Originally posted by newarts Some of us are more subject oriented than others. I like full face portraits, macros where the subject dominates the photo, etc - in many of my photos edge sharpness doesn't matter or can detract from my purpose.
A good case in point is the use of the Raynox close-up lens for macros; here's a nice macro posted here recently by Beregeded:
https://www.pentaxforums.com/forums/lens-clubs/74221-raynox-macro-club-23.html#post1709155
Looks great to me! But here's a test showing the edge sharpness for a Raynox 150 on a good Macro lens (Tamron 90:2.5):
Without the Raynox 150, the edges of the photo are just as sharp as the center. with the Raynox the edges and corners are poor. The top-most of the crops shows the upper-left quadrant of the full photo... Notice the in-focus area is in a disk about equal to the frame height.
But this is the sharpness distribution for the lens used to take the macro posted above - clearly the edge softness inherent in using the Raynox had no practical effect.
Edge quality matters a lot for wide angle lenses - one of the main reasons to use a wide angle lens is to see what's at the edges of a scene - otherwise why use a wide angle lens?
I've actually been shooting my LCD screen quite a bit recently. For some reason it seems a good way to test different macro configuration (lens reversing and such). I guess it's because it emits light and requires a shorter shutter speed than some things. Anyway, I can say that it's often difficult to get the entire FoV in focus, you have to have your front element
exactly parallel to the screen, the barrel of the lens
exactly perpendicular, you get funky diffraction issues that look really similar to what you posted. It's caused by the way LCDs are made, and the layer of protective glass over them. I'm not sure it makes for the best subject for testing edge to edge sharpness, unless you know this going in and are extremely careful to keep shooting tests until you get your camera perfectly aligned with the front of the screen.
Offtopic, but related in a way, here's a 1:1 crop of a 5:1 shot of a single LCD monitor pixel. Not quite as sharp as I want, I'll try it again sometime, but still kinda cool.