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Post by belgiumreporter on Jun 9, 2015 7:00:46 GMT -5
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Stephen
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Post by Stephen on Jun 9, 2015 12:02:42 GMT -5
I am not surprised the Nikon is better, but I am suprised that the Russian is the worst of them....they have a good reputation, and mine is dead sharp, but the focusing is Oh so critical!!
All mirror lenses do not like any kind of rough handling, but I am not suggesting any such handling from Belgian reporter!!, just saying that all mirror lens, bar solid glass designs, are very prone to collimation problems.
The Russians always treated the Mirror lenses as professional equipment, and built them with great care, but they cannot be in the Nikon league for collimation, checking and quality control.
The Optical company I worked for used several Russian cat lenses in specialist surveying equipment and the performance was spot on. We tried them on film for testing, and on optical bench testing, and the assembly standard was excellent. They dated from the end of the Communist period, and were a lot cheaper than Japanese competitors.
By the way it is possible to stop down a mirror lens, with a cap with a hole, and a centre disc supported on threads. The aim is an equal reduction of the light tube from the disc and cap. It increases the apparent depth of focus, but also sharpens the out of focus doughnut ring effects.
Stephen.
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Post by cooltouch on Jun 10, 2015 5:06:22 GMT -5
It's always nice to see comparisons like this. I would have liked to see more than just one shot of one subject, but what you show is illustrative enough.
I've recently conducted some mirror tests as well. I compared the Tamron SP 55BB 500mm f/8 mirror to the Sigma 600mm f/8 mirror. The Tamron came out on top handily. I was a bit disappointed. The Sigma is a recent acquisition. It has an EOS mount, which indicates to me that it was probably built late in Sigma's production run of the 600/8. I used to own one that I bought in 1984 in Canon FD mount, and it was considerably sharper than this EOS-mount one. I suspect that if I could have compared that early Sigma with my Tamron, things would probably have been much closer.
Because of this, I've done some investigating of Sigma's 600mm mirror. As it turns out, Sigma made a few changes to it over its rather long production span. Most all of them appear to be cosmetic, but it could be that the lenses were being assembled on newer equipment that held tighter tolerances during its early years, whereas as the years progressed, the manufacturing equipment began to wear, resulting in looser tolerances and thus lenses being assembled with more slop than they should have had.
That's one explanation, at any rate. Another is just looser quality control or, as Stephen mentioned, lenses having collimation that's off. Whatever the cause, it makes me wish that I'd never sold my original Sigma.
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Post by philbirch on Jun 10, 2015 13:59:48 GMT -5
An interesting test. Pentax C-mount lens???
Stephen explain the stop-down lens cap again. Front or back cap, what size hole and disc? Perhaps a quick diagram. Will this take the edge off softness like stopping down a stop?
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Stephen
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Post by Stephen on Jun 10, 2015 15:12:32 GMT -5
Take a cap and cut a hole in it at about the diameter of 1/3 of the diameter of the front element, and match it with a black card disc cut about 1/3 across the diameter of the glass from the centre out. The card can be supported by thread in two pieces glue to the card, and glued to the cap. This will cut the light by about 1.5 stops,(TTL will provide the exposure) You can cut the whole shape fom black card, with thin supports leaving four quarter holes.  The supports will never show, (so far out of focus). The depth of field increases, but not by much as it is so shallow with cat leneses anyway. It alters the out of focus doughnut rings, making them sharper. The rings can also be cut with a star shape, (say 8 spokes), and the centre as well, to soften the doughnuts, but still increase depth of field, and alter the F stop, or more accurately the T stop. Stephen. |
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Post by julio1fer on Jun 10, 2015 18:13:51 GMT -5
Interesting test. I wonder how you made sure that both lenses were accurately focused. The last shot is so much fuzzier than the other one.
In my limited experience (with a cheap Vivitar 500mm/f8 in K-mount), the practical problem is getting the right focus with that oh so dark image in the viewfinder, and besides DOF is so darned thin that you must get it right.
I found that, when I got good focus, lines were sharp but contrast was very low.
