Centre filters for UWA lens - rear element instead of front

[minh0204]

This might sound like a ridiculous idea, but I'm wondering why can't centre filters be made for the rear elements? Lenses like the Super Symmar XL 110mm has a 52mm thread on the rear vs 67mm on the front, thus a centre filter on the rear will be smaller and less expensive to make?

[Bob Salomon]

Except the fall off already occurred when the light rays pass through the rear element. Also, placing a filter behind the lens creates a focus shift = 1/3rd the thickness of the filter material.

[CatSplat]

Except the fall off already occurred when the light rays pass through the rear element.

A center ND doesn't stop falloff from occurring, it just darkens the middle to even out the exposure. It should have the same effect regardless of which end of the lens it's installed on.

[lenicolas]

Main problem I can think of is darkened image to compose and focus. But if you’re using a zone focusing camera then why not.

[Alan Klein]

Main problem I can think of is darkened image to compose and focus. But if you’re using a zone focusing camera then why not.

What's a zone focusing camera?

[Drew Wiley]

For one thing, they're bigger than the lens diameter, and might not fit inside some bellows. They need to be made oversized to prevent vignetting. That's a hard place to install and remove it without risking resetting focus, especially since ultrawides tend to have both significant falloff and finicky focus, being so close to the film plane - yet another issue - it might make the lens too close to the film. Then you've got the cost factor - how many people would want one?
You could have one custom made, no doubt, for an absurd sum. No need to go into the optical engineering issues relative to the intended task; maybe somebody else will pounce on that. Rear filters in general work poorly unless the lens itself is specifically designed for them in advance.

[lenicolas]

What's a zone focusing camera?

A camera that has no ground glass, prism, rangefinder or any other focusing system besides a distance scale on the lens.
You estimate (or mesure with tape or laser) the subject distance and set the focusing accordingly.
209062
Panoramic 617 and 612 cameras are a good example of zone focusing cameras that use LF lenses on helicoid mounts.
With mine I used to leave the center filter on all the time since you never look through the lens.

[Drew Wiley]

Just like rangefinders; you aren't looking through the lens when focussing. I made the mistake of forgetting to even remove the lenscap on my "Texas Leica" 6x9 in one instance a few days ago. Certain Technical cameras, like the Technika series, offered optional rangefinder focus as well as primary groundglass focus.

[Alan Klein]

Oh, so my Nikonos underwater camera was one of those too. You had to guess focus.

[Oren Grad]

That Technorama is a scale-focusing camera. Zone focusing is the technique of using depth-of-field calculations, tables, or markings on the lens to set the distance so as to achieve good-enough focus across a particular distance range of interest. Zone focusing is a technique commonly used with scale-focusing cameras, but of course it can be used with any camera/lens combination that has a means of manually setting a particular focus distance.

[lenicolas]

Thank you for the correction. English isn’t my first language. ^.^

[Rod Klukas]

One other item. The lens is designed to direct the light to the image plane/film/digital back in a certain trajectory. Years ago some photographers would use gel filters on the back of the lens. These were so thin that a single filter would cause negligible image issues. But a glass filter perhaps 2-3 mm thick, and with flat, supposedly top and bottom surfaces will bend the light differently than the lens designer intended.
And as Bob solomon correctly noted the image is focused 1/3 the thickness of the filter differently. That is why you never focus and then add a filter to the front of a lens for a sharp image, by the way.
So although some of these lenses do have threads, I would be checking before I just slapped a Center filter on it. Also the Angle with which the lens projects the image to the film plane is different than the angle of acceptance of the front of a lens. So to be truly maintaining your image quality you would really need a center filter tuned to that particular lens. Expensive and so small customer count, as someone noted above.
I can say that lenses which have matched center filters should have them if at all possible. I have tested many of the combinations and the increase in quality is quite noticeable.
Hope this is informative.
Rod

[Alan Klein]

One other item. The lens is designed to direct the light to the image plane/film/digital back in a certain trajectory. Years ago some photographers would use gel filters on the back of the lens. These were so thin that a single filter would cause negligible image issues. But a glass filter perhaps 2-3 mm thick, and with flat, supposedly top and bottom surfaces will bend the light differently than the lens designer intended.
And as Bob solomon correctly noted the image is focused 1/3 the thickness of the filter differently. That is why you never focus and then add a filter to the front of a lens for a sharp image, by the way.
So although some of these lenses do have threads, I would be checking before I just slapped a Center filter on it. Also the Angle with which the lens projects the image to the film plane is different than the angle of acceptance of the front of a lens. So to be truly maintaining your image quality you would really need a center filter tuned to that particular lens. Expensive and so small customer count, as someone noted above.
I can say that lenses which have matched center filters should have them if at all possible. I have tested many of the combinations and the increase in quality is quite noticeable.
Hope this is informative.
Rod
I've never considered that. I'm new to 4x5 view cameras. I've added polarizers and contrast filters to the front but after focusing. So focus changes when I do this? Do center filters have to be added before final focus as well?

