Behind the lens filter placementgreenspun.com : LUSENET : Large format photography : One Thread
I am unable to fit some of my lenses with filters in the normal position in front of the lens. Are there any problems to look out for if I can mount them behind the lens? TIA.
-- Bill (firstname.lastname@example.org), May 24, 2002
For imaging distant objects, the biggest disadvantage is that you should focus with the filter in place. This could be quite difficult with a dark filter. (For macro photography, one should focus with the filter in place regardless of which side of the lens has the filter.) The reason is that the filter shifts the distance by about 1/3 of its thickness--in front of the lens, with a distant object, this is completely neglible, while behind the lens, it is significant.
Secondly, when a thick filter is used in a converging beam of light, spherical abberation is introduced. The light behind the lens is converging; the light in front of the lens, arriving from distant objects, is not. I don't know how significant the spherical abberation is, especially stoped down.
If you must use the filter behind the lens, it might be better to use a thin one, such as gelatin or polyester.
-- Michael Briggs (MichaelBriggs@EarthLink.net), May 25, 2002.
There are threads in the archive that address behind the lens filter placement. But, here are the basic issues:
>> Behind the lens filtration alters the image by a small margin. It introduces some small amount of abberation. It comes from the fact that off-axis rays travel through more filter material than on axis rays. So, the change in focus depends upon the angle of the ray from the axis.
>> Behind the lens filtration also changes the focus by about a third of the thickness of the filter being used.
>> In front of lens filtration can introduce additional flare, because extraneous light can be reflected off the lens, then reflected off the filter and back into the camera. This can result in a hot spot in the middle of the image. Behind the lens filtration doesn't exhibit this effect nearly to the same degree. I would think that this effect could be mitigated by using a compendium bellows to minimize the amount of extraneous light from the front. One could also use coated filters to minimize this effect.
>> In front of lens filtration does not change the focus of the image on the ground glass. Nor does it introduce abberation, unless one has a very poor filter.
Since I intensely dislike flare, and since I have a neat Xenophon behind the lens filter holder that meets all my filter holder needs for $50, I prefer behind the lens filtration. One can adjust for the change in focus by focusing the camera with the filter in place. I use gelatin filters, which are very thin, so the effect is minimal. As for the abberation, with very thin gelatin filters, the effect is applicable only for small focal lengths.
Given all the different lens sizes, it's nuts trying to figure a way to mount one's filters on all those different sized lenses.
I've read that, if one wishes to use two filters, the optimum approach is to put one behind and one in front, and this eliminates light being reflected between the two filters.
-- neil poulsen (email@example.com), May 25, 2002.
Beware of behind the lens filters when using super wides. I have tried it with my 47mm (use a roll film back) and any dust on the filter shows on film. I have had to go back to front mount with this lens. With the 65mm it is OK if the filter is very clean. I also use the Xephonon 3x3 and 4x4s.
-- Richard Stum (firstname.lastname@example.org), May 25, 2002.
I remember observing Paul Caponigro using gel filters behind the lens of his 5x7 Deardorff: He simply placed in such a way that they were caught, jammed, between pleats of the bellows.
Works fine as long as long as you are gentle when inserting filmholder, pulling darkslide etc.
-- Hans Berkhout (email@example.com), May 26, 2002.
For a couple of ultra-wide lenses (120mm, 168mm) that I use for 8x10" with odd filter sizes, potential vignetting problems, and which don't protrude far beyond the lensboard in back, I've epoxied 3x3" resin filter holders (Cokin style, made by Ambico--I have a whole box of them) to the back of the lensboard. This works very well and solves vignetting problems I was getting from attaching even oversized filters to the front of the lens, as the step-up ring would vignette. The filter holders also have black 3x3" lenscaps that slide in to protect the lens when not in use.
-- David Goldfarb (firstname.lastname@example.org), May 26, 2002.
Most (if not all) of the Sinar system shutters have a filter holder intended for 100X100mm gels. I have often used filters behind the lens and so have many other studio photographers that I have visited.
The comments about being careful with focusing etc are all valid, so focus with the filter in place.
-- Björn Nilsson (email@example.com), May 26, 2002.
As an alternative to placing your filters behind your lens consider having Steve Grimes make a filter holder that will enable you to place all of your filters in front of your lens.
Steve made one for me for use with Lee 100mm filters. With Steve's "Utiltity Filter Holder" you can readily use a polarizer with up to three contrast filters, all of which sit in front of any lens you chose.
(I have no interest in Steve's business, I just appreicate the superlative quality and genius of Steve's work.)
-- Robert J. Triffin (RJTRIFFIN@RCN.COM), May 26, 2002.