Why No Barrel / Pincushion Distortion LF Lenses?

[Andre Noble]

I was just critically viewing some 35mm slides shot with a 20-35 Nikkor zoom. I was very suprised at the obvious geometric distortion (barrel) at the 20mm end in architectural shots.

On the other hand, I cannot recall a single image I've taken with Nikkor 90 and 210 mm LF lenses which showed even a hint of barrel or pincusion distortion.

I do not recall geometric distortion ever being discussed in regards to LF lenses.

Why?

[Doug Dolde]

Probably hasn't been discussed because it doesn't exist :-)

[Michael S. Briggs]

LF lenses typically have low distortion. Lenses with significant distortion tend to be complex or asymmetrical designs, such as zooms and retro-focus wide-angles.





When a symmetrical (about the stop) lens is used for symmetrical imaging, which means a reproduction ratio of 1:1, several abberations including distortion automatically cancel. This effect still works fairly well even in quasi-symmetrical lenses used for the non-symmetrical imaging of distant subjects, in which the image is much smaller than the subject. Most LF lenses are approximately symmetrical and so should have low distortion. Zooms and the retro-focus wide-angles needed for SLRs for mirror clearance are non-symmetrical lenses and so the designer has to work harder to minimize distortion.





A related question: Why no retrofocus wide angles?... at http://www.largeformatphotography.in...ic/498882.html

[Steven Dusk]

The new ultra wide angle, retrofocus design LF lenses are still zero geometric distortion. The reason is that if they weren't, they wouldn't sell any.

[Bruce Watson]

Steven,

What "ultra wide angle, retrofocus design LF lenses" would these be? Who's the manufacturer? Where can I find out more?

[Steven Dusk]

Bruce, examples are the Rodenstock 35mm and 55mm. They are a retrofocus design so that there enough room for a little tilt and swing before the standards hit each other.

[Glenn Kroeger]

Bruce:

Schneider also now make 24mm XL and 35mm XL digital lenses which are retrofocus.

[Struan Gray]

Although it is low, LF lenses do show distortion. I was recently shopping for a 90 mm lens and among the many advantages of the f4.5 lenses over their f6.8 or f8 siblings is that they have roughly half the distortion. Paul Butzi has Rodenstock literature on his website (www.butzi.net/rodenstock/rodenstock.htm) that shows this clearly. For me, the advantage wasn't worth the extra money, weight and bulk, but it's not completely insignificant.

Incidentally, it is worth noting that it is the change in the amount of distortion at different distances from the optic axis that produces curved lines. A uniform distortion really just means that the focal length is a bit different from that stated. It's the differential that gets you.