double image

[uphereinmytree]

Hello, I have a tiny 90mm bausch & lomb tessar from a folding camera that is now on a 4x5 (just covers nicely) and has a peculiar double image away from the center. The center is not distorted and sharp but as the image falls off to the corners everthing becomes increasingly doubled like a rangefinder. I believe all parts to be original. Any ideas? thanks

[Eric Woodbury]

Folding camera has a smaller image circle. You may be beyond the acceptable limits of the lens, i.e. the double image may be chromatic aberration. Try it with a strong filter to see if one of the images goes away. Something like a 25 Red.

[uphereinmytree]

The red filter had no affect on the doubling although that still may be the issue. I thought I had a nice fast tiny 90mm for 4x5.

[Glenn Thoreson]

The lens is not corrected for the format. I think you're pushing it beyond it's limits.

[rdenney]

That lens would have been designed for 6x9 format or smaller.

These days, lens manufacturers build the barrels to mechanically block light outside the official coverage of the lens. They do this to prevent producing image quality that does not meet the standard they have set for the image circle. That means that if it makes an image at all, that image meets the manufacturer's standard. In the old days, however, manufacturers didn't do that. They built the barrels to allow all the coverage that the optics would permit, with the notion that it's the photographer's responsibility to know the coverage limits according to the photographer's standard. A lot of old lenses therefore go very soft and suffer other aberrations at the edges of their coverage.

This would be especially true for a lens intended for a smaller format, in a camera with no movements. The manufacturer would not be concerned about the lens being used outside its official coverage.

Rick "well covered" Denney