What's the right forum to ask about field curvature?

Juan Valdenebro · May 4, 2021

I'm interested in learning about field curvature as we find it in 35mm lenses for 35mm rangefinders... Articles, links, books, and of course general comments from forum members...
I just thought there was an optics/lens design forum, but maybe I'm confused and it was not here at Photrio...
Thanks.

ic-racer · May 4, 2021

Pages 24 to 27: "Three Dimensional Characteristics" in "How to Read MTF Curves."

https://lenspire.zeiss.com/photo/app/uploads/2018/04/Article-MTF-2008-EN.pdf

Dan Fromm · May 4, 2021

Field curvature? Lens designers try to minimize it. And don't bother looking at the pdf mentioned in post #2 above. It mentions but does not explain curvature of field.

Ask the person who posts here as Nodda Duma (Jason Lane) where to look for explanations and information. While waiting to hear from him, look in https://web.archive.org/web/20141229200841/http://toothwalker.org/optics.html

Nodda Duma · May 4, 2021

More accurate explanation:

https://www.telescope-optics.net/curvature.htm

telescope centric, but substitute “camera lens” whenever you see the word telescope. The physics apply to any imaging lens.

Juan Valdenebro · May 4, 2021

Thank you all, I just read those three: formulae apart, they're more or less what I understand...
Any idea on field curvature differences between 35mm rangefinder lenses?
I've for long imagined lens designers can decide -as with optical aberrations and their relative correction- slightly different levels of field curvature, but I don't know if that's true, or if all 35s behave exactly the same way if we think of field curvature...

wiltw · May 4, 2021

I cannot add to the discussion on a somewhat academic basis, but I can point out that most lenses today have a fairly flat field. The Tamron 28-75mm f/2.8 lens from about 15 years ago was decidedly NOT flat field, and had pronounce field curvature, which was revealed in lens tests by photozone.de back then. And this became not uncommonly discussed in forums.

Juan Valdenebro · May 5, 2021

Isn't it just the contrary? Even best and current 35mm lenses in M-mount by Leica and Zeiss have strong field curvature?

reddesert · May 5, 2021

Here's a good summary of the common optical aberrations from Edmund Optics: https://www.edmundoptics.com/knowle...tes/optics/comparison-of-optical-aberrations/

If you want a technical book on aspects of optics a common reference is "Modern Optical Engineering" by Warren J. Smith. An older book that may be of interest, again technical, is "Lens Design Fundamentals" by Rudolf Kingslake.

Field curvature is just one of several aberrations that designers have to take into account and trade-off when designing a lens, and field curvature is often fairly strongly dependent on focal distance - that is, a normal lens optimized for long subject distances might have a fairly flat field at infinity, and a somewhat curved field when focused at <10x its focal length. An enlarging or macro lens would be different.

Lens designs of a given type certainly don't all have the same amount of field curvature, and it would be rash to make generalizations, other than that the focal surface is usually concave toward the lens.

darinwc · May 5, 2021

From a personal experience point of view, many vintage lenses have curvature of field, and it is mainly noticeable wide open.
That is not necessarily a bad thing!

Some lenses have a curvature of field inward (from center, going toward the edge of the frame, it is sharper closer to the camera). This is beneficial for rangefinders or SLRs with a central focus aid. What happens is that you focus at the center of the field, and then swing the camera another direction to compose the image. For example, you focus on a models face, then recompose your image so the models face is off-center and the background is in the frame. The models face will stay in focus. That will not happen with a flat-field lens.

Some lenses have just the opposite, where the field of focus curves outward, getting farther away as you get toward the edge of the frame. These are easier to use with SLR's because you can see exactly what is on focus. You can get some interesting near/far effects with these... theoretically. I shoot mostly in daylight so f8-f16 usually, where the field curvature really is not noticeable.

*Lens tests that use a flat test chart or a flat surface make it impossible to tell the difference between field curvature and unsharp edges. So you really need to compare lenses with real-world subjects with a mix of near/far objects. And with that in mind, comparing lenses becomes much more subjective!

wiltw · May 5, 2021

Juan Valdenebro said:
Isn't it just the contrary? Even best and current 35mm lenses in M-mount by Leica and Zeiss have strong field curvature?

Ten I have to wonder why the strong criticism of ONE lens (the Tamron 28-75mm) if 'they all do that!'

I have photos of a flat wall in my home which is covered with grass cloth. All points in the frame are in good focus when I shot the wall wide open. Proof that 'they all do that' is not true.

Dan Fromm · May 5, 2021

Juan Valdenebro said:
Isn't it just the contrary? Even best and current 35mm lenses in M-mount by Leica and Zeiss have strong field curvature?

Why do you believe that?

Sirius Glass · May 5, 2021

Dan Fromm said:
Field curvature? Lens designers try to minimize it. And don't bother looking at the pdf mentioned in post #2 above. It mentions but does not explain curvature of field.

