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5 Days Ago   #1
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Tamron 500mm f/8 SP Mirror (55B) problem

Hi all,

Recently, I got Tamron 500mm f/8 SP Mirror (55B) and I found that IQ is far below that I expect. Lens produces some kind of starry pattern, see photos.

Lens itself seems to be in perfect conditions, as I can't see any defects on mirrors and glass.

Any ideas about root cause and how to fix it?

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5 Days Ago - 1 Like   #2
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Perhaps this kind of lens is not suited for moving targets. I have 500mm mirror as well and I can only take a good picture with a static object and with that plane I can imagine you moved with it to keep it in the picture. And the second one I do not know, perhaps too dark and a slow shutter-speed?
5 Days Ago   #3
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It doesn't matter how I shoot, from the hand or from the tripod. It is not motion blur. I have also Samyang 500/8 and it doesn't produce such halo. Tamron always gives halo from bright/high contrast objects. Two photos above are just one of the best examples.

The same happens with sun flare on the water or cars.
5 Days Ago   #4
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Probably there's some tension on one of the mirrors that causes it to warp slighty. If you feel comfortable with disassembling the lens, google "howto relax mirror lens".

5 Days Ago   #5
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I have the same lens, and have not noticed this. As another member said, it's best on stationary subjects. The airplane was way out of focus, and I don't see any "starry" effects in that photo.
5 Days Ago   #6
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The second photo is a good example of a "diffraction pattern", and looks like something is misaligned in the lens. That pattern, if taken off a centered star should be perfectly circular about the star and the star image should be sharper and more concentrated than what you obtained. Some mirror lenses are better than others and a lot aren't too good. They can be manufactured cheaply but unless they are done right, the IQ isn't the best. Since a mirror lens has a central obstruction (the secondary mirror), they also produce a rather unusual out-of-focus bokeh pattern which is a bit apparent in your first photo. There is also some "ghosting" on that first image (along the front of the aircraft and upper engine) which at first looks like a chromatic issue (unusual for a mirror lens) but is probably an alignment problem or just due to a bad lens design or construction.

It's hard to say what is causing these issues and even harder to say how to fix them. You could check out romay's suggestion, but if the lens is under warranty, I would ask for a replacement. If it's a Tamron, I would expect better.
5 Days Ago   #7
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QuoteOriginally posted by Bob 256 Quote
The second photo is a good example of a "diffraction pattern", and looks like something is misaligned in the lens. That pattern, if taken off a centered star should be perfectly circular about the star and the star image should be sharper and more concentrated than what you obtained. Some mirror lenses are better than others and a lot aren't too good. They can be manufactured cheaply but unless they are done right, the IQ isn't the best. Since a mirror lens has a central obstruction (the secondary mirror), they also produce a rather unusual out-of-focus bokeh pattern which is a bit apparent in your first photo. There is also some "ghosting" on that first image (along the front of the aircraft and upper engine) which at first looks like a chromatic issue (unusual for a mirror lens) but is probably an alignment problem or just due to a bad lens design or construction.

It's hard to say what is causing these issues and even harder to say how to fix them. You could check out romay's suggestion, but if the lens is under warranty, I would ask for a replacement. If it's a Tamron, I would expect better.
The 55B is a 1980ís vintage lens.
5 Days Ago   #8
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Bob 256 comment is likely the correct diagnosis of the problem. When I looked at the second shot I immediately saw the diffraction pattern. If the bright spot was off in the upper left corner of the shot and that is a very tight (close to 100%) crop then you are seeing coma and a lot of mirror lenses suffer coma which is why telescopes used for astrophotography will also include a coma corrector. Not being familiar with this lens I am unsure if it included a coma corrector or if it does how much/well it corrects the coma.

5 Days Ago   #9
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My old Celestron 750 has adjustment screws for re aligning the mirror. With time and travel it would get a little out. I found instructions on the web about making adjustments. It has been years since I used it so I don't remember the websites. Doing a search should get you something. If your lens is adjustable you will find that very small ,like 1/10 turn or less, makes a big change. Also loosening one screw means you must snug another. Its trial and error until you get the hang of it. Also the adjustment screws need only a very light snugging as they put pressure on the mirror. If you tighten them mirror damage can result.
5 Days Ago   #10
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While I highly respect the comments on many various topics from Bob 256 & MossyRocks, I disagree that the second image is showing diffraction. The distortion rings (I'll call them that) are very similar to the image distortions on the upper left side of the airplane shot. Look closely at the distortion around the upper/left engine pod - it's the same as around the bright spot in image 2. The bumps (lumps?) in the contrail from that engine and along the adjacent top side of the fuselage are also similar, and NOT diffraction.

I have tried hard to take images of true diffraction - it's HARD to do!! You need a very small bright source (generally no bigger than a few arcseconds in size) to get diffraction rings. A typical LED dot light is a millimeter or more in size. To appear one arcsecond in size, that LED has to be 200 meters (two football/soccer fields) away!! The resultant Airy pattern first maximum is then typically 10-12-15 pixels in total diameter (depends on camera; but NOT lens!). Take a look here for a discussion of diffraction, showing how small it really is at the pixel level for our cameras: Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

I think something is horribly distorted in this lens - likely the primary mirror, which is probably over-tightened somewhere, and bent out of shape, essentially putting somewhat circular ripples into the mirror shape.
5 Days Ago   #11
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There is a club specific to adaptall lenses [Adaptall Mount Club (Tamron)], Suggest you post there.
4 Days Ago   #12
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I am pretty familiar with these tamron mirrors. With that optical result I am surprised there isn't visible evidence of something amiss. Can you post some pics of the lens? Have you had a look at it/through it carefully with a flashlight? Are there any signs that it has been tampered with?

My guess would be a knock that has misaligned either the 4 element correction group, or the secondary mirror. Or an incorrect reassembly of the correction group.
Just a thought: there is a doublet in the correction group (right by the mount end) that can suffer separation, visible as rainbowing, but I have nevere seen that to have any significant optical effect.
2 Hours Ago   #13
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Ok, thank you everyone for comments and suggestions. I initially suspected multiple reflections due to something wrong inside the lens. Now, I'm almost sure in the root cause.

There is glass separation in the secondary mirror. The mirror itself consists of mirror + glued lens(es), and now they are partially separated (see photo). Due to asymmetrical separation, resulting picture is also asymmetrical. Multiple reflections on the glass-air interface are producing that pattern.

I can try to shadow "bad" part, leaving only central part of the mirror to work, but in this case lens will become F/16 , or something like.
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