Stephen, nice information on reducing f. Does it improve optical quality in a noticeable way? (just helping to get more forgiving focus would be enough).
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Post by belgiumreporter on Jun 11, 2015 5:28:20 GMT -5
Thanks for your input guys, this thread is becomming increasingly interesting to me.
As i've stated this was a very brief test, if i find some time in the upoming days i'll conduct some more test wit my two "cats". I was somewhat disapointed with the results. The results you see here are a "best out of ten" i adjusted the focus a tiny bit on some shots to rule out front or back focus, even though the image was sharp in the finder and display . Next time i'll be using the d3 and see what that will give.
PS. Phil, the Pentax c-mount is a 25mm 1:1,4 lens that i'm planning to use on my Pentax Q the moment my C to Q adapter arrives from China.
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Post by philbirch on Jun 11, 2015 12:52:18 GMT -5
It is interesting. I have acquired one via Lloydy, my first cat lens. It is extremely hard to focus and the quality leaves a lot to be desired. It's soft. I never knew Pentax made C-mount lenses - presumably for cine or video.
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Stephen
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Post by Stephen on Jun 11, 2015 16:02:44 GMT -5
Pentax (Japan period), were a major supplier of C mount lenses under the brand name of Cosmicar, for security cameras, lots are for sale cheap on Ebay, but be warned, they do not properly cover the frame on Olympus or Canon, but work on the Pentax Q, with it's smaller format sensor. Despite the coverage issue lots of people use them on Digital for street photos in low light.
Stephen.
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Post by belgiumreporter on Jun 12, 2015 6:31:52 GMT -5
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Stephen
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Still collecting.......
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Post by Stephen on Jun 12, 2015 7:21:58 GMT -5
That Russian lens must be out of collimation, was it new or Second hand? It must be about 30 to 40% down on the Nikon, which is quite surprising. The Mirror in the Soviet design is very soundly mounted, and the whole design is Russian, they were world leaders in such mirror designs, both for lenses and for telescopes. Like all Russian equipment there is the risk of a bad one slipping through the checks, but this type of lens were low production specials, and a lot went to military uses. The standard was always better on the photosniper as well as the Mirror lenses.
One point...They may have changed the design, but some Russian mirrors required a plain UV filter to be in place, it is critical to the optical formula in some versions....I know it is plain glass, but it affects the total refractive index of the rear glass group. Some Russian cats have a slot for filters, some screw on the back. Generally they come with a set of fillters, and if it contains a UV, then always use it.
Stephen
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Stephen
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Post by Stephen on Jun 12, 2015 7:35:09 GMT -5
Looking at pictures of the kit supplied with the 3M5A, it looks like the 500mm lens comes with front filters, and these will not affect performance whether they are used or not. The versions we tested at work were the MTO 500 and 1000, and I think had back filters. At the time, the Russian lens was also much more contrasty than other makes the factory tried. I cannot remember exactly the makes, but I think there was a Minolta and Nikon, and a Vivitar 300mm solid cat, which performed best of all.
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Stephen
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Post by Stephen on Jun 12, 2015 10:48:35 GMT -5
In passing, if you have a sub standard cat lens, and I have a modern Korean one that is a bit poor to say the least, they can be converted to a quite decent telescope with adaptors, but the "true right way up", and "non reversed image" type adaptor to make into a spotting scope are quite pricey. Nikon make one especially to go with the Nikkor, at Nikon prices!
However I came across a reference to using the back half of a cheap roof prism monocular, the £5 to £10 pocket types sold on Ebay, by removing the front element and placing the back half of the monocular on a tube behind the Mirror lens mount. It should work in theory, the roof prism reverses perfectly, and the eyepiece lens should give a good light image.
But I have not tried it myself as yet, but will take one of the Monuculars apart and see if it works properly. It needs no modification to the lens, the mount could be obtained from a cheap set of extension tubes to match the lens mount.
A 500mm would be about 50x in theory, a decent length for a spotting scope. If you are after an astro telescope then any spare tele eyepiece can be added to the back, with reversed image of course.
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