Filters and convertible lenses?

[Steve Goldstein]

I have a couple of convertible lenses, one in Copal 0 and one in Copal 1, both of which are converted to the longer focal length by removing the front cell from the shutter. Does anyone make standard adapters that would allow me to use filters with these? The Copal 0 front cell thread is 29.5x0.5mm, for Copal 1 it's 40x0.75mm, and I've been unable to find filters or stepup rings that will screw in to either shutter.

I'm also interested in something to fit an Acme #3 Synchro (not the Kodak version) to use with a 215mm f/4.8 Acuton.

Yes, I know I could get custom step-ups made at SKG, and may ultimately do that, but was hoping to find something off the shelf. Given the past popularity of convertible lenses, I'm a little surprised nothing seems to exist.

[lenser]

Assuming the rear cells fit the front shutter threads (which mine do), mount he rear cells in front. Cokin makes a very nice adapter (two sizes) that fits any size front cell set up by tightening three nylon tipped screws. Then the normal Cokin filter holder (either of their sizes) fits onto the adapter and you use the filters (theirs of course) of choice for your shot. Lee and other systems may have the same, but I have not researched those.

[Steve Goldstein]

Copal 0 rear cells will screw into the front. Copal 1 will not; it figures, as that's the one I'm more interested in. I'm not sure about Acme Synchro #3 shutters.

[Brian Ellis]

Just hold a filter by hand in front of the shutter. You don't need to attach it to the shutter or the lens.

[Bob Salomon]

Do it the way Rodenstock recommended for the Sironar and leave the front element in and remove the rear element. That way the shutter blades are also protected rather then exposed. But since performance of a converted lens drops drastically better use a really good filter to minimize the degradation, unless that is what you are lookig for.

[Steve Goldstein]

Brian, hand-holding a filter will work in a pinch. I'll keep that in mind.

Bob, I like Rodenstock's solution, though it does involve more handling (removing the lensboard). Ilex literature is specific about using the rear cell for the convertible Acutons, and I believe the convertible Symmars were intended to be used the same way. Using the front cell alone might give a different focal length from the one specified, but it does solve the filter problem. I'll try both ways and see if there's a dramatic difference with the lenses I have.

[John Koehrer]

Why not Series size adapters? That's more likely what was used back in the day.
As I recall there's someone on the bay that has a slug of them available.
Of course then you would need Series size filters.

[Helen Bach]

What about using thin-ish filters behind the lens? Either stuck on with tape, screwed in or mounted in a frame.

Best,
Helen

[Brian Ellis]

Brian, hand-holding a filter will work in a pinch. I'll keep that in mind.

Bob, I like Rodenstock's solution, though it does involve more handling (removing the lensboard). Ilex literature is specific about using the rear cell for the convertible Acutons, and I believe the convertible Symmars were intended to be used the same way. Using the front cell alone might give a different focal length from the one specified, but it does solve the filter problem. I'll try both ways and see if there's a dramatic difference with the lenses I have.

Not to beat a dead horse but there's no reason to confine hand-holding to a pinch. It's the only way I use filters any more and it works fine, plus it saves massive aggravation in worrying about different size filter threads, buying adapters and finding them when you need them, etc. Of course it's a personal preference thing so if you want to use another system that's fine too, there's certainly no right or wrong.

[Bob Salomon]

What about using thin-ish filters behind the lens? Either stuck on with tape, screwed in or mounted in a frame.

Best,
Helen

Thick or thin, they both effect the lens performance, filters belong in front unless the lens was designed for filters in or behind the lens.

[Helen Bach]

Thick or thin, they both effect the lens performance, filters belong in front unless the lens was designed for filters in or behind the lens.

Whether the effect is significant in practice is debatable, and there are some optical advantages along with the disadvantage. Here is one such debate we both participated in: Adding Filters to the rear of a lens (http://www.largeformatphotography.info/forum/showthread.php?t=31300) and another: Rear Filter Mount for 55mm APO Grandagon (http://www.largeformatphotography.info/forum/showthread.php?p=267500).

Best,
Helen

[Bob Salomon]

Whether the effect is significant in practice is debatable, and there are some optical advantages along with the disadvantage. Here is one such debate we both participated in: Adding Filters to the rear of a lens (http://www.largeformatphotography.info/forum/showthread.php?t=31300) and another: Rear Filter Mount for 55mm APO Grandagon (http://www.largeformatphotography.info/forum/showthread.php?p=267500).

Best,
Helen

Helen,

A lens does what it is supposed to do when the rays pass through it and go to the image plane. Placing a filter behind it will negatively effect what the lens was designed to do. At the least it changes the point at which the rays come to focus at the image plane by 1/3rd the thickness of the filter. Then any lack of parralelism between the front and the rear surfaces will distort the image. Any uncoated glass to air surfaces will introduce additional flare, dust, smudges, fingerprints, scratches will effect the final image.

There simply isn't any reason to put a filter behind a lens unless the final image quality is not the ultimate desired result from a lens. Or unless the lens was designed for a filter to be in the lens or behind the lens.

[j.e.simmons]

If you do put the filter behind the lens, focus with the filter in place. I use gel filters in a holder adapted from a Cokin holder.
juan

[Helen Bach]

Helen,

A lens does what it is supposed to do when the rays pass through it and go to the image plane. Placing a filter behind it will negatively effect what the lens was designed to do. At the least it changes the point at which the rays come to focus at the image plane by 1/3rd the thickness of the filter...

Bob,

I know all that. I guess that you didn't refresh your memory of what has already been discussed. I had hoped that by giving the links we could avoid rehashing old stuff.

The statement that the focus changes by 1/3 of the thickness of the filter isn't strictly true - 1/3 is the minimum displacement, and it is greater than that away from the axis. (I mentioned this in more detail in the linked threads) The difference may not be significant, if you focus with the filter in place.

A friend and colleague uses a Deardorff with convertible Symmar or plain Angulon (not Super) lenses and an 81B Cokin resin filter taped to the rear cell. In the same studio I use a P2 with Apo-Sironar-S and Super-Symmar XL lenses. I never use any filter, unless I need a centre filter. It is very rare for our results to have any significant technical difference. It's good to have an understanding of how to approach technical perfection (and it can be an interesting subject in itself), but it is also good to have some practical perspective on what you can get away with.

Best,
Helen

[Bob Salomon]

Bob,

I know all that. I guess that you didn't refresh your memory of what has already been discussed. I had hoped that by giving the links we could avoid rehashing old stuff.

The statement that the focus changes by 1/3 of the thickness of the filter isn't strictly true - 1/3 is the minimum displacement, and it is greater than that away from the axis. (I mentioned this in more detail in the linked threads) The difference may not be significant, if you focus with the filter in place.

A friend and colleague uses a Deardorff with convertible Symmar or plain Angulon (not Super) lenses and an 81B Cokin resin filter taped to the rear cell. In the same studio I use a P2 with Apo-Sironar-S and Super-Symmar XL lenses. I never use any filter, unless I need a centre filter. It is very rare for our results to have any significant technical difference. It's good to have an understanding of how to approach technical perfection (and it can be an interesting subject in itself), but it is also good to have some practical perspective on what you can get away with.

Best,
Helen

To "get away" it would be best to set a scene with lots of fine detail all over up to and incjuding the edges and the cporners as well as leadin to the center. And then shoot it first without the filter in back, then with the filter in front and then without a filter. Do this twice. Once with a low end sandwich filter, once with a high end round and polished filter and maybe once more with the cheapes filter you can find. Then compare them. Then do it again with a polarizer.