 10ft any other lens can and do introduce horrible AF errors often causing cameras AF to fail when the AF algorithms can no longer find a solution with the DoF envelope .
Unfortunate those casual 'adjusters' tend to be the ones scrxwing over their cameras as they tend to point at a target with a wide aperture adjust till OOF one way take note of position adjust oof the other way take note and place 'adjustment ' such that the lens stops between the two noted positions = one totally miss-set camera/lens combo .