Are some mirror lenses better than others?

Discussion in 'Digital Cameras' started by Paul Ciszek, Jul 10, 2012.

  1. Paul Ciszek

    RichA Guest

    You don't know what you are talking about. I've owned all the gear, I
    know exactly how they perform. I've also owned over 200 telescopes
    ranging from $50 cheapos to $10,000 SCT's so I'm pretty familiar with
    them. Vivitar Series 1 stuff is vastly over-rated, like many lenses
    old, misty-eyed men fondly remember from days of yore. Modern lenses
    of good quality walk all over them.
     
    RichA, Jul 11, 2012
    #21
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  2. Paul Ciszek

    Paul Ciszek Guest

    I would only be using a telephoto outdoors, for wildlife, landscapes,
    and some of the larger astrophotography targets such as the moon.
     
    Paul Ciszek, Jul 11, 2012
    #22
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  3. Paul Ciszek

    Martin Brown Guest

    This is probably obvious but the trade is always between having a faster
    physically big refractive lens that will work well in low light and
    allow faster shutter speeds for wildlife (and controlled depth of field)
    against the compact but slow mirror lens which is hard to focus.

    You should be able to pick reasonably priced manual focus long focal
    length prime lenses by watching the secondhand lists for a while.

    If you can force your camera to preview at f11 that will give you a
    taste of what it is like focussing with an f10 mirror lens.

    In astronomy everything is effectively at infinity so depth of field
    issues do not matter so much but vibration and exposure time do.
     
    Martin Brown, Jul 11, 2012
    #23
  4. Paul Ciszek

    Paul Ciszek Guest

    After shopping around for a bit, it looks like the best ones I am
    going to be able to find are the Nikon, the Tamron, and the Minolta,
    all 500mm F/8. Wayne Grundy's review claims that the Tamron is
    sharper than the Nikon; he did not review the Minolta. So, between
    those three, does anybody have any strong feelings? The autofocus
    of the Minolta/Sony will probably be useless to me, but if it is a
    good lens otherwise, I will consider it. Sharpness/detail is my
    primary consideration; I like to use the moon as a test subject,
    if that is any help.
     
    Paul Ciszek, Jul 11, 2012
    #24
  5. Paul Ciszek

    PeterN Guest

    Possibly a little careless in post.
     
    PeterN, Jul 11, 2012
    #25
  6. Paul Ciszek

    PeterN Guest

    Nikon.

    Here is Ken Rockwill's review.
    Note: he states it is not great for photography. Note also it is manual
    focus.

    It is fairly inexpensive and I would not jump though hoops to sell it.
    If someone is in the NY area, and they want to try before they buy,
    fine. I have not made any effort to sell, nor will I, subject to the
    above.
     
    PeterN, Jul 11, 2012
    #26
  7. Paul Ciszek

    Rich Guest

    (Paul Ciszek) wrote in
    No, any light source, specular reflection that is out of focus will
    likely show them because what you are seeing is the central obstruction
    (secondary mirror) that's in the centre of the mirror lens.
     
    Rich, Jul 12, 2012
    #27
  8. Paul Ciszek

    Paul Ciszek Guest

    Where does infinity begin?
     
    Paul Ciszek, Jul 12, 2012
    #28
  9. Paul Ciszek

    Paul Ciszek Guest

    I have gone ahead and purchased the Tamron. The good news is, the
    Adaptall adapter for micro-four-thirds supposedly guarantees the
    ability to focus at infinity.
     
    Paul Ciszek, Jul 12, 2012
    #29
  10. Paul Ciszek

    Savageduck Guest

    Out there.
     
    Savageduck, Jul 12, 2012
    #30
  11. Wow! Amazing that you have the time to researcg what to buy or to test
    what you've bought at that kind of rate!
     
    Chris Malcolm, Jul 12, 2012
    #31
  12. Hyperfocal infinity? Well, one of my standard tests for long lenses is
    to photograph a large tower clock that's about two miles away. When my
    500mm f8 reflex is focused on that the stuff behind it is clearly out
    of focus. So it's certainly miles away.
     
    Chris Malcolm, Jul 12, 2012
    #32
  13. Paul Ciszek

    Paul Ciszek Guest

    Wow. Apparently only for astrophotography can I safely assume that things
    are at infinity.
     
    Paul Ciszek, Jul 12, 2012
    #33
  14. Paul Ciszek

    Martin Brown Guest

    I'd suggest a more terrestrial daytime target like a pylon or church
    spire. The moon won't show you the out of focus point spread function. A
    picture of a bird in a tree almost certainly will.

