This is the 17th post in a series of Nikon D850 tests. The series starts here.
Horshack has made an argument that if there’s bias to a PDAF system (as far as I know, CDAF systems don’t have biases), that can make the focus variations look larger when compared to a system where the average of many PDAF attempts is correct.
Here’s my version of his reasoning. All credit to him, and if there are errors, they are mine.
Consider a PDAF system with no bias:
This is an actual raw-blue-channel MTF50 curve for the Nikon 58 mm f/1.4 wide open versus step number using the D850 Focus Shift Shooting feature with minimum step size. I’ve colored a part of the curve red. Think of a 1-step variation in focused distance, and see that the effect of the MTF50 numbers is not great.
If there is the same one-step variation at a point that is offset from the peak, the variation in MTF50 is much greater.
The theory sounds good, but what about the practice?
Consider these three plots from the immediately-previous post:
From the looks of the green curves, you’d expect that the standard deviation (sigma) would get larger as you proceeded from left to right.
But you’d be mistaken.
My conclusion is not that the theory is wrong, but that we need a lot more samples to prove it right. And no, I’m not gonna redo the tests with 1000 exposures at each bias. I also suspect that there are uncontrolled systematic biases in the tests I’m doing, probably related to how far defocused the lens is between each capture. I don’t know of a good way to control this that doesn’t involve a lot of work.