Field curvature

Discussion in 'Open Discussion' started by Mellow, Sep 21, 2012.

  1. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    I was reading Amin's comments on the Sony RX100, which got me re-reading dpreview's comments on testing it, which reminded me of a comment I read about how some camera labs re-focus on the corners when determining resolution to account for field curvature (this last statement may be apocryphal, but that's how I remember it).

    Still, I don't get it. Without getting all technical and all that, isn't field curvature just a reason why some lenses aren't very sharp away from the center? Does it really matter what the origin of the softness is?

    In other words, if the RX100 has a lot of field curvature at short focus distances, isn't that the same thing as saying the corners will be very soft at close focus distances? And, if so, why don't the reviews just come out and say it?

    Sometimes I get the impression that some people exempt field curvature from the list of lens flaws. A lens might be described as very sharp, but suffer from a lot of field curvature. Isn't that an oxymoron?

    I apologize if I'm just broadcasting my ignorance about optics in general.
     
  2. addieleman

    addieleman Mu-43 All-Pro

    Aug 5, 2010
    The Netherlands
    Ad
    Couldn't agree more. When I make a picture I have to choose between a sharp centre or sharp corners when a lens suffers from field curvature. And let no one tell me that I could make more than 1 shot and merge them later in Photoshop, because that won't help me when leaves of tree are moving in the wind while taking those shots!
     
  3. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
    It does matter depending on what your subject is, because field curvature means that the edges/corners have a different focus point. If you're shooting you're shooting a flat, resolution chart, then the corners will be soft. If you're shooting a 3 dimensional subject, like a plant, then things in a different plane of focus than the center will be in sharp focus in the corners.

    Field curvature is a problem with macro lenses, but not much of an issue for most other photography. That's why it makes sense to refocus a lens to see if it can form a sharp image in the corners.
     
  4. addieleman

    addieleman Mu-43 All-Pro

    Aug 5, 2010
    The Netherlands
    Ad
    I have several legacy lenses that have field curvature at or near infinity, preventing me from making pictures that are equally sharp across the frame. When using a Minolta 28/2.8 I noticed by accident that stuff at about 5m was sharp in the corners while it was focussed at infinity. So it can be a real problem at larger distances too.
     
  5. zapatista

    zapatista Mu-43 Top Veteran

    668
    Mar 19, 2012
    Denver, Colorado, USA
    Mike
    The RX 100 images soften in the corners (after "correction") at 28mm equivalent. The softness, even at f1.8 is not nearly as bad as the 16mm e-mount lens...not even close. However, the 16mm lens is just soft towards the corners and might not have much to do with field curvature.
     
  6. Djarum

    Djarum Super Moderator

    Dec 15, 2009
    Huntsville, AL, USA
    Jason
    I usually see issues with field curvature with cameras at close focus. Its much more obvious at least. I suppose it depends on the group of elements that move and in which direction as they focus. I guess its apparent on the RX100 as Amin describes. I have seen images at infinity however that have pretty soft corners too with this camera even stopped down. Its all about compromise.
     
  7. Mellow

    Mellow Mu-43 All-Pro

    Aug 27, 2010
    Florida or Idaho
    Tom
    I see your point. So if there's field curvature but the lens is sharp, at least SOMETHING will be in focus away from the center. However, there are lots of things besides resolution charts that are flat--buildings come to mind. For these, significant field curvature would be a problem, right?
     
  8. Djarum

    Djarum Super Moderator

    Dec 15, 2009
    Huntsville, AL, USA
    Jason
    Yes it would. Stopping down should help. Focusing at infinity on landscape can cause a problem too, since ideally, everything past a certain point "should" be in focus, but its not.
     
  9. RobWatson

    RobWatson Mu-43 Hall of Famer

    I recall several examples in which the film holder was shaped to match the field curvature just so best focus across the field was possible. Had to do that with silicon ...

    With film it makes sense to refocus and example the edge to see if one has to modify the film holder.

    Now if our dear friends would work on a programmable microlens array that could be deformed on demand it could manage field curvature and possibly some IS as well. Our lens in our eyeball does this so why not?

    For those with an IP bent ... the fact that I can think it up means it is obvious and there for not patentable.
     
  10. dhazeghi

    dhazeghi Mu-43 Hall of Famer

    Aug 6, 2010
    San Jose, CA
    Dara
    Yes, and it's often a bigger problem than simple unsharpness for landscapes since at least until recently, if you stopped a lens down 3-4 stops, even the edges invariably sharpened up. With field curvature no matter how far you stop down, you may never get the corners properly sharp. It makes choosing where to focus a real challenge.

    DH
     
  11. RobWatson

    RobWatson Mu-43 Hall of Famer

    I think this does not happen ... the field curvature is fixed and as the depth of focus increases eventually the DOF exceeds the deviation by field curvature.

    As for things like distortion and other abberations they very well may not chagne much, or at all, with aperture so will never go away. Things like coma, astigmatism and spherical abberation can mimic 'out of focus' but really are something different. A great many optical properties go into what we call "sharpness" and focus is only a small part of the overall picture (nifty pun intended).

    Thread rift begin ...

    Field curvature is actually more properly referred to as the Petzval surface named after the fellow that first described the mathematics of said surface. Curious bit of history between Messers Petzval and Voigtlander and some accusations of fraud and patent infringement. Google and wikis will give some details for those interested in such things.
     
  12. b_rubenstein

    b_rubenstein Mu-43 All-Pro

    Mar 20, 2012
    Melbourne, FL
    A building may be flat, but unless the camera is lined up perfectly square with one of the sides different parts will be at different distances, and DOF is more of an issue than field curvature to get everything in focus. Again, field curvature is a serious defect for macro lenses, or doing something like copying artwork. Not much of an issue for typical, real world photos; where things like chromatic aberration, astigmatism are far more visible.
     
  13. nseika

    nseika Mu-43 Veteran

    260
    Nov 22, 2010
    Jakarta, Indonesia
    Lois