repleo Loc: Boston

So, if I have read this correctly, the 'softness' at the corners is due to distortion / defraction etc rather than difference in focal distance? Yes/No?

therwol Loc: USA

After years of experience with lenses and also knowledge of many I've not owned, I can say that lenses in general are not as sharp in the corners as in the center, and this has to do with design limitations. The "classic" lenses of yesterday tend to improve greatly in the corners when stopped down a couple of stops from the maximum aperture. Some of the new designs like the Sigma 50mm F/1.4 ART are tack sharp across the entire frame at any aperture, but you pay for that performance.

therwol Loc: USA

Yes, and it all has to do with lens design. Some are better than others. The best ones in this regard tend to be much more expensive than the worst.

bpulv Loc: Buena Park, CA

Tinplater, you are correct. To understand what is really happening, it will help you to understand the difference between a general purpose lens and a process lens. A general purpose lens is the lens type you are most familiar with. If a general purpose lens that is not corrected for linearity is critically focused on the center of a flat lens chart that is 10-feet away, for example, the only spot on that chart that that will theoretically be in focus is the center of the chart. To understand why, take a 10-foot long piece of string and have someone extended it from your nose while keeping it taught while moving it up and down, side to side, etc., you will find that a point 10-feet away prescribes an arc similar to the inside of a sphere; i.e., a general purpose lens resolves a spherical world to a flat surface. Modern lenses are corrected for a spherical world to the flat sensor. When you measure the corner sharpness of a lens using a lens chart, you are actually measuring the linear correction quality of that lens.

A process lens, on the other hand, is a special lens that is designed to resolve a flat surface to a flat sensor or film plane. Process lenses are used for high quality copy work, usually for the printing industry. Photographing the real world with a process lens would produce an image that when focused in the center would be back focused at the corners; the opposite of a general purpose lens.

bpulv Loc: Buena Park, CA

NO! Softness at the corners is due to poor linear correction in the lens design.

therwol Loc: USA

I stand partially corrected on my response to the question, but I did say it has to do with lens design. You can't really fix it other than perhaps finding the f stops where it may not be as noticeable. I think it comes down to how much you want to spend on a lens. There may not be an exact correlation there, but I believe that newer lenses out today do a pretty good job of minimizing it. Every lens has to evaluated on its own merit. Some of Nikon's earliest micro (macro lenses) that you can pick up for pennies have excellent corner sharpness at all f stops. I know because I own two of them, 55mm f/3.5 and f/2.8.

bpulv Loc: Buena Park, CA

There are two tools we have to correct linearity of lenses; none of which are perfect. Nikon DSLRs and I am going to guess that Canon DSLRs have built in linearity correction. On the Nikon, a chip in the lens tells the camera body which model lens is attached. The camera body uses a lookup table to find the correction factors necessary to fine tune the linearity inconsistencies of the lens. Nikon periodically has firmware updates for their cameras with information on newer model lenses.

The second tool is in Lightroom Classic CC. Although the tools correct both linearity and chromatic aberration, it will not improve corner sharpness. The program includes a lookup table that on command applies the proper linearity correction for the lens and camera that took the picture by using the appropriate metadata that is embedded in the file being edited. Go into the "Edit module" and open the "Lens Correction Panel." Click both "Remove Chromatic Aberration" and "Enable Profile Correction" checkboxes and you can actually see the change in linearity as it is applied.

therwol Loc: USA

Agreed. I have a Nikon D810 and a 24-120 f/4 VR which in my observation has objectionable linear distortion at the wide end when compared with my 24mm f/2.8 D prime. (By the way, that prime is pretty soft in the corners until you stop it down a couple of f stops.) The distortion correction tool in the camera does correct for linear distortion pretty well, but it doesn't improve sharpness in any way. In fact, the fudging of the pixels to straighten out the picture seems to possibly decrease sharpness a bit. I've not studied it closely, but I think it's true.

If anyone wants my opinion of the 24-120, it is enormously convenient but not the sharpest lens on the block when you start pixel peeping. My much older 28-105 D seems significantly sharper below 85mm except at the far corners, and it has virtually no linear distortion at any focal length. The only way to prove it would be side by side pictures, but I don't have time at the moment.

bpulv Loc: Buena Park, CA

I completely agree with you on the 28-105 D. I have two of them. The first one I bought for a film Nikon years ago. Three years ago during our transit of the Panama Canal, I was changing lenses and it dropped from waist level to the steel deck. The plastic filter retaining ring broke but, the lens continued to function properly throughout the trip. The first thing I did when I got home was buy a replacement. It is exceptionally sharp, a great travel lens and as a bonus, it is a macro lens too.

Jim Bob

Exactly. The true test of a lens.

John_F Loc: Minneapolis, MN

From the optics, yes. But a lens need not be a perfect spherical surface and that is where the rub comes from.

John_F Loc: Minneapolis, MN

Yes. Except that delete defraction, to which you meant diffraction, right.

Tronjo Loc: Canada, BC

Here is a good article about lens field curvature as a reason for corner softness: https://photographylife.com/what-is-field-curvature
Flattening the field is a matter of design compromises and this was done in the newer Nikon 24-70 VR lens: some of the center sharpness was compromised for getting better corner sharpness (flatter field).

Another source of corner softness is element decentring, which actually leads to the four corners being differently sharp (or soft). One can do nothing about lens design field curvature, but can fix element decentring by sending the lens back to the manufacturer for centering. If you want to check if your lens has a decentered element try this method: https://www.lensrentals.com/blog/2012/05/testing-for-a-decentered-lens-an-old-technique-gets-a-makeover/. There are also other methods that you can read about by Googling the topic.

tinplater Loc: Scottsdale, AZ

Thank you! The article really helped me understand this concept and the fact that lenses can be designed to attempt to correct for this problem. I would be interested to know which lenses in common use by photographers have best solved this problem (not subjectively but objectively).

Tronjo Loc: Canada, BC

The lens MTF curves published by manufacturer will tell you about the sharpness uniformity across the field when the lens is focused at infinity: the flatter the better, the smaller the difference between sagittal and meridional curves the batter, the closer the numbers to 1 the better. Most reviewers though, test the lens at specific subject distance using resolution charts. These results may differ from MTFs tell us and dependent on the camera used and the testing conditions. Such sharpness (resolution) results, as absolute numbers, are not comparable site-to-site and often lens-to-lens reviewed in the same site, but are valuable information about the sharpness across the field. I believe three sites about lens reviews: Photography Life, Cameralabs, and Lensrentals. Lens rentals are particularly helpful because they provide sample variations charts. From theses you can see that generally OEM lenses are more consistent than third party. DXO classification I take only as information.