I just bought a used Sony 40mm f2.5 lens and tested it on my Sony A7rIV.
It s sharp at every aperture, over the whole frame from f2.5 on. However, there is a portion on the lower edge of the frame which is less sharp - the last 5% from the edge I would guess. Oddly, both corners are still very sharp.
I tested this several times, and it is repeatable, even when I put the focus point at the bottom of the screen.
Even odder, it is worst at f4, almost gone by f5.6.
wolfloid wrote:
I just bought a used Sony 40mm f2.5 lens and tested it on my Sony A7rIV.
It s sharp at every aperture, over the whole frame from f2.5 on. However, there is a portion on the lower edge of the frame which is less sharp - the last 5% from the edge I would guess. Oddly, both corners are still very sharp.
I tested this several times, and it is repeatable, even when I put the focus point at the bottom of the screen.
Even odder, it is worst at f4, almost gone by f5.6.
What could this be?
Can you still send it back? It's not odd that it's worse at wider apertures and gets better at f5.6 - the increased depth of field at smaller apertures can somewhat lessen the effects of decentered elements.
I had two copies of the Sony 20mm f1.8 lens - both were sharp in the center and 4 corners, but mid-field on one side was sharper than mid-field on the other side - and yet the sharper mid-field performance was on the left of the first copy of the lens and on the right side of the other copy. They both were new and went back immediately.
How important is the bottom 5% of the frame, and how are you judging its lack of sharpness? Is the focus at the bad edge terrible, or just a little less sharp? Probably the latter, in which case it will be unnoticed by everyone except you. Personally I think the bottom 5% is almost completely unimportant (and you said the corners are OK anyway, so it’s not the whole length of the frame). You could send it back and then receive one which has a different, also unimportant issue. Try photographing other subjects at other distances. I think you are worrying unnecessarily, but people do worry I know. The fact that it is worse than f2.5 at f4 but better at f5.6 is a bit of warning as to the reality of your analysis.
The fact that it is worse than f2.5 at f4 but better at f5.6 is a bit of warning as to the reality of your analysis.
Well, you are thinking what I would normally think. But it is repeatably true, and surprises me. Had it been the expected poor at f2.5 but then gradually better as I stopped down, I would not have posted. It is precisely that worsening at f4 that I cannot explain, but thought somebody might be able to, that made me post.
I have three weeks to return it, and so I probably will.
wolfloid wrote:
[...]
I have three weeks to return it, and so I probably will.
Yes, return it if possible.
The lens is expensive as it is. You don't want to keep a lens behaving in such an irregular way. That is especially true as we can't understand why and you weren't looking for a lifelong riddle when buying it.
(Yes, I have the 40G and I like it.)
