Mirror Lenses

Probably because Pentax is the only major company ever to produce a mirror zoom, and the approach was supplanted by improved refractive lens zoom technology? If you have more info, please share!

Personally, I'm curious about Schiefspiegler optics and how that approach could be adapted to lens technology. Since that approach hasn't been embraced by the astronomy community, I suspect it might be more trouble than it's worth. But still, I'm drawn to the off-beat and unusual.

jblaschke wrote in post #14988696
I was under the impression that Tamron had two different versions of the 500mm, the later one supposedly far superior.

My Canon 500mm supposedly takes 82mm filters--I can see the threads, which are distinct from the lens cap threads--but the 82mm filter I bought it too thick to fit. The threads look like they'll fit perfectly, but the metal frame itself is maybe half a millimeter too broad to fit past the cap threads. I know I'm missing something. That front element is so big I'd like to protect it...

In thirty years of shooting, buying & selling vintage gear, I've never seen a second (earlier) version of the Tamron 500mm. Just the SP/Adaptall2 model. Maybe I missed it, but I'm only aware of the "55BB" model.

No, I absolutely would not put a "protection" filter on the front of any of these mirror lenses, even it can be fitted with one. Way too likely to compromise image quality.

Yes, on most some filter is supposed to be on the rear, is part of the optical formula. Should have a "Sky" or "Skylight" on there, at the very least. Using these lenses on a digital camera, it might be worthwhile to invest in a high quality multicoated filter for that purpose. I don't think the originally supplied filters were multicoated, which was fine for film but can be a problem with digital (search for discussion of rear element refelction issues with DSLRs).

Mirror lenses are a fixed aperture, so you don't have to worry about stopping down and exposure changes. Many use neutral density filters to reduce light, if needed, which doesn't change DOF at all, just reduces light levels. But, once again, it's fixed and there is no stopping down.

You can use a couple methods to arrive at a correct exposure. The camera doesn't need to "know" what the aperture is... in fact it doesn't really "care".

Use "stop down" metering... which means simply setting your camera to M or Manual mode and choosing your ISO (try 200 or 400 to start with, using f8 lens in broad daylight), and then adjusting your shutter speed until the indicator is centered on the meter scale. Alternatively you also could use a handheld meter and make all the settings directly, keeping in mind that the lens aperture will always be f8 (unless one of those ND filters is fitted).

If using another type of lens that has a variable aperture (such as the macro lens I used for the bee image in the post above), you need to do your metering with the lens stopped down to the aperture you plan to use. Lock in your settings, the you might want to open the aperture back up for focusing, stopping back down at the last moment to take the photo.

Another method is to set your camera to Av or aperture preferred auto exposure mode. Point the camera at your subject and the correct shutter speed should automatically be set. You might need to dial in some + or - Exposure Compensation depending upon the tonality of the subject. If shutter speed is too slow (you'll probably need 1/800 or faster to handhold a 500mm lens on a crop DSLR, 1/500 using it on a full frame camera), adjust your ISO higher. The camera doesn't really care what aperture is set... it simply measures the light coming through the lens and adjusts the shutter speed accordingly.

Using Av, you don't need to do the metering and settings in advance. Just be sure you don't have AE Lock on, and the camera will automatically adjust exposure when you stop the lens down to the desired aperture at the last moment.

Also might be able to use Auto ISO that's now available on some models... but I really don't know. Never tried it. Do note that you give up Exposure Compensation when using Auto ISO.

You can't use any other auto exposure mode on this or any other adapted, manual focus/manual aperture because all other AE modes need to vary the aperture to work.

AFAIK, there have been few mirror/catadioptric zooms offered. It would be a very difficult and expensive design to offer any kind of usable image quality, I imagine... Plus most zooms of the day were no longer than 200mm or 300mm.... While most mirror lenses are 500mm and longer focal lengths, where the "folded light path" gives the greatest advantage for a much more compact and affordable lens design. In other words, there would be little to be gained in size, weight and cost reduction with a cat zoom... while there's a lot of potential to save in all three factors with a long cat telephoto prime.

There were some interesting variations on mirror lenses. Konica offered 1000mm f8 and 2000mm f11 that were far from compact and used a bellows focus mechanism at the rear, but unfortunately meant their minimum focus distance was pretty huge. They also both had variable aperture, using a dial at the rear that moved various size "waterhouse" stops into the light path. That was really a rather brilliant design. There was also a provision for drop-in filters, since the front element was larger than any available filters of the day. The 1000mm cost about the same as a compact car in the mid-1970s.... supposedly only two or three dozen were sold. There were only two of the 2000mm lenses ever made and none were ever sold. I have no idea the cost, it would have been a special order item. I saw one on display at a photography trade show many years ago and it was huge... around 30-35lbs, if I recally correctly.

Yes, the Canon FD reflex has a drop-in filter that's part of the optic formula. I believe the 300mm and up refractive lenses used these as well. The set is pretty rare these days. Modern drop-ins don't appear to be compatible.

I'm trying to figure out the front threading to see if a CPL could be added. I'm not trying hard, mind you, but it's at the back of my mind.

jblaschke wrote in post #14990836
Probably because Pentax is the only major company ever to produce a mirror zoom, and the approach was supplanted by improved refractive lens zoom technology? If you have more info, please share!

Personally, I'm curious about Schiefspiegler optics and how that approach could be adapted to lens technology. Since that approach hasn't been embraced by the astronomy community, I suspect it might be more trouble than it's worth. But still, I'm drawn to the off-beat and unusual.

Never knew pentax made a mirror that was a zoom mirror. Can't find any info on it. do you have any by chance?
The one I have is a 500-800 F8-f12

More or less what it did was the everything from the mirror to the from glass moved when you changed the zoom of the lens. which also created the variable aperture that was of course based upon the current focal length.
the one I own is labled a cambron. I know there are others labled zkyor or starblitz. It's not the greatest lens by any means but I've seen worse also. My biggest problem is I believe it has something out of alignment. With a reflective lens if you shine a light down the barrel that light will shine in a nice perfect circle. might be not focused tight but at least the circle should be present. On my lens it's like something is tilted as it creates not even the donought but rather like a spiral shape and as you move the focus it's almost as though one side of the light will focus at a different distance then the other side. This is even if they are both the same distance from the lens.

There is some info about the Pentax 400-600mm Reflex f8-f12 here.

Google found quite a few mentions of it.

melcat wrote in post #14989709
I owned one of these a long time ago, and you were supposed to always have a filter in the optical path. There were 3 filters for B&W photography and a clear one for colour. Let's hope you have the clear one.

Yes, it's the clear one.

30.5 mm (tiny)

amfoto1 wrote in post #14990903
In thirty years of shooting, buying & selling vintage gear, I've never seen a second (earlier) version of the Tamron 500mm. Just the SP/Adaptall2 model. Maybe I missed it, but I'm only aware of the "55BB" model.

Sorry I missed this earlier. Tamron made a "55B" version that looks identical to the "55BB." The difference is that the "55BB" had some slight optical revisions made to the design to improve the overall performance. From what I understand, most people don't even know of the earlier version. I seriously considered getting the 55BB up until The Wife surprised me with the FD 500.

And to answer the other question, no, I do not own the Pentax mirror zoom. I'd like to, for novelty's sake, but I'm not actively looking.