Top lenses versus top telescopes for telephoto work

Discussion in 'Digital Photography' started by Rich, Dec 25, 2006.

  1. Rich

    Rich Guest

    IMO, a good apochromatic telescope while not possessing a fast f-ratio
    will beat any camera lens when it comes to long tele work. For
    instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
    both operating around f5, the Takahashi scope shooting wide open, it
    will easily beat the Nikon using a high resolution digital camera body.
    The difference would be even more apparent using a Canon 1DsMkII with
    its 16 megapixels. The Tak also supports 6x7 medium format. These
    telescopes range in price from entry-level "semi-apos" costing $300 for
    a 360mm f5.5 to over $4000 or more for a colour free 500mm-800mm f4-6
    that can be "sped-up" via dedicated compressor lenses.

    The reason these scopes have to be so good optically is that shooting
    celestial objects with stars in the field is a severe test of optics.
    Any residual off-axis aberrations show up in the star (point source)
    images. Most of us have seen the flary, blurred out images at the edge
    of a field of lights seen when night shots are taken with camera lenses
    used wide open. These scopes can't allow that.

    The reason these instruments haven't been made use of by more
    photographers is that they don't operate (no IS, focus radically
    different) like camera lenses. However, for wildlife photogs who use
    blinds and wait for the opportunity to get good shots, they would be
    ideal.

    Interesting thing; In 1977, Modern Photography tested a 550mm f5.5
    telescope put out by an American company called, "TeleVue." They put
    it up against other long teles from Nikon, Canon, Leica, etc. It beat
    them all. In fact, its MTF figures where so high they looked like those
    from a macro lens. That telescope wouldn't compare to what is avalable
    today from TeleVue or other makers of high end scopes.

    The closest these instruments have come to integrating with the general
    photographic community is though "Birders" who often use high-end
    spotting scopes that they can attach their cameras. However,
    "digiscoping" using P&S cameras and terrestial spotting scopes is not
    the same as using a DSLR on a non-terrestrial telescope operating as a
    "prime" lens.

    The apochromatic telescope (otherwise known as an apochromatic
    refractor) made its debut around the late 1970s with the introduction
    of brands like the American Astro-Physics, Takahashi, Nikon and Pentax.
    Nikon left the business some time ago and it's telescopes are highly
    sought-after collector's items. Pentax is about to enter the American
    market with its apos.
    They make a 400mm f4 called the 100 SDUF II.

    You will never see f2.8 or f2 apo telescopes, the need for diffraction
    limited performance negates that possibility. But at f4-8, they
    represent the pinnacle of imaging systems.
     
    Rich, Dec 25, 2006
    #1
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  2. That's part of it. Another factor is that telescope objectives are
    normally used as part of a system for visual observing, and the
    magnification is changed by changing eyepieces. A typical good
    refractor has a 2 inch focuser, and might be used with a very low-power
    eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
    is expected to produce an image nearly 2 inches in diameter with good
    quality.

    Or it may be used with a series of successively shorter focal length
    eyepieces to give higher magnification. A 4 inch diameter objective is
    capable of 200X useful magnification if the objective is diffraction
    limited, and refractors are *expected* to be diffraction limited, at
    least in the central portion of the image that would be visible at such
    high power. This is somewhat comparable to finding a zoom lens that
    performs well over a 20X zoom range.
    I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
    "semi apo" - not as good as the 4" TeleVue apo refractor referred to,
    but also a lot cheaper. It was sharp, but it's also quite a pain to use
    compared to a camera lens: (These apply to most astro telescopes used
    as camera lenses)

    1. Focusing is by rack and pinion, which is really too quick to get
    precise focus with a camera. (It's fine for telescope use, where
    your eye itself provides some degree of final fine focus).

    2. There's no auto diaphragm. In fact, there's no diaphragm at all,
    so you're always shooting wide open.

    3. The optical tube is longer and more awkward than a camera lens
    of comparable focal length.
    Dave
     
    Dave Martindale, Dec 27, 2006
    #2
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  3. There was a craze about 25 years ago for using telescopes with camera
    adapters in place of a long telephoto lens. Two problems limited the
    popularity and the craze died out.

    First, as you say, relative aperture was limited, so exposures were
    slower in low light. This was not too big a deal, as these things HAD
    to be used with tripods anyway. Secondly, they did not have adjustable
    apertures- exposures HAD to be set by varying shutter speed only.

