Convertable Lens and Filter

[chris_patti1]

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.

[n_dhananjay3]

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

[sean_yates]

Ron Wisner has this to say:

 

<p>

 

"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."

[armin_seeholzer]

Hi Chris

 

<p>

 

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!

[sean_yates]

I e-mailed Richard Knoppow recently, and he had this to say:

 

<p>

 

"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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

Meaning, that the filter probably does less damage on the

back of the lens than on the front given the type of lens.

 

<p>

 

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.

 

<p>

 

 

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.

[neil_poulsen1]

So, I have a couple of questions with respect to Mr. Knoppow's

response.

 

<p>

 

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"?

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.)

 

<p>

 

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?

 

<p>

 

Thanks for including Mr. Knoppow's response.