A reader suggested that the disappointing showing of the PC-E 24mm f/3.5 Nikkor D ED might have been caused by field curvature. In order to check that out, I made an aperture series focusing in the upper right corner.
The upper right corner, blown up 3:1:
Looks a lot better, doesn’t it?
Now let’s look at the center, where I didn’t focus:
Yep. That’s field curvature all right. The center is really fuzzy at f/4, and gets progressively better right through f/11, and it isn’t really crisp even there.
I checked to see if the shift adjustment center marker was off. It was fine. The tilt axis was set up as right/left, so, if the tilt was off, the upper left corner would suffer:
That’s not it. Field curvature it is.
By the way, Roger Cicala has written a nice article on field curvature. Here’s a key quote:
The takeaway message is that stopping down a lens doesn’t flatten the field. You have to stop down enough that the depth of field becomes greater than the curvature.