Edge sharpness / wide open - real, or imagined ??

[daveleo]

This is a general technical question, and please don't turn it into a brand war, thank you.

(Correct me , as necessary.)

The "plane of focus" of almost all lenses (except "flat field lenses") is not actually a 2D plane, it's really a spherical surface.

Now ...... imagine that there's this photo that was shot "wide open" (narrow DOF).
Assume that the center object in the image is the object focused upon, and in fact it comes in perfectly focused. But the objects near the edges and corners of the image are "soft" (loosely defined for this thread).
But then, notice that the objects at the edges are not anywhere near the "sphere of focus". I mean they are not within the DOF of the "sphere of focus" of ANY lens.

So my question is: when we see soft edges in an image and people comment "the lens is soft at the edges" ...... it could be because the objects at the edges are outside the spherical DOF of that particular focal length and aperture on that particular sensor (film) size.

Correct?

 

[Huss]

Field curvature doesn't create softness per se, but affects what is in focus. To answer the question empirically you can look at lab lens resolution tests. Those show that lenses have the highest resolution in the center, getting weaker to the edges.
As a lens is stopped down, resolution across the frame increases. This is why pretty much any lens looks decent at f8.

I've attached a test for the Sigma Art lens (which also compares it to the Zeiss Otus) so you can see empirical resolution test results across the frame, at all apertures.

https://photographylife.com/reviews/sigma-50mm-f1-4-dg-hsm/4

I have found that field curvature gives lenses a distinct look, and can help isolate the subject matter. Perfectly corrected lenses can feel sterile. This is why the Nikon 58 1.4 G is popular, it is anything but a sterile lens.

But I agree with you on the point that some people may confuse lens softness with areas that are actually out of focus due to field curvature.

 

[ferider]

Most lenses that I've used have a relatively flat field, Dave. And for those, like Huss said, there are other causes for softness in the corners.

For the few lenses that I have tried with pronounced field curvature, what you say does happen. Depending on how the lens is optimized, you might even have the center OOF, then a ring in focus, and the corners OOF again, even at infinity. Post #3 of my Canon thread http://www.rangefinderforum.com/forums/showthread.php?t=155193 shows you the Canon 50/1.2 as an example, and how it looks.

Note also that field curvature is not a feature of the basic lens layout - it depends how it was optimized. For example, I have several 50mm Sonnars that are pretty flat (Nikkor and Canon); then again, I have a late Zeiss Sonnar with a nicely spherical focus field.

Roland.

 

[Ronald M]

A field of digital focus points is important so an off center subject can be brought into focus without recomposing.

For interest, Leica lenses are not flat. years ago I tried printing from cardboard mounted slides. 50 2.0 Summicron V2 and V3 focused the whole slide. Leitz made Focotar and focotar 2 had distinct curvature, but the Schneider made large front element was almost perfectly flat. 60 macro elmar R 2.8 is not flat. 65 3.5 chrome is sort of flat, the later black is much more flat. 100 2.8 APO is really flat w/wo macro adapter. This is a killer lens on film and digital.

Leica sacrifices field flatness to correct other optical errors. This is not noticeable normally with three dimensional subjects.

One Zeiss camera had a moveable pressure plate and I assume the lenses were made for flat field. Some 4x5 film holders were made to hold film very flat. They were expensive, very expensive.

 

[tunalegs]

Saab by Berang Berang, on Flickr

Here's a photo that exhibits the effects of extreme field curvature in rather interesting ways.

Notice the car is in focus, and that the ground, all the way up to the camera itself - is in sharp focus. Yet the tree branches and plants around the car are not in focus.

Without field curvature, we'd expect the exact opposite. The tree branches should be in focus, and the ground should get softer as it comes closer to the camera.

With most lenses, slight field curvature is potentially hidden because foreground objects are closer and obviously therefore should look softer - so soft corners on the bottom of the frame are masked by this. The top corners are in the sky which is either just a flat color, or in the distance and so again, the effect of soft corners is masked pretty well. It's only when a flat object is photographed that these soft corners become apparent.

 

[xayraa33]

Saab by Berang Berang, on Flickr

Here's a photo that exhibits the effects of extreme field curvature in rather interesting ways.

Notice the car is in focus, and that the ground, all the way up to the camera itself - is in sharp focus. Yet the tree branches and plants around the car are not in focus.

Without field curvature, we'd expect the exact opposite. The tree branches should be in focus, and the ground should get softer as it comes closer to the camera.