Ask the person who posts here as Nodda Duma (Jason Lane) where to look for explanations and information. While waiting to hear from him, look in https://web.archive.org/web/20141229200841/http://toothwalker.org/optics.html

Except for fisheye lenses.

reddesert · May 5, 2021

Field curvature is the curvature of the surface of best focus and is not the same thing as distortion, which is deviation from rectilinearity.

Juan Valdenebro · May 5, 2021

I don't remember if it was Erwin Puts who said 35mm Leica and Zeiss great lenses commonly have considerable field curvature today... I've read that several times.
But my question has not been answered...
Do 35mm prime lenses for small format rangefinders behave differently, in a relevant way to photographs (not to microscopical testing)... ?
And which are those different ways and different lenses... ?
I have never found information on it.

wiltw · May 5, 2021

Just found this https://www.lensrentals.com/blog/2014/08/some-m-mount-field-curvatures/

"One thing that optical bench testing gives us that is hard to find elsewhere is a clear map of field curvature. We had a client interested in determining field curvatures for a several M-mount lenses and thought there would be a few among you who also wanted to see them."
"One thing these graphs will show (that you probably don’t really want to know) is that a lot of lenses have a very slight bit of tilt to the field. These are all good copies, tested multiple times. The tilt that is noticeable on these graphs isn’t noticeable in real-world photography, at least not without a great degree of pixel peeping."
"The other thing that you may not have thought of is that the sagittal and tangential fields often have different field curvature."
" You may notice the Leica 18mm is very mildly tilted, although this is not something you’d notice in a photograph. The Voigtlander 21mm is a good example of a lens with quite different sagittal and tangential curvatures."
"The Leica 28mm f/2.8 gives us a nice example, at least in the sagittal field, of a lens with double (sometimes called Sombrero) curvature."
"The Voigtlander 35mm f/1.4 (and remember, this is stopped down to f/4) shows some pretty wicked curvature."
"We’ll be presenting field curvature graphs on all of our lens reports going forward. I’ll also apologize in advance to all of you who want to see the curvature of some specific lens or other. We have over 150 more lenses that need to be tested, minimum of 8 copies of each one, and the zooms at 3 different focal lengths minimum."

This seems to mirror what I said, SOME, but not all, lenses exhibit noticeable field curvature.

Some general information on the topic
https://photographylife.com/what-is-field-curvature
https://www.dpreview.com/opinion/7031211310/roger-cicala-field-curvature-pt-2

And to dispell the myth that 'all Zeiss lenses do that' https://www.studiobinder.com/blog/carl-zeiss-lenses/

"ZEISS Otus 55mm f/1.4 ZE Lens
The Zeiss Otus 55mm is the cheapest of all the Otus Series lenses for Canon, and it has a scorching max aperture of 1.4. The Zeiss Otus gives you edge-to-edge sharpness and low field curvature, and it has the Zeiss lens has the T* anti-reflective coating that makes Zeiss lenses so unique."
"ZEISS Otus 28mm f/1.4 ZE Lens
The Zeiss Otus 28mm is almost $1000 more than the 55mm, and it shares the same max aperture of 1.4. The Zeiss Otus gives you edge-to-edge sharpness and low field curvature, and it has the Zeiss T* anti-reflective coating that makes Carl Zeiss lenses so unique."

Juan Valdenebro · May 5, 2021

Yes, Cicala's/lensrentals MTF graphs have been around for some years, but despite their multiple and possibly accurate graphs, I never found real photographs showing real life differences between the most used rangefinder 35s by Leitz/Leica/Zeiss/Canon/Nikon/Voigtlander in a comparative or comprehensive way. Thank you.

Nodda Duma · May 5, 2021

Not surprising, as they are all essentially the same design with very minor differences, with extremely flat field curvature from corner to corner

Juan Valdenebro · May 5, 2021

Once I read the classic 1.8 35 Nikkor used a type of glass that helped diminish field curvature...
I wonder how...
Just as I wonder why if the 1.4 35 Voigtlander is said to be based upon the 35 lux version 1, it did end up having such ugly intense barrel distortion...
We all know aberrations and their control imply compromises: one thing gets better, and then other one or two get worse, but it would be nice to hear more explanations on that type of stories... The first 35 lux had huge coma, and it's kind of soft at f/1.4, but that's better than strong barrel distortion...

Oren Grad · May 5, 2021

The discussion of field curvature is coming from digital camera discussion forums and test sites, because late-model, high-resolution digital sensors, when used with careful technique, mercilessly reveal departures from field flatness. And yes, that absolutely includes Leica rangefinder mount lenses in general and Leica-branded lenses in particular, many of which trade off field flatness in the interest of compactness or other design objectives. Once you go down the high-resolution digital rabbit hole for yourself, you'll quickly find that flat fields are the exception rather than the rule. How much that matters depends on what kinds of work you're doing, how much you enlarge, and how critically you view your prints.

You may find this piece by Roger Cicala helpful, in addition to the links that wiltw provided:

https://www.dpreview.com/articles/1351719699/roger-cicala-field-curvature-for-fun-and-profit

I've done Roger's DIY field curvature test, though not on any M-mount lenses. On digital only, though - film will be a very lossy medium for the test, because MTF for a film capture run through a consumer-grade scanner is going to be very poor for the low-contrast fine detail that the test depends on.

