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[MikeS]

Hi All.

I've seen lots of mention of using filters with convertable lenses (when they're converted) and everyone always says to mount the filter on the back of the lens, then mentions that because of the filter, that focusing will change when stopped down. My question is: Why doesn't anyone use filters in front of the lens? I have a push on series VI filter holder, and with a little tape around the push on part it fits in the opening where the front element used to be just fine. It would seem to me that this would kill 2 birds with one stone, it would put the filters in front so no focus shift would happen, AND it would protect the shutter from the elements. Is there any reason why this shouldn't be done? If not, I was thinking of having the folks at SK Grimes make up a special adapter that would screw into the shutter, and go either to series VI or maybe even 49mm (a size I have filters in) Thanks!

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-Mike

[Les]

First off let me state what I've learned/been told about convertable lenses in the 20 years I've been an LF photographer.

The lens goes in the back to reduce aberations and astigmatism. but putting the lens in the back will lenghten the bellows draw, won't change the focal lenght, it just needs more bellows because the nodal point is now in front of the standard.

Dagor and minicus lenses have a focus shift. It doesn't matter where you put that filter, you need to focus stopped down. The old Symmar convertables were a reversed Dagor and will shift focus. Protars don't have a focus shift problem.

When shooting black and white, it's better to use a strong yellow filter to reduce color fringing. It goes on in front.

Now when using my prime lenses such as a Commerical Ektar or Nikkor SW, WF Ektar, etc the filter goes in back for several reasons:
A. For most normal and longer lenses, you can use a smaller filter, saves money. 4" wrattens cost a bundle.
B. With the filter in the back you reduce the chances of reflections and flare. Wratten filters are flexible, and if not put on dead flat, can pick up a ghost.
C. By using a 3" techincal gell holder, some small pieces of wood and velcro I have a universal filter system...no need to buy 4 sizes of B&W filters at $60-$100 a pop.

With that in mind, my answer to your question is "go ahead and do exactly what you want to do, but focus stopped down."

[glennfromwy]

Rather than "converting" the lens, I would just buy a prime lens of the needed focal length to begin with.
Convertible lenses such as the Symmar are notoriously bad when using only half the cells. A search for lens resolution tests may give you an idea of just how much. I have a 210mm f:5.6 uncoated Symmar that I like very much, but converted, it just plain stinks.

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Glenn

"Wyoming - Where everybody is somebody else's weirdo"