Field curvature and focus calibration?

[NateWeatherly]

Advertisement (gone after registration)

Trying to learn the ins and outs of my lenses and was wondering if anyone could help me understand how Field curvature is handled when adjusting a body and/or lens.

 

Take, for instance, my 50mm Summilux pre-asph. Seeing as the plane of focus in the center is further back than in the field, how SHOULD the lens' behave at infinity on my M2? Would the lens cam normally be tuned to bring the field into focus, or the center? I'm assuming that the field makes more sense at infinity, since otherwise it would be impossible to bring the field into focus, but it seems like you'd probably want accurate center focus at closer distances.

 

Anyone know how this is handled?

 

Thanks!

[Vec]

The rangefinder should be calibrated for the center of the field of view. If the images in the rangefinder coincide then that should be in focus -- and since it's in the center... The aspects of field curvature are inherent to the lens itself.

 

What camera a lens is mounted to is immaterial to the lens. Even if it's mounted to a modern live-view camera, the character of the lens, including the field curvature, will be the same.

 

From what I understand the 35 FLE has some pretty severe field curvature. So severe that if you're wide open shooting at infinity the edges would be out of focus because it would be severely front-focused. That said, I have that lens and I've never had any issues -- I just use it to work around that.

Edited July 15, 2017 by Vec

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[pgk]

From what I understand the 35 FLE has some pretty severe field curvature. So severe that if you're wide open shooting at infinity the edges would be out of focus because it would be severely front-focused. That said, I have that lens and I've never had any issues -- I just use it to work around that.

 

The problem here is quite simply the amplification effect of internet chatter. Words like 'severe' seem to be used to amplify or emphasise something which is rarely a problem in practice. And field curvature in the lenses commented on is such a 'problem'.

 

Think about the M2. It is a film camera and film is multi-layer so has a depth and/or needs to be perfectly flat in the camera which it probably isn't. Add field curvature and obviously we are going to have problems getting anything 'sharp'; except of course that reality illustrates very well that this is incorrect.

 

In some, fairly rare instances, field curvature might be a problem - taking a very flat subject perpendicular to the camera at a fast aperture say. But how often do we do such a thing? And if this is what you do I would suggest that the equipment being used needs to be carefully assessed and changed if needed.

 

Don't worry about it - as long as the lens is adjusted correctly (centre ) you are fine.

[lct]

All lenses have some field curvature more or less especially at full aperture. The effect is more visible on fast and sharp lenses because they are sharper than others on the plane of focus. The 35/1.4 FLE is a good example but is not significantly worse than other fast and sharp Leica lenses like 50/1.4 asph or 28/2 from this viewpoint. For better results, try the 50/2 apo. 

[pgk]

The effect is more visible on fast and sharp lenses because they are sharper than others on the plane of focus.

 

But only on planar subjects. If you shoot landscapes set at infinity and at f/1.4 then corners may be soft - but its rare  to do so. Same with planar subjects wide open - this really is an internet testing problem/phenomenon which is rarely going to impinge with real life photography except on unusual occasions. I have a number of f/1.4 lenses and to date its never been a significant problem in real world use of them - honest!

[Steve Spencer]

But only on planar subjects. If you shoot landscapes set at infinity and at f/1.4 then corners may be soft - but its rare  to do so. Same with planar subjects wide open - this really is an internet testing problem/phenomenon which is rarely going to impinge with real life photography except on unusual occasions. I have a number of f/1.4 lenses and to date its never been a significant problem in real world use of them - honest!

 

I disagree to some extent. It depends on the type of shot you shoot. It also depends on the nature of the field curvature. For example, for environmental portrait that I often shoot at fairly wide apertures I really dislike inward field curvature in which the edges are corners are in focus closer than the centre. This form of field curvature negatively affects the focus fall off and makes it harder to use selective focus to isolate the subject. An example of a lens with this sort of field curvature is the Zeiss ZM 35 f/1.4. I like that lens very much, but the field curvature is a problem that you need to understand and work around with that lens. Specifically you need to avoid anything in the foreground closer to the camera than the subject. In contrast a lens with outward field curvature in which the centre is in focus closer than the edges on corners has it own issue with portraits. Used well this sort of field curvature can wrap focus around the subject isolating them very effectively, but it can also lead to shots in which the background is sharp where you do not expect it to be and in which the look of the focus fall off can be ruined. I like a lens with a mild version of this sort of field curvature, but an extreme version is pretty awful for portraits and IMO ruins the look. 

A whole different set of issues arises for a landscape shooter. If you want the whole scene in focus outward field curvature in which the corners and edges are in focus further away than the centre can leas to shots in which it is very difficult to get the corners closest to the camera in focus. In contrast an inward field curvature in which the corners and edges are closer in focus than the centre can make getting these close corners sharp even at wider apertures, but it is extreme can make it impossible to get distant corners in sharp focus. For landscapes I prefer a lens with mild inward field curvature. So for these types of shot the Zeiss ZM 35 f/1.4 in my view is excellent with its mild inward field curvature.

And then there are landscape shooters who prefer to have just a small part of the shot in sharp focus and for these shooters getting a lens with a flat field of focus is a critical part of their lens selection. As act notes the 50 cron AA is great for these types of shots.

[pgk]

Advertisement (gone after registration)

I disagree to some extent.

 

You are welcome to do so, but I still maintain only a very few will ever find field curvature a real world problem - paradoxically it is a real problem for one of my specialisations, underwater photography and even a small apertures but that's another story and the solutions are complex. FWIW I have shot landscapes wide open on both 35 and 50 Summiluxes and the results have been fine .

[Vec]

When I was saying "severe" it's just from referring to the internet chatter echo chamber. I have the 35 FLE... never once have I have came across a picture I took with it (and I use it for ~ 50% of my shooting and it's my favorite lens) and thought that "damn... if only for the field curvature this would be an excellent picture."

 

Assuming things are within spec (i.e. the lens impinges on the roller arm the correct amount, and the roller arm moves the rangefinder the correct amount), you're golden.  :-)

 

If you're shooting landscapes at f/1.4 the field curvature might make the edges soft. Simply don't shoot at f/1.4 if it's bothering you that much.

 

No matter what you're shooting, Leicas or cheap disposable cameras, making a good photograph typically entails understanding the equipment you're using. There are plenty of interesting YouTube videos where they give a pro photographer a cheap camera. The limitation isn't the gear.

 

Yes, we pay a lot for the lenses we use. But Leica is still bound by the same laws of physics (and economics realistically) that everyone else is. Engineering is an exercise in tradeoffs. You want small fast lenses? Be prepared to put up with some level of aberrations or pay even more (50mm Summicron APO anyone?) to achieve far smaller tolerances. Lens design is a very complex art form.