topic Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ? in EF & RF Lenses

mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

frogpoet — Fri, 03 Jan 2014 14:23:10 GMT

so I am a "fuzzy donuts" nut..

Mirror / reflex lenses... How short do they get ? 55mm ?100mm ? 200mm ?

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

trulandphoto — Fri, 03 Jan 2014 19:42:31 GMT

@frogpoet wrote:
so I am a "fuzzy donuts" nut..
Mirror / reflex lenses... How short do they get ? 55mm ?100mm ? 200mm ?

Since Canon doesn't make any I'm curious why you ask here.

Wouldn't it be just as easy to do a search yourself?

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

TCampbell — Fri, 03 Jan 2014 22:10:16 GMT

I'm wondering why you would be interested in "short" focal length mirror lenses.

These lenses fulfill a very special purpose and you would normally not want to use one for general purpose photography.

A "mirror" lens (technically these are "catadioptric" lenses which means it uses a combination of lenses and mirrors. The front piece of clear glass is a "corrector plate" and is technically a lens even though the corretion is minor.

These lenses have no ability to communicate with the camera -- they have a fixed focal ratio which cannot be changed. They are manual focus only.

They commonly come in 400mm, 500mm, or 800mm focal lengths.

When a lens uses glass elements, the element works by bending light. Whenever you bend light by passing it THROUGH a lens element (as opposed to using a mirror), the light refracts based on it's wavelength. Red light does not bend as much as blue light for example. This means a simple single lens element cannot focus all wavelengths of light to the same focal plane. To correct for this, additional lens elements are included -- often MANY lens elements.

When lenses get longer, the amount of light making it to through decreases -- and to compensate, the lens needs to have a much larger physical diameter. When using lens elements, these elements also need to be physically larger. As the lens elements get larger, the prism effect which causes dispersion (different colored wavelengths bend different amounts) becomes even more extreme. This is why VERY long focal length lenses become VERY expensive.

"Mirror" lenses side-step the whole problem by bouncing the light off mirrors... there is no dispersion when bouncing a light off a mirror (you are not technically passing light "through" a lens element. This drops the cost of lens construction dramatically!

Mirror lenses typically just have "t-mounts" and you have to buy a T-adapter for your camera. As mentioned previously... you cannot adjust the focal ratio and the lens is manual focus only. There is no communication whatsoever between camera and lens (the camera behaves as if it has no lens attached.)

The out-of-focus areas of the image will blur based on the shape of the aperture opening... which for a mirror lens is actually donut-shaped. This means out of focus areas have black spots in the middle -- resulting in a very unpleasant quality in the out-of-focus areas.

An 800mm mirror lens might cost $200 (more or less) and you'd need the T-adapter (perhaps another $15-30 depending on who makes it.) An 800mm lens actually designed for an EOS mount (with autofocus, aperture control, etc.) would cost substantially more (likely thousands).

Due to the inability to change focal ratios, the manual focus constratint, and the lack of auto-focus... there is a considerable handicap in terms of shooting speed and flexibility. You would not likely want to use one as an every-day lens.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

amfoto1 — Thu, 09 Jan 2014 17:41:13 GMT

It's rather rare, but Tamron made an SP 350mm f5.6 "mirror" lens with the interchangeable Adaptall2 mount (which is available in EOS/EF mount). It's an excellent lens, as is their SP 500/8 "mirror", which is also Adaptall2. IMO, the two Tamron SP mirror lenses are among the best ever made (and I tried probably half a dozen different mirrors over the years). Both are long discontinued, but you can find the 500/8 used for under $200 and a "chipped" EOS/Adaptall2 from China costs about $40 off certain auction sites. The 350/5.6 is a lot harder to find. They are well made and incredibly close focusing (about 5 feet, with the 500mm).

If you go looking for one of the Tamron SP mirror lenses, try to be sure it has the originally included tripod mount. That's nearly impossible to replace, if it's missing. The lens hoods also can be hard to find and expensive to replace. And they originally came with a set of filters including one or two neutral density to allow you to use the lens an effective stop or two lower light. If I recall correctly, they use 30mm filters (rear mounted), which is a standard size and means you can buy additional, if necessary.

