Tilt an old lens a teeny bit off axis to improve image. Anyone ever heard of this?

[Oren Grad]

P.S. other aberrations like curvature of field are not sensibly the reason. Even a plano-plano singlet has good enough field curvature at f/16 to not be noticeable with a 1-5 degree tilt...and it’s not like you wouldn’t refocus to compensate anyways. Field curvature isn’t magical, it’s just a curved plane of best focus.

Jason - leaving aside alignment defects and the possibilities opened by aspherical designs, are there any lenses with traditional spherical-section elements that have field curvature patterns that are not radially symmetric? Hard to see how that would be possible if each element is radially symmetric.

[Nodda Duma]

Jason - leaving aside alignment defects and the possibilities opened by aspherical designs, are there any lenses with traditional spherical-section elements that have field curvature patterns that are not radially symmetric? Hard to see how that would be possible if each element is radially symmetric.

Yes, actually, because you cannot ignore alignment errors. Perfect alignment only exists on paper. Real, manufactured lenses are not ideal and have associated tolerances they have to meet to ensure required performance... lens designers consider this to be more important than the actual design itself, if that makes sense.

There’s a whole list of contributors, including lens wedge and element centration when a stack of optics are put in a barrel. So you can see symmetrical differences in aberrations off axis (corners typically, where you’re getting close to the edge of the image circle the design corrects for), and for more complex designs we’ll evaluate this asymmetry during tolerance analysis in the design phase for boresight and image performance, selecting a compensating element to actively adjust the position of during assembly to meet requirements.

For simpler optics (like visible spectrum objectives) we control aberrations with alignment and fabrication tolerances, and (usually of concern) characterize the distortion map, which I have yet to see one being perfectly symmetrical.

[cowanw]

I wonder if old lenses are more prone to rub marks at the center of the front curved surface.

[Oren Grad]

Yes, actually, because you cannot ignore alignment errors.

Fair enough - a good, reality-based answer. Actually, we see this clearly with the combination of cutting-edge lens designs and the latest high-resolution digital cameras, where the effects of sample variation in lens element alignment and lens mount / sensor alignment in the camera body, layered on top of the complex field curvature patterns generated by piling on the aspherical surfaces, are painfully obvious and a real problem for those who seek to take full advantage of the capabilities that these new products seem to offer.