Soft corners wide open? Or DOF / FL?

[MOREGONE]

So I hear about some lenses being soft in the corners when shooting wide open. I personally have never noticed what I thought were soft corners so if someone could post point me to a solid example it would be helpful.

But my real question or thought is, isn't it to be expected? I am trying to sell my Tamron 28-75 and a would be buyer seems to think it is soft in the corners.

I always think about the area in focus on plane going perpendicular to my sensor. But now that I am thinking more about it, when shooting shallow DOF and focusing, the distance play a big role. So if I am shooting 28mm f2.8 my subjects face is going to be at a different distance. And with a shallow DOF I would understand why the corners are soft / bekeh-y.

I hope someone gets what I am trying to work out.

Thanks

 

[480sparky]

It's not a DOF issue. It's the simple laws of optics. All lenses are softer in the corners than at the center. And the shorter the FL, the more it becomes noticeable. Yes, stopping down improves things until diffraction takes over.

 

[Derrel]

I have read that if a lens has significant curvature of field, meaning the focus plane is curved, that it's possible to focus so that the edges are sharp, but the center of the field is soft! And also, vice-versa!

 

[theHomelessJedi]

It's not a DOF issue. It's the simple laws of optics. All lenses are softer in the corners than at the center. And the shorter the FL, the more it becomes noticeable. Yes, stopping down improves things until diffraction takes over.

Agree completely. Something a lot of folks don't know is that fast lenses werent necessarily made to be shot wide open. I know the Canon 85mm f/1.4 has a sweet spot around f/4 - f/8 as far as shallow DOF with less softness on the corners goes.
Just my personal experience.

 

[tirediron]

For s**ts and giggles, I've done home-brew lens sharpness tests with all of my lenses at one time or another (a sheet covered with 10pt upper-case 'X', tripod shot and checked for square level done from a couple of feet inside min. focusing distance) and EVERY lens I own has some degree of softness in the corners from my 24-70 & 70-200 to my 85 1.4!

 

[KmH]

The simplest lens has 2 spherical surfaces and is thicker in the middle than it is at it's outer edge.
Consequently light passing through the middle of the lens focuses to a slightly different point than light passing out towards the edge of the lens.

Additional lens elements are then needed to correct for that and other optical aberrations.
The most basic combination is a doublet that consists of the crown lens and flint lens.
Particularly with zoom lenses it can be very difficult to correct the focus completely without adding some other kind of optical aberration(s) like coma, barrel or pincushion distortion, chromatic aberration, field curvature.
So some of the glass elements in a lens are there to correct focus and some are there to control the other types of optical aberration.

How much effort and money a lens maker is willing to devote to the design and manufacture of a lens has a lot to do with how accurately and completely the various optical aberrations are corrected.