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Purpose of rear glass element in CAT lens?
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PostPosted: Mon May 26, 2014 5:12 pm    Post subject: Purpose of rear glass element in CAT lens? Reply with quote
Hello.

The average mirror lens for camera has design like this:

http://en.wikipedia.org/wiki/File:MinoltaRFRokkorX250f56text.svg

Checked on 500/8 Vivitar and 1000/10 MTO-11.

I've tried lens booster with MTO-11 and it steps down to very repsectable 600/6.3 (according to my calculations).

However, for me remains unclear, what is the purpose of rear glass assembly in these lens?

I want to speedboost this lens even more, to something like 400/4. So might removal of rear glass may help in this, or I'm going into wrong direction?


PostPosted: Mon May 26, 2014 5:27 pm    Post subject: Re: Purpose of rear glass element in CAT lens? Reply with quote
CuriousOne wrote:

However, for me remains unclear, what is the purpose of rear glass assembly in these lens?

It is a filter: http://www.cssplay.co.uk/lens.html


PostPosted: Mon May 26, 2014 5:30 pm    Post subject: Reply with quote
I have seen people moving lens groups to increase the focal length/coverage or increasing the back focal length of a mirror lens.


PostPosted: Mon May 26, 2014 5:58 pm    Post subject: Reply with quote
No I don't meant filter (which is flat), I meant optical element just before filter.


PostPosted: Mon May 26, 2014 6:13 pm    Post subject: Reply with quote
CuriousOne wrote:
No I don't meant filter (which is flat), I meant optical element just before filter.

Those optical elements correct the aberrations of the mirrors. If you remove them, you will ruin the lens.


PostPosted: Mon May 26, 2014 6:29 pm    Post subject: Reply with quote
On the MTO 1000mm/10 I read somewhere the last lens elements are a field flattener.
Here a short DIY way how it works without this lenses at ~700mm:
http://www.astroklaus.de/html/mtomod.html


PostPosted: Mon May 26, 2014 7:36 pm    Post subject: Reply with quote
Thanks, this is quite interesting, will give it a try!

But I have to note that mine is MC MTO-11, it has different construction - the rear lens assembly are built into one block, which can be removed completely.


PostPosted: Mon May 26, 2014 9:24 pm    Post subject: Reply with quote
ZoneV wrote:
On the MTO 1000mm/10 I read somewhere the last lens elements are a field flattener.
Here a short DIY way how it works without this lenses at ~700mm:
http://www.astroklaus.de/html/mtomod.html


A field flattener must be put at, or very near the focal plane. This is not the case with MTO 1000 or the Minolta 250mm F5.6, for which the rear elements are far from the focal plane. For these lenses, certainly the rear elements affect not only the flatness of field, but also other aberrations such as spherical, coma, astigmatism, etc.
As I said before, the removal of the rear element will very likely ruin the performance of the lens. On one occasion I tried to remove the rear element of a lens CZ Sonnar 180mm F2.8. The result was horrible.

A photographic lens is a highly complex optical system, whose performance depends on the delicate balance of aberrations of the individual elements. Even a small change in the relative position of an element is sufficient to destroy the balance of the aberrations. The removal of an element should be viewed as a sacrilege, but nevertheless, some brave people do all kind of "experiment", such as using only one half of a lens double-Gauss.


PostPosted: Mon May 26, 2014 9:38 pm    Post subject: Reply with quote
Gerald wrote:
The removal of an element should be viewed as a sacrilege, but nevertheless, some brave people do all kind of "experiment", such as using only one half of a lens double-Gauss.


Y'know, Boyer advertised and sold f/2.8 6/4 double Gauss type Saphirs as convertible lenses. Convertibility requires that each cell be corrected well enough. What well enough means is an interesting question.

More generally, symmetrical and nearly-symmetrical (not Tessars, whose cells are quite different) lenses have been sold as convertibles, usually for formats 2.25" x 3.25" and larger. There are few if any convertible lenses among the small format lenses popular among posters here.

A few mirror lenses have internal turrets with filters at the rear. With them the default filter is a piece of clear flat glass. It is necessary so that changing to a colored filter won't force the user to refocus. Remember that placing a piece of flat glass between the lens and the film plane shifts the plane of best focus.


PostPosted: Mon May 26, 2014 9:53 pm    Post subject: Reply with quote
Gerald wrote:
...
A field flattener must be put at, or very near the focal plane. This is not the case with MTO 1000 or the Minolta 250mm F5.6, for which the rear elements are far from the focal plane. For these lenses, certainly the rear elements affect not only the flatness of field, but also other aberrations such as spherical, coma, astigmatism, etc.
As I said before, the removal of the rear element will very likely ruin the performance of the lens. On one occasion I tried to remove the rear element of a lens CZ Sonnar 180mm F2.8. The result was horrible.
...


So you are also doing some "experiments"? I think on a Sonnar 180 the rear element is no field flattener Smile

For a 1000mm lens ~60mm in front of the focal plane is not too far away.
Yes, ideally field flatteners should be directly in front of the focal plane - but this results in very high cleanliness demands for that glass.

I do not say this modification will work without problems on every mirror lens, but from the image that one guy has on his website it worked for that kind of lens.


PostPosted: Mon May 26, 2014 11:46 pm    Post subject: Reply with quote
ZoneV wrote:
So you are also doing some "experiments"? I think on a Sonnar 180 the rear element is no field flattener Smile


You're correct, the rear element is a weak lens, but don't mess with it! Laughing

It was just out of curiosity. Razz I was re-greasing the CZ Sonnar 180mm F2.8 when I discovered that the rear element could be easily removed. That element has its own barrel, which can be unscrewed by hand.
I did a quick test with the rear element removed, but I had no illusions. I already knew that the outcome would be bad. Crying or Very sad


PostPosted: Tue May 27, 2014 8:22 pm    Post subject: Reply with quote
Gerald wrote:

A field flattener must be put at, or very near the focal plane. This is not the case with MTO 1000 or the Minolta 250mm F5.6, for which the rear elements are far from the focal plane. For these lenses, certainly the rear elements affect not only the flatness of field, but also other aberrations such as spherical, coma, astigmatism, etc.


I don't think that is true about the focal plane. It depends entirely on the design of the field flattener. Usual field flatteners available for telescopes (i.e. for simple doublets or triplets) at focal lengths 500-700mm have a working distance from the sesor somewhere around 50-70mm. The important is that this distance is set correct! Catadioptic systems don't have spherical abberation, that's why the front corrector element is for!

Gerald wrote:
CuriousOne wrote:
No I don't meant filter (which is flat), I meant optical element just before filter.

Those optical elements correct the aberrations of the mirrors. If you remove them, you will ruin the lens.


They correct abberations, but only to a certain extent, field curvature is much more important. A maksutov-cassegrain (mirror lenses with a strongly concave front corrector plate, with secondary mirror directly on the plate) or a Schimidt-Cassegrain (the ones that have almost a flat corrector and a separate housing for the secondary mirror) have excellent on-axis correction for abberation and are usually diffraction limited in the center. Field curvature (and of course astigmatism induced from the curvature) is much more problematic.

The problem I think it is in the small mirror lenses for 35mm cameras is that the maksutov and/or schmidt configuration itself would provide only a very small fully illuminated field. Therefore, the focal length of the mirror assembly has to be much less and then the rear group acts as teleconverter/field flattener to produce a fully illuminated 35mm field. And still vignetting is quite strong for such a slow lens.
This might as well explain why the Zeiss Mirotar 500/4.5 performes so good. Much larger - easier to illuminate 35mm field, huge secondary mirror for a large illuminated field, less field correction needed - less abberations induced.


PostPosted: Tue May 27, 2014 11:22 pm    Post subject: Reply with quote
sammo wrote:
Gerald wrote:
CuriousOne wrote:
No I don't meant filter (which is flat), I meant optical element just before filter.

Those optical elements correct the aberrations of the mirrors. If you remove them, you will ruin the lens.


They correct abberations, but only to a certain extent, field curvature is much more important.

First of all, field curvature is an aberration! Make no mistake; a field flattener is an optical sub-system that suffers from… field curvature! It must be so, because the function of a field flattener is to compensate the curvature of field of the main system. In other words, an ideal field flattener should introduce a curvature of field of equal magnitude, but opposite sign to that of the main optical system.

A pure field flattener should be placed on the focal plane for its aberrations, others than curvature of field, have no effect on the image. When a field flattener is placed before the focal plane, all its aberrations are important.

