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

[jodphoto]

In the 70's we used a lot of old glass on 8x10 Ektachrome and shot catalogue stuff to size. (Yes kids, no scans, color separations were made by contact printing).

There was a prevailing, if arcane, convention, when using old glass, to tilt the front board a teeny bit off axis to improve image quality. We would never shoot straight through the center of a lens. This was not to correct focus or perspective control, but specifically to optimize the lens performance.

Has anyone ever heard of this and can you provide a technical explanation that doesn't ring as folklore?

There's a lot riding on your answer. (A beer, anyway.)

[Corran]

Curvature of field.

[Richard Wasserman]

Was this true for more than one camera? If only one was involved perhaps the front and rear standards were not quite parallel?

I also like Corran's answer

[Oslolens]

Reflections? Apprentice-joke?

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[reddesert]

Lens aberrations depend on the focus distance. So it's possible you were using lenses computed for "normal" ranges like infinity to 1:10, but focusing it at a large reproduction ratio of 1:4 to 1:1 (seems possible for catalog shoots onto 8x10). That could enhance curvature of field, and tilting could compensate for it. Hard to know if it was for real unless someone tried it both ways - I think when just looking at the ground glass, it would be hard to separate out the effects of change of focus plane vs curvature of field.

[Mark Sawyer]

Unless you're using Petzvals or meniscus lenses, I doubt it was field curvature. Regardless, front tilt won't remove field curvature. No movement will. Could be focus shift from stopping down, but that's corrected by re-focusing. Sounds more like folklore.

[jodphoto]

Reflections? Apprentice-joke?


Oslolens, reddesert, Corran, Richard, and Mark,

Thanks for your responses. I was a newbie, but the shooters wouldn't do that on a job as a prank. The lenses were Tessars types, Commercial Ektars, Dagors, some Artars (for jewelry). My perception at the time was that shooting straight through wasn't as good optically with old glass. Old, meaning single coated, cleaning marks, slight haze, that sort of thing. We would have dozens of sets going and would swap lenses between them so it wasn't one lens or camera. We shot mostly at 16, 22 or 32, and with exposures between 4 and 20 seconds. With Deardorffs, mostly the back was at a right angle to the bed, and the only movement would have been the front tilt, but severe movements (like scheimpflug principle) was not really done with these old lenses. Also the sets were gobo'd right to the edge of the image so no light sources could be seen by the lens. It was advised to tilt the lens a degree or so as a precaution and never shoot straight through them. Maybe it helped with internal non image forming light or flare.

If it was folklore, it was definitely believed. So it looks like I owe my buddy a beer. Thank you all for weighing in.

[Mark Sawyer]

Well, "Tessars types, Commercial Ektars, Dagors, some Artars" are all pretty flat field, so it's not field curvature. Maybe the camera was out-of-kilter, and needed a degree or two of front tilt to straighten it out. Certainly so if you needed to do so with all those different lenses. Or if you were shooting three-dimensional shots, it might have helped with the depth of field. Other than that, I'd still say folklore.

[Nodda Duma]

Stopped down, I agree that there may be benefit, knowing what I know of stray light and ghosting. So yes, you could be on to something.

At very small apertures light rays are intersecting optical surfaces at near-normal angles of incidence, and in those situations the ghosting artifacts would be more prominent and possibly apparent as glow or halos around very bright objects (glint, etc), or reduction in contrast depending on the particulars of the design. Fun fact is comparatively, ghosting is *less* prominent with the lens opened up because more rays along the ghosting ray paths are hitting surfaces at steep angles and getting thrown out of the area of the film / imager, so compared to the primary path and image, wide open the ghosting is significantly less prominent.

So what you said they advised — tilting at a very small angle at the smaller apertures you shot at — would throw those on-axis ghost artifacts out of the image and so there is very likely something to that. Note “artifacts” as I use it are any effect arising from internal reflections or stray light...so loss of contrast, halos, or even the J.J. Abrams movie lighting effects that he puked all over the Star Trek reboot movie..though those were CGI created...the actual look of the artifact depends on how close the focal length of the ghost path is to the focal length of the primary path.

It would affect different lens forms differently, so improvements would range from none to some.

Another solution besides tilting would have been to open the lens up to a wider stop However, the increased aberrations would soften the image, which is the whole reason to shoot stopped down. Tilting the lenses slightly when stopped down is a nice solution to both image quality and paraxial ghosting. Very elegant. I’m going to actually pass this tidbit on at my optics team meeting this week... Congratulations on a story worthy of sharing with the professional designers I mentor.

So tell your buddy that a lens designer said he owes you a beer. :)

-Jason

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.

[Corran]

I of course defer to Jason and his optical knowledge. I threw that out there though because I feel like I've seen a bit of focus curvature at the periphery on occasion - usually with older, simpler lenses. Very slight, but I was thinking perhaps that could magnify when doing close-up work, and a bit of tilt could correct it (and I feel a bit of tilt would naturally be preferred with most 3D objects?). Perhaps there were other factors at play in my situations - filters on the lens is something I thought of at the time.

Anyway, the flare discussion above is also interesting, because I definitely even recently saw exactly what you describe - not a hint of flare while focusing wide-open, and then stopped down, horrid flare from light sources very close to the periphery ruining my photo. Interesting!

[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.