How do you see through the lens with filters. Doesn't it make too dark?

Finally, I'm using a Schneider center filter on a 90mm Nikkor lens that is suggested for this lens. Nikkor has no center filters for their lenses of their own. So what does that mean? How does that effect my procedures?
Thanks.

[minh0204]

I've never considered that. I'm new to 4x5 view cameras. I've added polarizers and contrast filters to the front but after focusing. So focus changes when I do this? Do center filters have to be added before final focus as well?

I think Rod made a typo, he meant one should not add a filter to the rear of the lens after focusing.

This is the same issue as putting a Fresnel lens between the ground glass and the lens, it will shift the focus.

[Drew Wiley]

Alan - Don't worry about that. I already did plenty of testing with the 90/4.5 + CF + contrast filters too. Just go ahead and focus first, then add the filters. But REAR mounting filters on a lens is a no no unless otherwise specified. There are some modern lenses which are less than ideally corrected, especially at the widest apertures, and can exhibit a bit of focus shift between different colors of very deep contrast filters, even high quality glass ones mounted on the front. So when in doubt, carefully test. It's more likely to be a problem with MF lenses.

[grat]

If you want a truly ridiculous question, why can't you take the equivalent of a "flat" image using lens "X" (ie, evenly lit diffuse exposure), which should show the falloff fairly well, load that image into software, invert it, and print it on a transparency?

You should now have a lens-specific center-spot ND "filter", and assuming it's the right size you should be able to put it in front of the lens.

I try to have at least one crazy thought a week, and figured I should get it out of the way early this week. :rolleyes:

[Drew Wiley]

Grat - can be done. But you wouldn't have a truly hue-neutral gray if color shooting were involved. Reflections could also be an issue. And then there's the issue of light-scattering in those silver grains different than how official ND filters are made to specifically avoid that. But try it just for fun.

[Bob Salomon]

If you want a truly ridiculous question, why can't you take the equivalent of a "flat" image using lens "X" (ie, evenly lit diffuse exposure), which should show the falloff fairly well, load that image into software, invert it, and print it on a transparency?

You should now have a lens-specific center-spot ND "filter", and assuming it's the right size you should be able to put it in front of the lens.

I try to have at least one crazy thought a week, and figured I should get it out of the way early this week. :rolleyes:

Fine, if you can put up with the less then neutral color and use it on a camera with no movements.

[Dan Fromm]

Finally, I'm using a Schneider center filter on a 90mm Nikkor lens that is suggested for this lens. Nikkor has no center filters for their lenses of their own. So what does that mean? How does that effect my procedures?
Thanks.

I wrote an article that addressed this. There's a link to it in the list. Read it. If you don't remember what the list is or where to find it, please ask.

[Alan Klein]

Alan - Don't worry about that. I already did plenty of testing with the 90/4.5 + CF + contrast filters too. Just go ahead and focus first, then add the filters. But REAR mounting filters on a lens is a no no unless otherwise specified. There are some modern lenses which are less than ideally corrected, especially at the widest apertures, and can exhibit a bit of focus shift between different colors of very deep contrast filters, even high quality glass ones mounted on the front. So when in doubt, carefully test. It's more likely to be a problem with MF lenses.

I shoot MF RB67 also. So, are you referring to filters on the front or the rear?

[Drew Wiley]

Only filters on the front, Alan. That's the only place they belong. There are a handful of really huge MF telephoto lenses which are so big (nearly 10 inches front diameter) that filters simply won't work on the front, so are specially engineered for rear ones. That's the case with certain very long focal length Pentax 6x7 lenses which almost resemble telescopes. I don't know if similar RB lenses exist or not; but I'm quite sure you wouldn't own anything that huge, heavy, or expensive anyway.

[Bob Salomon]

Only filters on the front, Alan. That's the only place they belong. There are a handful of really huge MF telephoto lenses which are so big (nearly 10 inches front diameter) that filters simply won't work on the front, so are specially engineered for rear ones. That's the case with certain very long focal length Pentax 6x7 lenses which almost resemble telescopes. I don't know if similar RB lenses exist or not; but I'm quite sure you wouldn't own anything that huge, heavy, or expensive anyway.

Also with some very short lenses like the 30mm Zeiss Distagon for Rollei and Hasselblads.

