Petzval without swirl

[Andrew]

sorry for the silly question, but is the image in the original post made with the lens you're unhappy with or is it OF the camera and lens you're unhappy with?

if it's made with the lens, your background is too close to the subject and the background is too uniform to effectively show the swirl effect anyway. I'd reinforce the comments about stretching the lens because I thought a 9 inch lens had no swirl when I tried it on 4x5... that was because the format didn't include the periphery of the image circle but it's there it in larger formats.

[goamules]

Also, if your Darlot happens to be a slow one, it won't swirl. The Petzvals faster than F5 do it, but the slower ones usually don't. Swirl is one of the common aberrations, called coma. Several designs do it, but always at the extreme edge (unless someone has misconfigured the glass, then you'll sometimes see a ring if swirl between the middle and edge).

[John Berry]

Take back airspace elements out.
flip the rear element, then flip the two elements as an assembly. Reinstall and you should have a huge curvature of field that will give you swirls.

[goamules]

But you'll also have the swirls in the wrong places, extreme coma, and less sharpness anywhere in the frame. I don't recommend misconfiguring lenses just to show aberrations.

[Dan Fromm]

Supposedly swirl is caused by a kind of mechanical vignetting

Will

Not vignetting, uncorrectable (with the Petzval design) off-axis aberrations.

[Mark Sawyer]

Not vignetting, uncorrectable (with the Petzval design) off-axis aberrations.

Any idea what the aberration is, Dan? All I can think of is coma, but it doesn't look like coma...

We've never come to a conclusive answer about this, but I've suspected mechanical vignetting. But that doesn't explain how the mid-range swirls in a misconfigured lens occur.

[CCHarrison]

My research led me to:

off-axis uncorrected coma combined with spherical aberration...

Other Dan

[goamules]

I've always said the elongated orbs in the edges are coma, but I'm starting to think it may be Astigmatic difference or Saggital Astigmatism. Probably combined with normal Coma (which I believe usually has a pointed "trailing edge" as in an astronomical comet. Either way, I think it would be hard for anyone but an optical scientist to predict or prevent it.

http://www.nikon.com/products/instru...tion/index.htm
http://www.handprint.com/ASTRO/ae4.html