With most lenses, slight field curvature is potentially hidden because foreground objects are closer and obviously therefore should look softer - so soft corners on the bottom of the frame are masked by this. The top corners are in the sky which is either just a flat color, or in the distance and so again, the effect of soft corners is masked pretty well. It's only when a flat object is photographed that these soft corners become apparent.

I find field curvature affect in photos rather pleasing most of the time.

Chromatic aberrations in lenses produce a pleasing 19th century photograph look which I am partial to. Maybe that is why the Petzval lens is one of my all time favourites.

 

[daveleo]

Saab by Berang Berang, on Flickr

Here's a photo that exhibits the effects of extreme field curvature in rather interesting ways. ..........

Hard to believe that a camera lens would be deliberately designed with (for) this effect. Wouldn't this be caused by a misalignment of the elements?

------------------------------------

In general, I think many people (myself included) incorrectly mix the terms "soft" and "out of focus".
.
I think to correctly assess how "soft" the edges are, you need to focus on objects near the edges, without recompossing. I generally don't see people doing that.

 

[tunalegs]

Hard to believe that a camera lens would be deliberately designed with (for) this effect. Wouldn't this be caused by a misalignment of the elements?

I doubt there was any intention of having the lens produce such an effect.

This was taken with a Halina Prefect box camera and the largest aperture. At f/16 the effect is mostly subdued. This is just the result of a cheap, primitive lens being pushed beyond its limits really.

 

[Doug]

I have a lens that produces results somewhat like the SAAB photo above. In examining its photos I make out that the field of focus is shaped like a shallow bowl, turning toward the camera more strongly in the outer zones. The lens does suffer some loss of sharpness in the corners especially at wide apertures but I've noticed that objects in the corners are sharper when nearer than those at the focus distance. (The observation comes from photos shot with a full-frame camera whereas the lens was intended for APS-C, and the effects seen outside the view of an APS-C sensor)

One other observation.... I suggest that a curved field of focus also means that central focus shifts as the aperture is changed... and these two characteristics are linked.

 

[uhoh7]

But I agree with you on the point that some people may confuse lens softness with areas that are actually out of focus due to field curvature.

You really see this alot lately, as people throw RF lenses on Sonys, which add FC. However it works the other way too, people will take a test shot with closer details on the edges, and declare "hey, this M wide is great on the A7rii"

I think if you really know how the lens is going to work, FC may go either way, i.e. CV 35/1.4 vs 28cron, you might be able to get it to work for your purposes from time to time.

Much nicer is just a even lens, like the 50 cron v4 at F2.

But things get very wild with the mid-zone dips in some of the greatest M lenses, like the FLE and 50 Lux Asph. The edges are pretty even but the mid zone is wild at faster speeds. These lenses can really sing or fall flat based on the subject location and focus.

A classis comparison would be the 75Lux vs pre-asph 90 cron at F2. The Lux is far stronger in the mid zone (of course "stronger" here just means in the POF maybe), but which is the sweeter Portrait lens?

 

[daveleo]

Thank you all for the input. This is a topic I need to understand better and appreciate more. Especially when reading (let's say) overly simplistic comments about a lens edge sharpness that you find on the internet.
I am surprised to realize that the "sphere of focus" often is not a sphere, and it may have a center that is not at the camera.
I agree that "imperfections" can give a lens some character, and "perfect specs" can make sterile and boring pictures.

 

[Doug]

...
I am surprised to realize that the "sphere of focus" often is not a sphere, and it may have a center that is not at the camera.
I agree that "imperfections" can give a lens some character, and "perfect specs" can make sterile and boring pictures.

True, odd to consider that the "surface of focus" is often not a plane nor a spherical surface but may have ripples part way out!

As to the value of imperfections, I generally disagree... sterile and boring pictures result when you take a sterile and boring picture, IMO, and no "artistic" imperfections can save it! I would prefer a relatively plain and neutral imaging from the equipment and let me put some soul into it both at the time of exposure and in post. 🙂

 

[goamules]

Some early fast designs do have a curved field, like a Petzval. Most lenses since the 1800s worked hard to correct first the field curvature, then chromatic aberration (actually fixed partially first, in 1820s maritime telescopes), then the astigmatism, then the other aberrations like spherical. Sometimes you'll see extreme curvature in early Cine lenses, that had to be very fast but wide angle. But most 35mm camera lenses are quite flat.

With Large Format cameras with movable backs and fronts, you can get the focus to follow any plane you want, and get things out of focus that normally wouldn't be, correct perspective, and do more.