In another camera system I had a 250mm or 300mm mirror lens for a while... I forget the exact focal length or what brand it was... Sun or something like that. It was a T-mount and very compact, but it wasn't particularly good optically. I bought it hoping it would make a nice travel lens, but didn't use it very long due to the poor IQ.

I can't recall ever seeing a mirror lens shorter than about 250 or 300mm. Since the whole idea of the "folded light path" of a mirror lens is to reduce the size of a telephoto lens significantly, I'd be surprised to find any in a focal length shorter than, say, 200mm. You might search online for "catadioptric lenses" to see what you can find.

If you really like the "donuts" of cat lenses, you might like some of the vintage Russian lenses that have similarly wild bokeh effects, even though they aren't mirror lenses.

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

ebiggs1 — Fri, 10 Jan 2014 15:50:52 GMT

Actually Canon did make a CAT. The FD 500mm f8 Reflex and of course I have one. I have used it quite a bit. And now a fellow, Ed Mika, makes a great adapter for this lens to EOS. You use ND filters to change the light acceptance from f8.

Not with standing the short comings listed above, Canon did a fine job of eliminating most of them. And this lens was/is quite capable of outstanding photographs.

I would certainly look for one of these used before any off-brand lens, if you are interested in this type lens.

Distance Scale: (m) 4 (magnification 0.14X) to 50.OO (ft) 15 to 200 oo
Focusing: Revolving helicoid
Minimum aperture: Fixed at f/8. ND filter is used for adjusting light volume. No depth of field scale.
Diaphragm: Automatic
Filter Type. Rear section filter holder, drop-in type (with 34mm exclusive filter. Regular 1X)
Hood: Built-in type.
Cap: S-83 (CA2-0698)
Function: Auto Aperture, Full aperture metering (AE operation when used with ALL Canon automatic SLR cameras)
Length x max. diameter: 146mm x 90mm
Weight: 710g

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

ebiggs1 — Fri, 10 Jan 2014 15:54:29 GMT

Tim do you happen to have a C90? They are great CAT's too. Might well be the most known CAT lens there is. They are

t-mount. I have not tried to see if there is a way to mount it to EOS but I suspect there is.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

ebiggs1 — Fri, 10 Jan 2014 15:58:06 GMT

Back in the day, most everybody made a mirror lens for their cameras.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

TCampbell — Fri, 10 Jan 2014 23:07:47 GMT

@ebiggs1 wrote:
Tim do you happen to have a C90? They are great CAT's too. Might well be the most known CAT lens there is. They are
t-mount. I have not tried to see if there is a way to mount it to EOS but I suspect there is.

If you mean the Celestron C90 (Mak-Cass scope) -- I don't, but I know about it. A C90 is a 3.5" (90mm of clear aperture) scope. But most Mak Cass scopes have a fairly high focal ratio. The C90 is f/14 (really f/13.89).

I have three scopes... a 14" Meade LX200-ACF (schmidt cassegrain design). That's an f/10 scope with a 3556mm focal length -- way too much for terrestrial use but great for astronomy. I also have a TeleVue NP101is apochromatic refractor (it's a Petval quad-element design with superb optics) -- but that's a 560mm f/5.6 scope -- so that is actually useful for terrestrial use as well as astronomy of wide-field objects. Lastly, I have a special Hydrogen alpha solar scope (Lunt 80mm).

You CAN easily mount any t-mount lens to your Canon EOS. Basically this is how you mount a DSLR camera to most telescopes.

What you need to do this is called a "T-Ring" or sometimes it's called a "T-Adapter". Many companies make these and they can cost anywhere from just under $10 to maybe a little over $30.

Here's a link to just one such example at Adorama so you can at least see a picture of what it looks like: http://www.adorama.com/PROLAT11EOS.html

I own several of these... most of mine are Baadar-Planetarium brand (I like the fit and finish and it's important that it have a nice solid "fit" to the mounting flange on the camera so that the camera doesn't wobble on the optical tube and your focal plane is nicely perpendicular to the optical axis of the lens (otherwise if it sags the focus can vary from top to bottom of your image.)