A photographic lens is not designed as a telescope, whose optical parts are usually optimized separately. A photographic lens is designed by using a global optimization, where the aberrations of the various parts of the optical system are not corrected individually, but these aberrations end up canceling each other.

The rear group Minolta 250mm F5.6 lens is a doublet whose surfaces have large curvatures. Surely this doublet suffers from strong aberrations, but these aberrations are precisely the amount necessary to cancel the aberrations of the front group with mirrors. If the rear group is removed, the cancellation of aberrations disappears and the image quality degrades.


PostPosted: Wed May 28, 2014 1:03 am    Post subject: Reply with quote
They are probably there to spread the image out further prior to getting to the film plane


PostPosted: Wed May 28, 2014 8:06 am    Post subject: Reply with quote
I am still confused why all field flatteners that I have seen have a specified working distance from the focal plane? If they are placed closer or further away no aberration apart from an unflat field can be seen.

The fact why I am comparing to telescopes is because both Maksutov- and Schmidt-Cassegrain designs are used for telescopes and there can't be much difference between lenses and telescopes apart from the rear lens group. And for your info, high quality telescopes are also designed as a complete system.


PostPosted: Wed May 28, 2014 1:38 pm    Post subject: Reply with quote
sammo wrote:
I am still confused why all field flatteners that I have seen have a specified working distance from the focal plane? If they are placed closer or further away no aberration apart from an unflat field can be seen.


Commercial field flatteners are fairly well corrected from most aberrations except curvature of field. It must be so because those general-purpose field flatteners are designed to work with telescopes that have already been corrected from most aberrations. Of course, this is not the case of the rear group of a photographic lens which is designed using global optimization.

This article has interesting information about flat flatteners designing:
http://www.telescope-optics.net/field_flattener.htm

Some excerpts:

Single-lens flattener

The simplest way to correct for field curvature is by placing a single thin lens just in front of the final focus. With proper choice of parameters, it flattens the field while inducing very low aberrations, except at very large relative apertures.

................................................

In general, lens aberrations are low if it (the field flattener) remains very close to the original focus

................................................

With such generic prescription, best location for the flattener varies from one system to another, but it should be within 10-20mm. At the optimum location, coma is near zero, and the median astigmatic surface is nearly flat (the error is actually slightly smaller with a very mild residual curvature, than with flat median surface, which requires more of astigmatism). Moving corrector away from the optimum location worsens the curvature/astigmatism error, and also induces coma (small changes in the lens spacing have little effect, the two lenses can be in full contact). Again, optimization would probably somewhat improve the best flattener's performance, put not significantly in practical terms, since the dominant aberration is the astigmatism flattening the field.


PostPosted: Wed May 28, 2014 2:53 pm    Post subject: Reply with quote
Did some preliminary tests with Vivitar 500/8 mirror.

Removed the rear element, it consists of two lens, convex and concave, separated with metal ring.

Tested on minolta film camera body and viewfinder. With these lens removed, the image becomes much more brighter, and magnification is being reduced by about 0.7x. However, infinity is gone and max focus distance is only 7-8 meters. Will continue experiments with NEX body.


PostPosted: Wed May 28, 2014 6:54 pm    Post subject: Reply with quote
Interesting experiment, although no infinity and a 20 foot maximum focal distance kind of takes some of the utility out of a mirror lens. That's shorter than the MFD of the Vivitar 600 solid cat Smile

I would be interested in seeing the results. The mirrors I have generally do better when you are shooting something semi close.


PostPosted: Wed May 28, 2014 8:14 pm    Post subject: Reply with quote
Yes, I'll try tomorrow morning to capture something meaninghful with and without rear element.


PostPosted: Thu May 29, 2014 6:05 am    Post subject: Reply with quote
Here are the test shots. It should be noted that this lens initially was of very bad quality, so bad sharpness and contrast weren't caused by modifications.

I think guys familar with shutter speed and iso ratings, according to these images, can estimate how faster/shorter lens becomes?

According to my estimations, with rear glass removed, lens becomes 250mm/F4, but I might be wrong.

The circular halo on SB image is caused by shiny surface of SB holding tube - I had to machine it down to fit inside M42-NEX adapter.

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