    This later problem meant that the lenses could not be used with
    automatic exposure, which was getting popular at the time. While auto
    focus was not around yet, AE was, and many folks had already come to
    depend on it by then.
     
    Don Stauffer in Minnesota, Dec 27, 2006
    #3
  4. Rich

    M-M Guest


    You present a good summary. I have been using these lenses for a good 25
    years now, first with film, now with digital.

    With P&S digicams, auto focus and exposure works fine- just mount the
    camera lens as close to the eyepiece as possible. The Coolpix series of
    cameras with the 28mm threads on the lens (9xx, 4500) are the best way.

    With SLR's, yes- you must manually adjust the shutter and focus and the
    aperture is set by the exit pupil of the telescope. But cameras come
    with light meters so I don't see the big deal. As you say, one must use
    a tripod and set up the subject carefully, have a lot of light and
    usually a remote shutter release. And often the photos will not be
    tack-sharp.

    But when you get one like this, it's all worth it:

    http://www.netaxs.com/~mhmyers/cdjpgs/eagle1L.jpg
     
    M-M, Dec 27, 2006
    #4
  5. Rich

    M-M Guest

    M-M, Dec 27, 2006
    #5
  6. While I agree that telescopes can beat out telephoto lenses,
    they do so only at near infinity focus. Camera lenses are
    a compromise in giving good image quality over a larger range
    of subject distances. A apochromatic telescope will not do as well
    on a closer frame filling subject, like birds. Then, in real
    world situations, the manual focus, fixed aperture telescopes
    would give poor overall performance "to get the image" than
    modern autofocus lenses. And even so, the super telephoto
    lenses are VERY close in quality to apochromatic telescopes, and
    I bet it would be hard to tell the difference of say a FSQ106
    versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
    images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
    http://www.clarkvision.com/gallerie...1/web/m31.c07.10.2005.Avg23-75s-v1.6-700.html
    At f/5.6, star images of most of the field are barely over on pixel,
    being limited by the blur filter of the camera.

    I agree that you can get nice images with a telescope, but the
    piggyback through the lens adds a lot of flair and depends a lot
    on the quality of the eyepiece and matching of field curvature
    between camera lens and eyepiece. Compare this eagle image with
    a super telephoto lens (costing several times more):
    http://www.clarkvision.com/galleries/gallery.bird/web/eagle.c09.11.2004.JZ3F4717.b-700.html

    The advantage of the super telephoto is the speed, autofocus, and
    flexibility to get action images that would be difficult with
    a telescope. (I own several telescopes, up to 12.5 inches aperture
    Dobsonian and a 8-inch home-made Cassegrain). Before spending
    $5,700 on a 500 mm f/4 I tried using a telescope, e.g. 900 mm f/6
    Newtonian. I could get great images, but too bulky and close
    focusing was a problem, and the closer the focus, the more
    the image quality dropped.

    Roger
     
    Roger N. Clark (change username to rnclark), Dec 28, 2006
    #6
  7. Rich

    Rich Guest

    The Oracle is a 16 year old telescope. Most apos now use two-speed
    focusers for course and fine focus so that issue is resolved. But they
    can't focus fast like a collar focusing lens or an AF lens.
    That is something they could implement, but most observers want full
    aperture.
    Yes, these kinds of scopes don't incorporate (generally) elements
    further down the tube to increase focal lengths, but generally rely on
    the main lens (objective) to provide the focal length. Additionally,
    few are below f5 (Pentax makes a 400mm f4 photographic model
    and Takahashi makes a Sky 90 which is a 450mm f5). Both are fairly
    compact.
     
    Rich, Dec 28, 2006
    #7
  8. Rich

    Rich Guest

    In a situation like that, what you can do is stop down the scope
    slightly to eliminate an
    additional spherical aberration that might be encountered due to the
    less than infinity focus.
    But also, this issue might not be a major one for a small (80-100mm
    aperture scope) and then only at very short distances. Questar
    Corporation used to supply a 1/2" wide diaphram to stop down their 90mm
    wide mirror-lens scopes when using them under object distances of 25
    feet or less.

    Then, in real
    Good image, because these lenses are used only at prime focus, and are
    close enough to diffraction limited to avoid any visible issues at that
    kind of magnification. But, unlike the camera lens, the telescope's
    focal length can be increased significantly without any noticeable loss
    of correction, contrast, etc. In-case the target needed this.
    Downside being the increase in focal ratio (speed) of course.
    Yes, I can't see someone hauling a Newtonian scope into the field to
    shoot birds, but it would make for an interesting conversation amongst
    the others there!
    But you can buy a refractor from TeleVue or TMB with an f5 focal ratio
    and 500mm focal length that would make an interesing comparison.
    http://www.televue.com/engine/page.asp?ID=297
     
    Rich, Dec 28, 2006
    #8
  9. That depends on the camera. In aperture priority mode, you set the
    aperture and the camera determines shutter speed from the light meter
    reading and how much it thinks the lens will stop down (if the meter
    reading is taken before exposure). To work with a lens that has no
    diaphragm, you just have to convince the metering system that the
    shooting aperture is the same as the viewing aperture.

    I know my old Minolta X-570 could do it. I'm sure the X-700 would too,
    since it was a superset of the X-570. These cameras actually metered
    off the film after the lens had stopped down to taking aperture.

    Dave

    PS: the "meter at shooting aperture" behaviour once caused me some
    puzzlement for a while. I once shot a bunch of photos at a world's
    fair using a lens whose diaphragm was stuck open. I didn't figure this
    out until well after the fair. The camera noticed the bright "stopped
    down" image and used an actual shutter speed that was much higher than
    what it indicated it was going to use, based on the aperture ring
    position. In the end, most of the exposures were pretty good, not
    grossly overexposed like you'd expect from a stuck diaphragm. But I was
    shooting with far less DOF than I expected to have, and some subjects
    ended up out of focus because of that. I didn't understand why until I
    noticed the stuck aperture; the camera doesn't record the actual shutter
    speed it used anywhere.

    Dave
     
    Dave Martindale, Dec 28, 2006
    #9
  10. But stopping down is a problem because you lose light and then
    can not capture action. The point of the super telephotos
    is to provide great image quality wide open over a large
    range of distances.
    But Questars were optically too slow for wildlife action photography.
    Super telephotos are designed to still give good image quality
    with teleconverters. Here is a star field with a 500 f/4
    and 1.4x TC:
    http://www.clarkvision.com/gallerie...eb/m45-700MM-8534-8561_C16B-add27-v3-800.html
    The stars a still very good.
    I have compared image quality of some of the apochromatic telescopes
    with my 500 f/4. The ones that have great performance, comparable
    to the 500 f/4 L telephoto, are, guess what, as expensive as the
    500 mm f/4 lens. You get what you pay for. The reason I looked
    into this is that I have one undesirable problem with the
    500 f/4: the aluminum tube changes length during long night
    exposures (e.g. over 20 minutes). A PLASTIC TUBE WOULD BE BETTER!
    ;-) But for wildlife photography in rough environments, the
    metal tube is great.

    Roger
     
    Roger N. Clark (change username to rnclark), Dec 28, 2006
    #10
  11. Rich

    Rich Guest

    True, in terms of speed, there are no telescopes to match a fast
    telephoto (f2.8, etc).
    And yet people did use them for that. Questar's Field Model, they even
    had one with
    electric high speed focusing. But, were talking 1350mm at a minimum
    for the 90mm model
    so that is a long tele. However, Questar's 700mm mirror camera lens
    was the best mirror lens of all time, in terms of definitiion and its
    speed was f8 and it wasn't a telescope.
    At 700mm that is pretty decent.
    Apochromatic telescopes are generally considered on a "price per inch
    of aperture" basis.
    So, a typical high-end one costs over $1000 per inch. A TeleVue 500mm
    f5 is about $3800.00. Some cost more, some less. TMB now has a 130mm
    aperture "signature" series that costs about $3800 for a 780mm f6 scope
    with superb optics, around $750/inch.
    A bargain today. Pentax's SDUF costs $3000 for a 400mm f4 lens and it
    will support a medium format film size. Inexpensive apos (from China)
    run about $100-$400 inch depending on the scope size.

    You get what you pay for. The reason I looked
    Not really, the plastic would actually contract more as it got cold, it
    would just take alot longer to do it. In other words, it would
    probably not even stabilize during the night.
    Alot of photogs seem to tape or wrap their lenses. PBS had a guy on
    shooting Yellowstone in winter. He took his Nikon 500mm and stuck it
    face down in the snow(!) while he used another lens on his camera!
    But, the lens cap was on it.
     
    Rich, Dec 29, 2006
    #11
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