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.

[Bob Salomon]

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.

Close.
Darken a room, point the lens out the window with the aperture at 5.6 and the shutter open. While pointing it out at a distant object hold a mount board or a shirt cardboard behind the lens and move the board towards or away from the lens till you see a sharp image. That is the extension needed to use the lens on your camera at infinity.
The measurement will be appx. Since you would not be using a loupe to focus the image sharply, but it will be close.

[bc_69]

Close.
Darken a room, point the lens out the window with the aperture at 5.6 and the shutter open. While pointing it out at a distant object hold a mount board or a shirt cardboard behind the lens and move the board towards or away from the lens till you see a sharp image. That is the extension needed to use the lens on your camera at infinity.
The measurement will be appx. Since you would not be using a loupe to focus the image sharply, but it will be close.

Thanks Bob and Domaz. I did this and achieved a sharp image at about 130mm distance. Next I remounted the lens on a Horseman rail camera and was able to focus on a bunch of flowers. It may be that the Gandolfi didn't have enough compression on the bellows for the distance I was at which was only about 1m. When I tried to use it on the Gandolfi it was for the first time and I moved the lens close and far, I should have been able to get a sharp image but for some reason I wasn't, I'll blame me and the bellows at this stage.

[Corran]

Just to be sure, I will mention that to focus closer, you must extend the bellows out. If 130mm extension gives you infinity focus, 260mm would give you a "1:1" magnification (life-size). Your 1-meter focus distance will be somewhere in between. There's a formula but it doesn't really matter at the moment. Set up your camera/lens again and start racking the focus out until you achieve focus.

[DHodson]

I'm with Corran on that. By definition, a 120mm lens focal length lens focused at infinity is 120mm. Focusing on anything closer than that requires you to increase not decrease the bellows so something doesn't sound right. Here's some formulae you might want to look at on Kenneth Lee's web page.

http://www.kennethleegallery.com/html/tech/bellows.php

Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

Hope this helps
Dave

(I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)

[Bob Salomon]

I'm with Corran on that. By definition, a 120mm lens focal length lens focused at infinity is 120mm. Focusing on anything closer than that requires you to increase not decrease the bellows so something doesn't sound right. Here's some formulae you might want to look at on Kenneth Lee's web page.

http://www.kennethleegallery.com/html/tech/bellows.php

Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

Hope this helps
Dave

(I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)

Depends on the flange focal length which isn’t necessarily the same as the focal length.

[Jim Andrada]

Or set your extension to approximately 250mm (plus minus a bit doesn't matter - do it by eye) and then move the subject toward and away from the camera and see if you get it in focus. This is just a rough test after all.

[bc_69]

Just to be sure, I will mention that to focus closer, you must extend the bellows out. If 130mm extension gives you infinity focus, 260mm would give you a "1:1" magnification (life-size). Your 1-meter focus distance will be somewhere in between. There's a formula but it doesn't really matter at the moment. Set up your camera/lens again and start racking the focus out until you achieve focus.

This did cross my mind as well but it was late and I ran out of time when I did the simple test. At 1m I was not getting the entire flower arrangement on the screen so I'll do more comprehensive testing in the next couple of days, mostly further from the subject until I have the entire set up on the screen and then see if I can focus normally.

When I did a quick comparison with the 135mm on the Gandolfi at the same distance they looked to be about what you would expect with the 120mm image slightly smaller which I took as a good sign, but I still have the nagging doubt about not being able to focus when I originally tried.

[bc_69]

Here's some formulae you might want to look at on Kenneth Lee's web page.

http://www.kennethleegallery.com/html/tech/bellows.php

Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

Hope this helps
Dave

(I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)


Depends on the flange focal length which isn’t necessarily the same as the focal length.

Yes it is on a 4x5.

The 1m test was just a test to see if I could get a clear image. In reality I will have the camera further back so the entire subject is on the screen, I will test this in the next couple of day.

[Dan Fromm]

Here's some formulae you might want to look at on Kenneth Lee's web page.

http://www.kennethleegallery.com/html/tech/bellows.php

Using fl = 120mm and 1000mm focusing distance would result in a bellows draw of around 135mm if I'm doing the calculations right.

Hope this helps
Dave

(I'm assuming we're talking 4x5 cameras here. Getting full coverage on larger cameras with a 120mm lens might be an issue on some cameras)

Not quite right. There are two solutions, not one, unless magnification is 1:1. Otherwise there are two magnifications for every film-to-subject distance.

If film-to-subject distance is 1000mm and focal length is 120 mm, the possible magnifications and lens rear node-to-film distances are 0.1627:1 and 139 mm and 6.15:1 and 858 mm. These are extreme examples. Even so, they're the reason why the best way to focus close up is to set extension to give the desired magnification and then move camera + lens or subject to focus. If necessary once the final setup is more-or-less reached focus can be fine tuned by adjusting extension.

Dave, image circle grows with magnification. Extension required, measured in focal lengths, grows with magnification. This is why in general the higher the magnification desired the shorter the lens used.