    I'd probably suggest looking for a used Tamron 500mm f8 then.
     
    Martin Brown, Jul 12, 2012
    #34
  15. Paul Ciszek

    Martin Brown Guest

    Especially whilst residing under a bridge.

    He is in my kill file. I only see his tripe when someone replies.
     
    Martin Brown, Jul 12, 2012
    #35
  16. Paul Ciszek

    Martin Brown Guest

    For a given camera/lens/sensor working at the limit it scales with
    (focal length)^2/(focal ratio)/(circle of confusion)

    So 500^2/8/0.02 = 25/16 x 10^6 mm = 1500m

    If the lens is close to diffraction limited and the sensor more than
    6Mpixels it could easily be 2-3x longer.

    I'm surprised it is quite that bad (but the hyperfocal infinity is
    certainly of the order of a km for long lenses). I'd have expected it to
    be just possible to get infinity and the church spire in focus on film
    (OTOH for a digital SLR the circle of confusion is smaller).

    http://en.wikipedia.org/wiki/Depth_of_field#Lens_DOF_scales

    My back of the envelope numbers based on classic 35mm film gives about
    3km as hyperfocal infinity for my old 1000mm f10. My favourite high
    contrast test targets are a weather vane at 500m and a pylon at 2km.

    I confess I have never really looked carefully at depth of field at long
    range with these long lenses.
     
    Martin Brown, Jul 12, 2012
    #36
  17. In my 35mm film shooting days my longest focal length was 300mm. I
    printed up to A4ish in size, sometimes from a slightly cropped
    image. I examined my best negs and slides with a low power microscope
    made for that purpose.

    When I started looking carefully at pixel level sharpness on 14MP crop
    sensor images of long lenses (a zoom at 250mm, a 200mm prime
    teleconverted to 400mm, and a 500mm f8 mirror lens) I was startled to
    discover how far out was the conventional wisdom from my 35mm film
    days about DoF, mirror slap vibration, hand-holdable shutter speeds
    (no, my hands don't shake more than they used to :), etc..

    Now I sometimes print A2, almost always from a slightly cropped
    image. In terms of the care I now have to take to get a really sharp
    image at modern digital resolutions it's like the difference between
    washing up dinner in the kitchen sink, and washing up the glassware in
    a biochemistry lab.
     
    Chris Malcolm, Jul 12, 2012
    #37
  18. Paul Ciszek

    Paul Ciszek Guest

    Already ordered. ;-) This is the picture that convinced me:

    http://www.flickr.com/photos/rain0975/3083568658/sizes/o/in/photostream/
    (Be sure to view the original resolution.)

    Now, this guy was using a teleconverter behind the Tamron, but I figure
    the teleconverter can't add detail--the fact that he was able to get an
    image with that much detail means that the Tamron can produce that much
    detail, and all the teleconverter does is scale the available detail to
    his sensor. Is my reasoning correct?
     
    Paul Ciszek, Jul 12, 2012
    #38
  19. Paul Ciszek

    Martin Brown Guest

    In a sense that could add some new detail in practice. A well made 500mm
    f8 mirror lens operating at its aperture diffraction limit will need a
    sensor with pixels smaller than 5um to be adequately sampled. Most times
    atmospheric seeing and air currents will limit you first.

    Allow for perfect conditions with a bit of additional oversampling and
    3um pixels would be ideal. Adding a multiplier effectively matches the
    lens to the sensor and prevents jaggies on edges. There is a lot of info
    around about choice of oversampling for astronomical imaging.

    Real astro CCDs don't have anything by way of anti-aliasing as the aim
    is to capture every last possible bit of signal.
     
    Martin Brown, Jul 12, 2012
    #39
  20. Paul-

    I have the Minolta 500/F8 AF lens, as well as a Celestron C-90 (1000mm
    F/11) with T-Mount adapter and a TeleSor 500/F8 with PK mount. They all
    are reasonably sharp but have noticeably less contrast than the
    non-mirror lenses I've tried with about the same focal lengths. They
    represent a trade-off between image quality and lens size/weight.

    Do not sell the Sony/Minolta Autofocus short. I found focusing to be a
    major problem with those with no AF, since it could be quite critical.
    The longer the lens, the harder it was to focus manually. (Back in the
    day of split image focusing, a lens of F/8 or smaller could only be used
    with ground glass focusing.)

    Fred
     
    Fred McKenzie, Jul 12, 2012
    #40
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