Filter - part of optical path? A requisite?

jjstafford · Jun 30, 2005

I note that in some lenses, a filter, even if it a clear one, is required because it is part of the optical path. For example, some very fast long lenses for 35mm, and some wide lenses where the filter is inside the lens.

My question concerns Metrogons and other aerial or mapping lenses that when used in their original mission were (to my knowledge) always used with a filter, usually yellow, sometimes red, and supplied with a clear filter as well. Does their formula require a filter?

BradS · Jun 30, 2005

Seems like any filter, once placed in front or behind a lens, is part of the optical path. I think a yellow, yellow-orange or orange filter is typically used in aerial recon photography to cut through the atmospheric haze.

jjstafford · Jun 30, 2005

Thanks, Brad. I think I need to explain it better. On some lenses the filter is required, even if it is clear because it is part of the optical formula... something like that. So, is a front-filter part of the optical formula on any lens, and in particular Metrogons or the late military Biogon 3" (of which there are two very differently scaled versions.)

David A. Goldfarb · Jun 30, 2005

The Metrogon filters also functioned as center filters, I believe, so they would be used both to cut through haze and to even out the exposure from the center to the corners. In that sense, they were designed to be used with the filter, but I don't think there is a focus shift issue, like there would be with lenses that use internal or rear-mounted filters.

jjstafford · Jun 30, 2005

David, I forgot about the graduated filters! Yes indeed, I went through The Big Box and found two graduated color filters - one yellow (true grad) and one red (semi-transparent silvered 'star' in the middle). Found two heated windows, too.

My worries are over. I can test for focus shift. I was concerned about the rest of the things I am ignorant of.

Back to the work bench - with more joy. I appreciate the help!

--
jj

Dan Fromm · Jun 30, 2005

John, I don't believe that Metrogons require a filter or piece of flat glass.

Of interest, perhaps a month ago I had the privilege, not joy, it was heavy, of holding a 180 Topogon in my lap. The outer-most element on each side was a piece of flat glass. Its then owner, who has friends at Zeiss, told me that according to them the flat glasses were part of the design and should not be removed.

Against that, I have a couple of Taylor Hobson lenses and an Elcan that were extracted from aerial cameras. All have pins for filter holders to attach to, and the two Taylor Hobsons arrived with yellow filters. All three shoot very well with no filter.

These lenses came in mounts that allow the lens to be collimated to its camera. Presumably any of them would have to be recollimated when setup changes, i.e., filter added or removed. And one of them shows signs of having been recollimated several times.

jjstafford · Jun 30, 2005

Dan Fromm said:
John, I don't believe that Metrogons require a filter or piece of flat glass.

Of interest, perhaps a month ago I had the privilege, not joy, it was heavy, of holding a 180 Topogon in my lap. The outer-most element on each side was a piece of flat glass. Its then owner, who has friends at Zeiss, told me that according to them the flat glasses were part of the design and should not be removed.

Against that, I have a couple of Taylor Hobson lenses and an Elcan that were extracted from aerial cameras. All have pins for filter holders to attach to, and the two Taylor Hobsons arrived with yellow filters. All three shoot very well with no filter.

These lenses came in mounts that allow the lens to be collimated to its camera. Presumably any of them would have to be recollimated when setup changes, i.e., filter added or removed. And one of them shows signs of having been recollimated several times.

Thanks, Dan. This is becoming confusing. FWIW, the lenses I have also have pins on the front which accomodate the filter holder, which sometimes includes heating elements to obviate moisture build-up.

I do not know what a collimating mount would be, unless it is the same pins.

To know that you found not significant problem by not using a filter is helpful. Me thinks we are on target... until another surprise arises.

Again, many thanks
--
jj

Filter - part of optical path? A requisite?

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