Lens Not Focusing, What is the Cause?

[bc_69]

Recently I bought a Sironar N 120mm. I only got around to mounting and using it yesterday. What I found is that it will not focus unless I have the lens practically on top of the subject and the lens board is wound back into the body of the camera housing. I tried different distances from the subject with the camera with no success. It appeared as if winding the lens back into the camera housing was the only way I could get close to a sharp image anywhere, with the best result when I was within a few inches of the subject.

Could it be the rear element is not the original for this lens? It looks genuine Rodenstock with similar design to the front element but it is smaller obviously by some margin. It screws in easily and neatly as well. I'm at a loss to explain it other than the possibility the rear element is not original.

Any tips of how to find the answer would be appreciated.

[John Layton]

When in this configuration...what is the film plane to lens board distance? Alternately...what type of camera? Maybe not enough bellows compression by design? Hmmm...your "winding the lens back into the camera housing" implies more than enough compression - but do at least measure the film plane to lens flange distance.

[bc_69]

I'm going to mount the lens on a different camera with a long bellows to see if that helps, but I can't see why given a 135mm lens works perfectly well on the other camera.

[Havoc]

Could be some missing shims.

[Corran]

I'm going to mount the lens on a different camera with a long bellows to see if that helps, but I can't see why given a 135mm lens works perfectly well on the other camera.

If you've used a 135mm lens, your 120mm should work with roughly 15mm less extension for infinity focusing.

Your rear element suspicion is probably correct. Shims wouldn't make that much of a difference in image quality, and wouldn't change the focus extension by any significant amount.

[bc_69]

Your rear element suspicion is probably correct.

Yes, I think so. It is interesting the rear element has all the same moldings around the outside of the barrel as the front one, which probably means it is Rodenstock but from a different lens. Would that be normal for the rear element from another Rodenstock lens to fit mine?

[Mark Sawyer]

I'd start by checking the focal length of the front and rear elements separately. The lens being a fairly symmetrical Plasmat, each should focus at about 240mm. (Tell me if I'm wrong, folks; Sironars aren't in my repertoire...)

[djdister]

I ran across a lens once that had an internal lens element reversed. You couldn't tell just by looking at the front or back of the lens though. Really difficult to focus...

[reddesert]

Some lenses do have smaller rear groups than front groups. Pictures of the lens, and how it is mounted and the camera setup, would be helpful in diagnosing the problem.

You can try measuring the focal lengths of the front and rear groups separately. This will be kind of a rough measurement since a single group may have a nodal plane that is outside the body of the lens. If you want to make a more precise measurement there are ways to do it, such as focus on a yardstick that is some large distance (tens of feet) away and measure its length on the ground glass, etc.

The threads on modern shutters are mostly standard so it is possible for groups to get mixed up, but it should be rare.

[domaz]

Take the lens off the camera, open the shutter. Position it so it's pointing at an open window with a wall behind it. You should physically see the image the lens projects. If it doesn't focus on the distant objects out the window at the expected focal length as measured from location of the aperture something is wrong with the lens.

You can also focus "aerially" by putting a loupe up to your eye and positioning the lens in front of you. You can actually see the aerial image of the lens in the loupe if you do it right. It's quite tricky to do but is a good way to check coverage.