View Full Version : Adding Filters to the rear of a lens

james zhou
9-Dec-2007, 19:21
This applies to using ultrawide lens such as Super Angulons or Super Symmar XL lenses. Many of them need a large diameter center filter so adding a filter to the front of the lens is problematic. Then there is an option of adding filters to the back of the lens, in fact al the SSXLs have threads for this. But I remember reading about problems of using filters this way (don't remember what problems). However, I do not understand why would there be any problems. Anyone can enlighten me on this?

Mark Sawyer
9-Dec-2007, 20:09
Adding a filter behind the lens shifts the focus ever-so-slightly, a percentage of the thickness of the filter. Not an issue with gels because they are so thin. That's the only issue I know of...

Gordon Moat
9-Dec-2007, 20:13
I had considering doing that with my 180mm Nikkor-W, which has 52mm threading on the rear group. I have plenty of 52mm filters, so I thought it would be worth the hassle. Unfortunately, what I discovered is that the threading is much finer, meaning none of the standard 52mm filter threads will work on it. You might find a similar issue trying to use filters on the rear of your lens.


Gordon Moat Photography (http://www.gordonmoat.com)

9-Dec-2007, 20:22
I've taped filters to the rear of barrel lenses that were either to large in the front for my filters, or the glass bubble protruded beyond the rim of the lens. Taping works just fine, and it's inside the camera, so nobody will be the wiser ;-)

Bob Salomon
9-Dec-2007, 20:30
When you mount a filter behind the lens you will reduce the optical performance of the lens. Any defects in or on the filter will reduce the quality of the recorded image. When you put the filter in front of the lens it affects the light the enters the lens. When you place it behind the lens you are modifying the performance of the lens. A filter should never be used behind the lens unless it is part of the optical property of the lens. For instance filter that are inserted inside lenses like the 30mm Zeiss Distagon for Rollei and Hasselblad or filter for certain long and fast Nikkor and Canon lenses for 35mm or the corrector plates for the Rodenstock Apo Sironar Digital HR lenses when used with film rather then digital.

Lenses have rear threads to hold the rear group of elements in the lens. Not to add filters to. That is why the 52mm threads above are a different thread pitch.

Yes you can put a filter on the back. But try the same shot with a front and a rear mounted multi coated quality filter on a high quality lens. Then make your decision.

Especially if you are after the maximum performance that a lens can deliver on a subject that includes very fine details. Right out to the edges and the corners.

Bob Salomon
9-Dec-2007, 20:31
"shifts the focus ever-so-slightly"

30% isn't all that slightly.

Alan Davenport
9-Dec-2007, 21:35
30% isn't all that slightly.

What Bob said... A filter will shift the focus roughly 1/3 of the filter's thickness. A filter that is 1.5mm thick will shift the focus about .5mm. That isn't significant in the object space (at least in non-macro object space) but becomes very significant in the image space. If you use filters behind the lens, they should be in place when focusing.

John Schneider
9-Dec-2007, 21:43
Not to belabor the subject, but Bob S.'s answer is spot-on. I did my own tests with a 10x loupe for focusing and filters of various thicknesses (gels up to 0.5" thick aerial filters, and talked to a Rodenstock engineer at length; both agree with Bob's answer.

james zhou
9-Dec-2007, 21:59
Mark, wouldn't focusing with the filter on the correct the focus shift problem?

Bob, thanks for the answer. I still don't understand the physics behind the problems. It's probably involve deeps physics. I can do some tests in the future and see... too bad for 210 SSXL, there are almost no filter for it (135mm front thread).

9-Dec-2007, 23:01
Gels behind the lens allow for smaller filters and pretty high quality. I used to have steel blocks adhered to the back of my lensboards and attached the gels with magnets.


Brian K
10-Dec-2007, 05:54
I used the Sinar 4x4" resin behind the lens filter system for years and had no problems, but I always focused with the filters on. Then again in those days I wasn't inspecting my film with a microscope.....

Mark Sampson
10-Dec-2007, 07:56
Hmm. Some years back I bought a "Xenophon" gel filter holder for my 121/8 Schneider. It's a frame that attaches to the back of the lensboard and has an opening just big enough to put a 3" gel up against the (large) rear element of the lens. It's a bit more complicated to use, and of course it's necessary to focus with the filter in place, but I've never noticed any sharpness degradation when using a gel.

Jim Noel
10-Dec-2007, 11:23
I have two Xenophons which I used for years with gels. As my supply of lenses increased I got away from them. Currently I attach gels to the rear with small pieces of tape. With the older lenses (100 years or so), I keep a #8 taped to the rear at all times. This is just enough to eliminate the softness caused by the blue rays focusing at a different distance than others wave lengths. Makes an amazing difference in sharpness.

Although I have good quality glass filters for most of my more modern lenses, I often use gels on the rear and see no difference in quality of the final image.

Helen Bach
10-Dec-2007, 17:45
Scotch ATG tape, commonly known as 'snot tape', can be used to hold gel and polyester filters behind a lens. (as mentioned by Jim) I've done it more often with cine lenses, and then mainly with very wide angle lenses.

The shift caused by a behind-the-lens filter depends on the angle of incidence of the ray - it is only 33% of the filter thickness for rays near to the axis (and for a refractive index of 1.5). The general formula is

L = t . (1 - cos i / (n . cos r))

where L is the displacement of the ray along the axis for the ray with angle of incidence i. The filter's refractive index is n.

But that's for nit-picky information only. In practice it probably won't matter with LF at typical LF apertures.


Mark Sawyer
10-Dec-2007, 18:08
Mark, wouldn't focusing with the filter on the correct the focus shift problem?

Yes it would, just like re-focussing after closing down the aperture on a lens that suffers from focus shift. Gels would be the "least offensive" of the rear filter options, but they're hard to keep in good condition in field use, at least for me... Still, if you adjust for that focus shift, in contact printing or relatively small enlargements, I doubt any decent filter on the rear or the lens would degrade the print's appearance visibly.

But then, I'm one of those who consider a .5mm adjustment "slight" (but mentionable).

james zhou
10-Dec-2007, 19:32
thanks everyone, i learned quite a bit.