Just note that while this fits the Canon body and mounts the lens, there are no electronics... you shoot in manual and you manually dial in the focal ratio of the scope (which is fixed since there are no aperture blades in the scope) and then adjust exposure by using only shutter speed and ISO. And of course the scope has to be manually focused as well.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

ebiggs1 — Fri, 10 Jan 2014 23:45:36 GMT

Yup been there done that.

I had a C8 also.

I don't think they make the old C90 any longer. It was a great little scope.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

TCampbell — Fri, 10 Jan 2014 23:48:48 GMT

They still sell it as a "spotting" scope (no go-to electronics ... just an optical tube with a tripod mount.)

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

frogpoet — Sat, 11 Jan 2014 16:12:09 GMT

just looking for other folk opinions and thoughts.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

frogpoet — Sat, 11 Jan 2014 16:40:24 GMT

Thanks,

Your reply to my querry was quite informative and easy to understand (no techno-jargon). I have a sigma 600mm on its way and am looking forward to playing with it. Since you seem to be well versed in the lens department, anybe you can enlighten me on a subjec that has me puzzled...ie... Is there any way to (stop-down) reduce the aperature in a mirror lens in order to expand the depth of field a bit or is that just a pipe-dream I have? I have seen nothing with a rating of f/8 or lower (4s, 6s, etc.) Is it possible to achive a deeper DOF in a mirror lens ? (pardon my innocence but I have an inquisitave mind...some would say I think like a 3year old).

Thanks again for your well written reply to my post.

Bruce

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

TCampbell — Sat, 11 Jan 2014 21:25:18 GMT

There is nothing internal to the lens or camera. You would have to make a mask to fit over the front of the lens.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

ebiggs1 — Sun, 12 Jan 2014 14:35:45 GMT

The lens will remain an f8. The only thing you can do, is limit the amount of light the camera sees but the lens will always be an f8. Adding an ND filter will just make the image darker, it will not change the depth of field. F-stop is the ratio of the focal length to aperture diameter. F-stop is not a measure of light coming in, it is the measure of the focal length to the aperture.

Re: mirror / reflex lenses... how short do they get ? 55mm ?100mm ? 200mm ?

amfoto1 — Mon, 13 Jan 2014 17:08:30 GMT

"Is there any way to (stop-down) reduce the aperature in a mirror lens in order to expand the depth of field a bit"

In the case of most mirror lenses, no.

There is no way to stop them down for increased depth of field. As others noted, you can reduce the amount of light reaching the camera with a neutral density filter (usually rear mounted or fitted in a drawer in the rear of the lens), but that doesn't change DOF at all.

Two exceptions I'm aware of were the Konica Hexanon 1000mm f8 and 2000mm f11 mirror lenses from the mid-1970s. They were both fitted with a disk of Waterhouse stops that allowed the aperture to be changed. I believe they offered as small as f32 or f45 apertures.

In case you are unfamiliar with them, Waterhouse stops are simply a series of precisely-sized holes, each bored in a piece of sheet metal, that were individually placed behind or within a lens via a slot to effect a different size aperture. Waterhouse stops were used with barrel lenses prior to variable apertures and leaf shutters. If interested, there's more into about Waterhouse stops here. and about making a set if them for a vintage lens here.

At least theoretically, you might be able to make something similar for any lens lacking an aperture.

The two Hexanons have a series of these stops inside, all arranged on a metal disk, so that different size apertures could be dialed into place. They were also somewhat unusual using a bellows focusing mechanism behind the main bulk of the lens. Unfortunately, both lenses are long discontinued, sold for the equivalent of a small car when they were new, and now are quite rare. Best estimates are that a few dozen of the Hexanon 1000mm were made and sold. They remain fairly valuable, a friend of mine just sold a very complete and near mint 1000mm for more than $2000 US.

It is believed that only two of the 2000mm were made and none was ever sold. One was in a camera museum and the other was last seen displayed in the Konica-Minolta headquarters, before the corporation's camera business was sold to Sony (around 2006, if memory serves).

There may be some other mirror lenses that used similar arrangements to allow changes to f-stop and depth of field, but I'm not aware of any.

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM