Convertable Lens and Filtergreenspun.com : LUSENET : Large format photography : One Thread
I have a Fujinon 250mm f/6.7 lens that I am trying to use as a convertable by removing the front element. Although the Fujinon was not advertised as a convertable, I can get a good looking image on the ground glass with the rear element only at a focal length of what looks like about 450mm or so. I've read that it is a good idea to use a yellow filter on "converted" lenses to help compensate for chromatic aberrations that are no longer corrected when the front cell is used. I've also heard that it's best to put the filter in front of the lens, but I can't remember the reason why. The problem is that the Seiko shutter's threads (into which the front element screws) is an odd size and thread pitch, and I haven't been able to find a filter to fit. Does anyone know from experience whether there is a real problem with putting a filter on the rear (I'll be doing B&W contact prints, by the way) or have any ideas about how I can get one on the front. Since this is an experiment, I don't want to spend a lot of money.
-- Chris Patti (firstname.lastname@example.org), March 18, 2001
Typically, you don't want to put anything in the optical path (i.e., the image projected by the lens) because any imperfection on the filter will degrade the optical image. Its the same reason it is recommended that you use variable contrast filters above the lens rather than below the lens on an enlarger. Having said that, if your filter is scrupulously clean, its worth chancing. It might be worth trying one of the barn door style holders which can be fit around the shutter with a gel fitted into the filter. Good luck. DJ
-- N Dhananjay (email@example.com), March 18, 2001.
Ron Wisner has this to say:
"Frankly, after many bench tests, I have never been able to find any effects from the use of good quality glass filters, as long as they are used on the outside of the camera. Inside, behind the lens, can introduce some astigmatism. I have found from experience that some plastic filters are not especially good, but gels are, of course, the best."
-- Sean yates (firstname.lastname@example.org), March 18, 2001.
Under the following Homepage:http://www.hevanet.com/cperez/testing.html You can see under the Symmar 150mm convertable thad the yellow filter is bad for the resolution. Just do your test without yellow filter. Good light, and tell us about the result!
-- Armin Seeholzer (email@example.com), March 18, 2001.
I e-mailed Richard Knoppow recently, and he had this to say:
"I think this is one case where breaking the rules is better than following them. In principle, if the rays of light going through a plane-parallel plate are parallel the plate has no effect whatever on them. Light from an infinite source are parallel. Rays which go through the plate at an angle are off-set by an amount proportional to the angle, the thickness of the plate, and its index of refraction.
Light emerging from a lens is convergent. In principle a plane parallel plate in a convergent or divergent beam of light will introduce some spherical aberration (even though there are no spherical surfaces, it is rather the light wavefront which is spherical), and some chromatic aberration since the index of refractionn varies with wavelength.
Now, if we consider the relative amount of convergence from a single element of a convertible lens with long focal length, it will be small compared to the thickness of a good quality filter. So, even though the filter _will_ introduce some aberrations they will be minor and probably negligible especially considering the amount of chromatic already present in these lenses.
Meaning, that the filter probably does less damage on the back of the lens than on the front given the type of lens.
Turner-Reich individual lenses seem to have considerable chromatic error. Probably any narrow filter will improve them. The Orange filter (probably a #15 or G filter) cuts out virtually all blue light and some green also so should sharpen up the T-R cells considerably. If you are using gelatin filters the effect will be so small that it probably would he hard to measure, they are so thin they have practically no effect on the optical path except for extremely short focus lenses.
A note: Ideally, a single cell should be used behind the diaphragm, generally the correction is better this way. However, single meniscus lenses have a slight telephoto or retrofocus effect, depending on which side you are. The principle planes lie outside or nearly outside of the lens, one of them usually about at the surface of the convex side and the other some distance away from it. What that means practically is that if the performance of the lens is acceptable when its on the front of the shutter the bellows draw will be significantly shorter. My Ansco/Agfa camera can not focus the longer element of my T-R lens when its on the back but has just enough capacity to focus it on the front. I see little difference with the shorter FL cell, which I can focus on either side. So, although putting the single cell on the back is good practice it can be used on the front too with little visible effect on performance.
-- Sean yates (firstname.lastname@example.org), March 19, 2001.
So, I have a couple of questions with respect to Mr. Knoppow's response.
What "damage" is done with the filter mounted on the front of the lens? The spherical aberration appears to come from the fact that, with the filter mounted behind the lens, light enters the filter from different angles (the light being convergent), and thereby, the offset is different at the differing angles. So, I see the "damage" done with the filter mounted behind the lens. But, if the light coming from the front is near parallel, where is the "damage"?
One "damage" with the filter mounted in front of the lens could be the potential for additional flare. (At least, according to Schneider LF techs.) I can see this, because there's so much more extraneous light in front of the lens than behind the lens.
Since so much of the distribution of wavelengths is being truncated by the filter, will the chromatic aberration with the filter behind the lens really be that serious? Filters used for B&W are pretty strong.
To what extent will focusing with the filter in place correct for the aberrations that the filter might introduce? (Mr. Knoppow probably already factored this in as part of his reponse.)
How short is "short" (in focal length) with respect to mounting gel filters behind the lens? At what point is the focal length so short that mounting a filter behind the lens will have a detectable impact on the image? Again, to what extent with this be corrected if one focuses the image with the filter in place?
Thanks for including Mr. Knoppow's response.
-- neil poulsen (email@example.com), March 20, 2001.