Dissembling large format multi cell lens?

[Ted R]

This is particularly for a 240mm Rodenstock S, there is some dust between the lens elements. It's unlikely to effect the image however I do have a lens wrench to pull apart the elements to blow them out. However I'm unsure if there are any seals, tolerances, gaps or torques that would effect by me unscrewing it. Worth noting this in the inner cells not the sides facing the shutter. Would it be as simple as unscrewing it, blowing it out and just tightings it back down again?

I like taking things apart :-) Sometimes I put them back together and they still work. In my (non-professional) experience with large format lenses they are precision assemblies however it would be unusual to find seals inside, and I have never heard of torque specifications for lenses though they may exist, hand tight is tight enough, most of the parts are probably brass and cannot take much force before distorting. Mark the edges of the glass to record the orientation so that things go back together with the same rotational alignment. If you encounter shims note the sequence and location very carefully.

[lecarp]

If you ever plan to sell that lens to someone, leave it alone or send it to a professional.

[Jac@stafford.net]

But how do you create a totally dust free room to reassemble.

Dust is a b*tch. I have it under control in the house with three forced-air HEPA filtered machines. However, most humans shed all the time, so wear clean cotton gloves and long sleeves, and a hat when working. I kid you not.

[Bob Salomon]

I can't show you a 240 but here is a factory cutaway of a 210 N in Copal 1. While it may not seem particularly complicated, it is, and if you screw up centering or spacing while playing the lens becomes basically junk.

[EdSawyer]

It's not that hard to do. I've opened and cleaned many lenses, from 35mm through RZ67 and large-format. They are not that complicated, no seals. As mentioned, keep the spacing and order correct and you should be fine.

-Ed

[Ted R]

PS having encouraged you to go ahead I must also add that I agree with the advice that a few specks of dust make no difference to the performance of the lens. The reason is that the location of the dust in the center of the lens where the shutter and iris blades are located is out of focus. Think about this. The iris is a circular aperture yet closing the aperture does not create a disc shape at the film plane, what it does is reduce the intensity of the light. Dust behaves the same way, it doesn't appear in focus, it reduces the intensity, by an amount that is too small even to calculate never mind measure. A thick coating of dust might be significant and might reduce contrast but a few specks are irrelevant.

[Jim Andrada]

I guess if the dust is between the front cell and the back cell, it's pretty simple. Just unscrew them. I wouldn't get involved in taking the cells themselves apart though. As others have said, "Beware, there be shims!" Occasionally, not always. Be careful and remember where they fell out of. Easier said than done once you discover them on the desk, or on the floor, or wherever, but such is life.

[ic-racer]

If you have a lens wrench go for it. After removing the lens group, place it in a bucket of acetone for about a week, this will soften the glue and allow you to wash away the dust speck. Afterwards, put the lens elements in your jig and re-glue. One problem that crops up is that when you check your work with your USAF test chart , say across a large room, you still may not have flat field even if it is re-glued correctly, as you really need to check it at infinity in your collimator.

[Math]

I agree with those who say leave it as-is: Getting the bit of dust out changes nothing, and changes of increasing the dust are large.

Complicated however, they are not, as long as you treat them carefully and have the right tools.

What I really can't agree with however is the idea of separating and re-gluing lens elements, you'll never get that flawlessly concentric! Nor would dust settle between the two elements, anyway. Even a lens with heavy separation will work just fine compared to something you re-glued yourself!