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  1. #11

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    No Powdered Sugar on My Doughnuts

    Quote Originally Posted by Pinholemaster View Post
    And the donuts tasted yummy.

    A telephoto 500 mm has an variable aperture, while the 500 mm reflex lens has a fixed aperture.

    The reflex lens is usually lighter in weight, and shorter in length.

    Optically you should test, but I've experienced more sharpness with a telephoto style 500 then a reflex design.
    Nice bokeh donuts with my 500mm mirror lens, But no powdered sugar.
    Tom Hoskinson
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  2. #12

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    A reflex lens is MUCH smaller and lighter that a refractive lens of the same length. My Tamron 500mm reflex is not much bigger than my 35-70mm, f3.6 Zuiko and about the same weight. It can be hand held with 400ASA film. This makes it a very practical user lens, and the ultimate value of a lens is whether you have it with you when you need it. The Tamron also focuses to less than 6ft, a remarkable close focus for a lens this long. A reflex lens is much cheaper than the equivalent length refractive. As for image quality, the Tamron 500mm is very highly regarded and performs well in objective testing.

    I really get a kick out of those people who throw around the word "bokeh" and act as if it something real. They make sweeping claims about one blurry thing being intrinsically better than another blurry thing! Many people prefer the textured "bokeh" produced by a mirror lens.

  3. #13

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    Don't astronomers use mirror lenses with great sharpness.
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

    Regards
    Bill

  4. #14
    David A. Goldfarb's Avatar
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    Mirror lenses, to the extent that they don't use lenses (and in practice they often do use lenses to make them more compact among other things), are inherently apochromatic, so they can be very sharp. Astronomers don't usually have to worry about things in the background, and everything is pretty far away, so I doubt they have to think at all about the out of focus area, because they don't have one.

    If you photograph birds in trees or any kind of wildlife standing in tall grass or reeds, I would say that indeed, double lines make a cluttered background more cluttered, so for such purposes, which are common uses for long lenses, bokeh is a real practical issue.

    There's also the fact that these lenses are usually f:8, while the big telephotos used for wildlife, sports, and such are typically in the f:2.8-5.6 range. A 500mm refractive lens of this type would usually be around f:4, and it would usually be used wide open or close to it, so a lot of the image might be out of focus, and you want that part of the image to look good.

    In some situations, those little donuts look good and can be used creatively, but they don't come up that often, which is why the wildlife shooters spend the money on the big lenses and go to the trouble of schlepping them around.

    Of course before there was any of this, there was this delightful camera--

    http://cgi.ebay.com/RARE-1908-Natura...d=p3286.c0.m14
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  5. #15
    Ole
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    As an aside, they may well be f:8, but due to the bit in the middle they are usually t:10 to t:12.
    -- Ole Tjugen, Luddite Elitist
    Norway

  6. #16
    AgX
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    Ole, that `bit in the middle´ is less than half of the front lens area. How could that lead to f 1:12 ?

  7. #17

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    I have a A Multi-Coated Russian Rubinar 500mm f5.6 Maksutov casegrain mirror lens that I use on my Contax 35mm SLRs.

    I agree with OLE, the Transmission Number (it is not an f number) is pretty large (thus a slow lens). That said, I am pleased with the optical performance of my LZOS Rubinar lens - its resolution and color characteristics are quite good.
    Tom Hoskinson
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  8. #18
    David A. Goldfarb's Avatar
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    I think the transmissive loss with these lenses is also due to the quality of the mirrors used and the optics involved. I used a Nikon 500mm reflex lens once in the early 1980s, and it was very sharp and pretty bright, but it was also physically fairly large for a mirror lens. I wasn't shooting wildlife at the time, but in retrospect, the images I remember from that lens were shots with simple, uncluttered backgrounds. More compact lenses (like the Soligor I owned) in general are optically like shorter lenses with built-in tele-extenders, and those usually add some transmissive loss. I had a 1000mm/11 Meade lens for a while as well. It could get one close to the subject, but it was kind of low contrast.

    Another factor with these lenses is that I think the compact size deludes users into thinking they can use the same small tripod with them that they use with their other lenses. Maybe it's not necessary to have quite as large a tripod as one would want with a long lens that weighs 10 lbs., but the angular rotation caused by camera shake is the same with a mirror lens as it is with the big glass, so a larger tripod and head are still in order.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  9. #19
    Jim Jones's Avatar
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    The image quality of my 500 f/8 Nikkor mirror lens did not compare well with some off-brand 400mm f/5.6 to f/8 lenses. Despite its close focusing and compactness, the advantages of conventional lenses were overwhelming.

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