Hi,
Enclosed is an image where I feel that background branches need to be sharp all out to the corners. I was fortunate enough to reshoot it with a better lens a couple of days later.
There is a small story attached to this image. I shot it using an 10 MP Alpha 100 (APS-C) a couple of years ago that was pretty sharp, so I tried to make a 70x100 cm print from it. To my surprise it turned out good enough. Not good for pixel peeping at 40 cm but viewed at > 80 cm it is OK. I wanted to reshoot the subject with a higher resolution camera.
Attachements:
1) Full image with weak lens
2) Corner crop from 1)
3) Corner crop from image taken with sharp lens (an old Minolta 80-200/2.8 APO)
4) A more central crop (along center of long edge) on the weak lens (Sony SAL 70-300/4.5-5.6)
This composition requires around 150-160 mm. As i want background be dominated by the dark pine forrest. Moving just a couple of meters forward ruins the image and moving back puts you on the middle of the road and puts a couple of power lines into the image. Situations like this is one of the reasons I almost always use zooms.
Best regards
Erik
And if one has a primary interest in sharpness of the subject focused on, maybe a bit of corner softness is less important that optimum performance near the center: the OOF affects are likely to overwhelm a bit of corner softness most of the time (how often is something in the extreme corner in completely sharp focus?) At center it seems to be a close context between f/4 and f/5.6 for both lenses, with f/8 in third place!