dsmeltz Loc: Philadelphia

So you only need to make an adjustment that gets over the large difference. Averaging does not stop at that. It tries to make an adjustment that is more accurate that is actually possible. You are still going to have misses based on the behavior of the camera and lens while focusing for the individual shot. So what you want to do is multiple shots that allows the focus system to get at least one solid hit. If I take six shots either allowing new focus for each or (in the case of very shallow DOF) moving in or out physically maintaining a set focus, my odds of getting the shot are much higher than averaging multiple adjustments and taking one shot assuming my micro adjustment is super accurate. Once you are close on the micro adjustment, camera technique is all you need.

SuperflyTNT Loc: Manassas VA

I doubt you’re really that dense so I’m done explaining it to you.

JD750 Loc: SoCal

Tony's point was IF the differences you’re adjusting for are greater than the minor differences in shot to shot focus (variance) then the lens is out of spec. Send it back. If the lens is within spec then the differences you are adjusting for are within the variance of the focus system. You will be chasing your tail.

Taking a lot of shots and averaging is a good way to find any bias, even within the lens variance and in theory, you could remove it with a micro focus adjustment. BUT that is a lot of work, best done with a compute and automated test software. The manufacturer of the lens should have this in place. Your lens should already be adjusted +/- test tolerance, when you get it.

If you results vary one possible reason for that is the variance in the phase detection calibration on your particular DSLR camera. Re-adjusting for that is time consuming. Is it worth the time?

Pistnbroke Loc: UK

Every lens I have ever had needed some form of fine focus adjust .
But to stick to Tamron .I have two 100-400 Totally different, one is +8 at 400 and the other -6 at 400.
By adjusting ( 16 values) the sharpness is vastly improved. When used at 400mm and 1.5m as a crude macro the focus is spot on rather than blurred rubbish. Until I had Nikon camera with auto fine tune I gave up and just set the 400mm in the camera menu but you can get all 16 numbers in about 20min with the Nikon. Settings from the D850 gives perfect results on the D7200. Give up now if you have a canon.
PS that averaging idea is rubbish not necessary. push the buttons ,read and move on.

SuperflyTNT Loc: Manassas VA

I’ll take Steve Perry’s advice over Tony Northrop any day. Steve actually teaches instead of just giving opinions.

dsmeltz Loc: Philadelphia

I am not the one being obstinate. There is a concept called the "diminishing value of new information" (Anyone moderately familiar with applied statistics should understand that concept.) It is clearly illustrated in the idea of doing numerous measurements to obtain an average for a setting when that function the setting controls will have significant variation under normal use circumstances. Since in actual field use, numerous things influence focus acquisition (available light, amount of contrast, other focus setting choices, etc....), trying to achieve false precision (another term you would be wise to understand) is a fool's game. Once you are reasonably close, there is no practical gain in refining the adjustment since the shift is well within the equipment's operational margin of functional inaccuracy. At that point tack sharp focus is better achieved by the photographer's technique than with a micro adjustment that is as likely to cause missed focus as it is to achieve it.