[Dan Fromm]

Drew, you may be thinking of the Reflex Nikkors (500 mm, 1000 mm and 2000 mm), which take small filters in the rear. They must be used with a filter in place. The filter set includes clear, red, yellow and several NDs of various densities. These are all for 35 mm F-mount SLRs.

[Bob Salomon]

Drew, you may be thinking of the Reflex Nikkors (500 mm, 1000 mm and 2000 mm), which take small filters in the rear. They must be used with a filter in place. The filter set includes clear, red, yellow and several NDs of various densities. These are all for 35 mm F-mount SLRs.

So did a 300mm Nikon which was not a reflex design. Heliopan even made a polarizer in a drawer for it.

[Drew Wiley]

No Dan, I was not thinking of Reflex Nikkor. Everything Nikon is tiny compared to P67 equivalents, which even at the rear of their long teles take rather big filters, at least in the case of the Pentax Takumar series. The newer EDIF series is a little different, but still huge compared to 35mm lenses. I have a Nikon F adapter for my 300EDIF; that's fun wildlife combination, though I rarely use it because I seldom print 35mm shots. Gratefully, their 300's accept standard 82mm filters on the front of the lens. It when you get to 600mm and beyond that things get crazy big.

[Alan Klein]

Alan - Don't worry about that. I already did plenty of testing with the 90/4.5 + CF + contrast filters too. Just go ahead and focus first, then add the filters. But REAR mounting filters on a lens is a no no unless otherwise specified. There are some modern lenses which are less than ideally corrected, especially at the widest apertures, and can exhibit a bit of focus shift between different colors of very deep contrast filters, even high quality glass ones mounted on the front. So when in doubt, carefully test. It's more likely to be a problem with MF lenses.

I wasn't clear with my question. What did you mean with MF lenses. Do you have to focus MF lenses after the filters are put on the front? Or is it acceptable to focus first and then add the filters on the front?

[Bernice Loui]

Sinar shutter has a rear filter holder for 103mm Sinar glass or square gel filters.
Zodiak 30mm f3.5 fish eye for Pentacon six has filters in the rear similar to the Zeiss fish eye for Hasslblad (image quality wise, both are surprisingly similar).

Canon 35mm fish eye, built in filter wheel at the rear of the lens.
Canon and many other telephoto lenses over 300mm and similar zoom lenses.
Canon FD 200mm macro has a filter holder at the rear of the lens.

Schneider 110mm & 150mm SSXL has a threaded rear element for filters..

Kinoptik 9.8mm T2.3, Tegea has a rear gel filter holder slot.

Rear of the lens filter is quite common.


Bernice

[Bob Salomon]

I wasn't clear with my question. What did you mean with MF lenses. Do you have to focus MF lenses after the filters are put on the front? Or is it acceptable to focus first and then add the filters on the front?
No, you focus with the filter in place as it is part of the optical system of the lens, if a filter came with the lens from the manufacturer. Otherwise you would be out of focus by the amount the filter shifts the focus plane.
A better example might be that glass plate on Hasselblad Polaroid backs. Since the film plane of a Polaroid back is different then the roll film back Hasselblad used the glass plate to shift the focus so it would be acceptably sharp for test shots with the lenses it was recommended for.

[Bob Salomon]

I wasn't clear with my question. What did you mean with MF lenses. Do you have to focus MF lenses after the filters are put on the front? Or is it acceptable to focus first and then add the filters on the front?

You should focus with the filter in place. Especially as some cheaper filters can play havoc with focus.

[Alan Klein]

Holy cow. I've never ever considered that. Half my pictures must have been focused before putting the filters on. I use good filters like B+W or Nikon. Is this something I can check live to see if I see a difference? After all, if it's still in focus after I put the filter on requiring no change, then it shouldn't matter for future use. After all, the whole point of spending extra money for a brighter lens that's heavier is to make it easier to see, compose and focus. If I have to add a 1 1/2 stop polarizer on beforehand, I've defeated the whole point and spent money for nothing.

[Corran]

Alan, you are spinning yourself dizzy asking so many questions and not properly reading the responses and/or getting too many responses from too many people who are talking about wildly different things.

You do not have to focus with a filter in place if used on the FRONT of the lens.

For any filter BEHIND the lens you must focus with it in place.

I seriously recommend less questions and more shooting. You seem to ask the same questions over and over and over and are just confusing yourself. And, most any of these question can be answered with a simple test with your own equipment. Have you noticed your images with filters on the front of the lens noticeably out of focus? There's your answer.

[Drew Wiley]

Alan - don't worry about it. You are using good modern lenses and high-quality filters apparently for both your medium format and large format applications. Just focus as needed and then put the filter on the front of the lens afterwards if it's too dim focussing with one in place. Don't put them on the rear element.

[Alan Klein]

Must be age. :(