Uneven sharpness

[blotafton]

I'd like to hear what you think about lenses that are sharp in the center but softer on either the left or right side. Many of my lenses exhibit this phenomena, from low quality zooms like a Makinon 24-50mm to the very best, C/Y Planar 50mm 1.4.

Is it just bad luck with sample variation, bad adapters or something else?

Others I can think of right now are: Fujinon 55mm f1.8 EBC, Rokkor 55mm f1.7, Vivitar 35-85mm 2.8, Sun 24-40mm.

I check this at infinity. Perhaps I shouldn't look so closely?

[miran]

Decentering. All lenses have it to some degree. It's just a question whether it's enough to notice or not. Generally speaking the wider the lens, the more you will notice it.

[Lightshow]

The design of the lens can cause it, not being assembled correctly, spherical aberration.
I tend to really like these types of lenses for portraits, and I wouldn't be surprised if some lenses were designed like this intentionally.

[blotafton]

Not so fun for landscapes though. These got to be counted as bad right?

Industar 50 collapsible.

Right side.



Left



Vivitar 35-85mm Right



Left

[luisalegria]

Yes, its a defect.
And its something you would notice most at infinity, so its not good for landscape.

[marcusBMG]

All lenses tend to be softer away from the optical axis, the question is how much and whether equally.

Roger Cicala on the lensrentals.com blog has written extensively about this sort of thing (example). It's not usually decentering (optical elements not exactly aligned on the optical axis) per se, people tend to use the term decentering to refer to discrepancies in resolution between left and right sides of the frame but the causes include the effects of tilts, optical imperfections/assymetries and improper spacings in the optical assembly as well.
I've had just a few more modern zooms: Pentax DA 18-135mm, Sigma 17-70mm, Sigma 18-125mm. All had discernable difference in sharpness between left and right frame when scrutinised at 100% 1:1, especially wide open.

[woodrim]

First, how do you know it's at infinity? With any lens at infinity, I wouldn't shoot wide open unless the light was such that required it. Stopping down should quickly solve the edge problem.

[y]

Many, many, many wide lenses suffer from decentering.

Take a test shot and turn your camera 180° around (upside down) and take it again. Then compare the sharpness of the original and the reversed image.

If you have a visibly decentered lens simply sell it and obtain a new copy. I've seen uneven sharpness on many wides (28 or less mm) or even modern 3rd party zooms. Ofc, decentering at wide-open doesn't harm that much as decentering at f/8.0 or even f/11.0. But still... its a defect which is visible!

[blotafton]

Yes decentering is a hot topic over at dpreview, the first place I came across the term.

The Industar photo is more or less at infinity but the vivitar is probably @ 30m. That's just one of the things I usually test with a new lens.

These are stopped down woodrim. The Industar is f/5.6 or f/8. I don't remember the setting on the Vivitar but the flare suggests that it's stopped down a bit.
It doesn't bother me that much if it goes away at f/4 or 5.6. But if it doesn't it's disappointing. At least one can get a feel of a lens determine if it's worth hunting down a new one.