Juan Valdenebro · May 6, 2021

Oren Grad said:
The discussion of field curvature is coming from digital camera discussion forums and test sites

Sometimes, but not exclusively.
I don't use digital cameras.
A couple of years ago I did a photograph of an old car, a rusty abandoned Mercedes, unused for years, parked inside a front yard, while I was doing street. I focused the car at 2 meters from camera (the back half of it) and behind the car, on the house facade, there was a number (address) I wanted to be part of the image, and I wanted it focused... I thought "It's a 35mm, so lots of DOF, and at f/8, for sure things a couple of meters behind focus, will be on perfect focus..."
When I developed and checked my negative with a loupe, I was surprised: the number on the facade was far from being on focus!
Some months after that, I was walking the same neighborhood, and light was brighter, so I did the same image at f/11 (1/125), went home, developed, and the number on the facade remained totally out of focus... Surprised again... What was happening? Finally I understood the number was placed on the upper left corner of the frame, so it was indeed far from the plane of focus: far from the curve of focus!
I was 47 that year (now 49) and I had never seen the effect of field curvature on any of my images, which used to be sharp across the frame: a 35 at f/8 focused to 3 meters...
I went back to the scene for a third time on a very bright day, and did it again at f/16. And the number was totally focused at last. The difference between f/11 and f/16 was unbelievable.
Now I care more about important subjects behind the point of focus when they are close to the corners: those times I use f/16, and two years ago I started to use a fix f/11 instead of f/8 for street. f/16 is amazing with a 35 when it's double-Gauss, because there's no diffraction, but light doesn't allow that all the time.
I don't care about graphs.

reddesert · May 6, 2021

Field curvature is just one of the major aberrations, and a major part of lens design is about trading off the different aberrations against each other and against other considerations (size, price, and so on). So for example, there' s no particular type of glass that minimizes field curvature. In that Nikkor, there might have been some glass that used in one element enabled a tradeoff that reduced field curvature, but it doesn't mean that plunking that glass into some other lens or some other element would have a good effect.

Juan, in your example of a 35mm lens on 35mm film, at f/8 and focused at 2 meters, the conventional depth of field (with a circle of confusion of 0.03mm) is roughly from 1.4 to 3.2 meters. Probably not enough to bring the house into sharp focus even with a perfectly flat field lens. Try for example the DOF calculator at https://www.dofmaster.com/dofjs.html In contrast, focusing at 3 meters, the DOF is from 1.9 to 7.2 meters. While curvature of field may have affected your photographs, the other issue is that one loses DOF very rapidly as one focuses closer.

Nodda Duma · May 6, 2021

You don’t want to see graphs yet you want to understand the aberration? I’m sorry, to truly understand what’s going on requires study of the underlying math and physics. It’s not something that can be put readily into layman’s terms to advance your understanding beyond myths, half-truths, and marketing BS. There’s just no way.

First thing to understand: Field curvature isn’t a magical cause of all a lens’ woes. Optical fabrication and assembly tolerances are significant contributors to image degradation in a modern lens, and could just as easily be the root cause to your experiences as anything. Any review or test of a lens which draws conclusions about the design based on a single sample should be immediately suspect: The reviewer has no idea where that lens falls in the performance distribution for that design, and for consumer-grade optics such as used for photography the distribution can be quite wide.

underlying that is the effect of balancing of all the aberrations, which for modern optics really must be viewed as Zernicke terms as the higher order aberrations are often balanced against and of similar magnitude as the residual third order aberrations.

if you really want to understand field curvature, and not just myths and legends, I highly recommend accepting the fate that it requires maths and follow the link I provided above. You will not gain a full understanding there, but it will shed light on the tools required to truly understand the phenomenon.

If however you are happy with myths and legends, then yes.. field curvature causes everything.

Dan Fromm · May 6, 2021

Juan Valdenebro said:
f/16 is amazing with a 35 when it's double-Gauss, because there's no diffraction, but light doesn't allow that all the time.

Why do you believe that lens design can tame diffraction? And which lens are you talking about? I ask because I'm not aware of any 35 mm lens, if that's what you meant, that is a double Gauss design. Many 50 - 60 mm lenses for 35 mm cameras are 6/4 double Gauss designs or variants, but 35 mm?

Craig75 · May 6, 2021

complete broken biscuits by op

grain elevator · May 6, 2021

In practice, getting an idea of the field curvature of one's lenses is easy. Take a picture of something textured and flat, stretching away from the camera. For close up, a newspaper or similar, for longer distance, flat lawn should work. At higher magnification, the plane or curve of focus will be obvious.
I doubt you will find much about the lenses' behaviour on film. Field curvature is different on digital due to sensor cover glass. Towards the corners of the sensor, light must travel for a longer distance through the cover glass, which has a different refractive index from air. No such effect with film.

What's the right forum to ask about field curvature?

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