Why does older CoolPix-995 work with scopes, and DSLRs will not?

Discussion in 'Digital Photography' started by Olin K. McDaniel, Jun 10, 2007.

  1. Perhaps this should be posted in a "Digiscoping" news group, but I
    don't believe there is any such.

    Anyway, for several years I've been using my CoolPix-995 successfully
    with a Swarovski AT-80-HD spotting scope, and would like to upgrade
    the camera to something with more resolution and a larger sensor.
    Specifically, a good DSLR with an Image Stabilized lens.

    Unfortunately, I keep hearing both (1) DSLRs are not suited for
    Digiscoping and (2) if you insist, you must remove the camera's lens
    and shoot without it, using a special adapter and a T-Mount. But it's
    almost impossible to find an explanation as to why these two
    statements are correct.

    Can anyone give a good technically sound and logical explanation as to
    why the CoolPix series and others like it CAN be coupled up close to
    the scope's eyepiece, and the DSLR types cannot without the above
    modification and loss of use of its lenses? It is these Image
    Stabilized lenses that make a workable arrangement using them so
    desirable for this application.

    Thanks for any explanation (preferably based on factual knowledge, not

    Olin McDaniel
    Olin K. McDaniel, Jun 10, 2007
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  2. Olin K. McDaniel

    John Bean Guest

    The eyepiece of the spotting scope is designed for something
    about eye-sized, and I could bang on about entrace and exit
    pupil sizes, but instead...

    Look at the diameter of the front of your Coolpix's lens.

    Look at the diameter of the frony of the dSLR lens tou want
    to use.

    Compare and contrast. Which is more like the size of the eye
    that it's about to replace behind the scope?
    John Bean, Jun 10, 2007
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  3. Olin K. McDaniel

    Mike Russell Guest

    Good explanation.

    Not to worry, though, the DSLR / T-mount solution is also very good, and
    will probably give superior results. Since the eyepiece and camera lens are
    absent, there will be fewer pieces of glass in the optical path.
    Mike Russell, Jun 10, 2007
  4. OK John, so the diameters are the cause of the incompatibility.

    But, Mike, if I've got to use the T-Mount and maybe the Swarovski's
    adapter - my real question is why don't they add a correction lens in
    that adapter tube? Why throw away the magnification the lenses, such
    as the Swarovski's eyepiece provides? These give 20X to 60x
    magnification, all within the eyepiece itself. (You did say "since
    the eyepiece and camera lens are absent". Check what you said.) My
    further question is - do these adapters really work without both such
    lenses? I thought they only required removal of the camera lens, and
    fitted onto the eyepiece which was left in place.

    Now, what I really suspect, but cannot confirm it since it's tough to
    get in touch with them directly, even via a web site, perhaps
    Swarovski may actually include such a corrective lens in their ~$280
    and $460 adapters. Can anyone on here tell me if that's true or not?
    That's essentially what my original post was all about.

    Now, back to you John. There are items being sold to make such a
    combo compatitible. The problem is - they fail to explain how they
    are solving the diameter difference problem, and how removing one or
    both lenses manages this. That's precisely what I'm looking for an
    understanding of.

    And perhaps I shouldn't expect the 3000mm focal length (35mm
    equivalent) I get with the CP-995 and the 20X eyepiece on the scope,
    but that's what I'd like to match. Simply that!!! What are the
    other options? $8000 lens plus $300 2X tele-extender? That gives me
    about half what I get now, but now I'm working at only 3 MP
    resolution. I want the best of both worlds, 8 to 10 MP resolution
    plus the 3000mm equivalent focal length. Don't we all? An adapter
    designed for DSLR cameras (like the 30D) and a good lens is the
    desired goal. Let's pursue this as if price was not the major
    obstacle. Can it be done? Or is it just not worth it?

    Olin McDaniel
    Olin K. McDaniel, Jun 11, 2007
  5. Olin K. McDaniel

    Roy G Guest

    For someone who uses telescopes, I would have thought you would know a
    little more about Optics.

    The Eyepiece lenses do not produce a "real" Image, It cannot be brought to
    focus outside the Telescope.

    The P & S Camera lens will be able to bring this virtual Image to focus at
    the sensor.

    A DSLR with a lens attached would be able to do that also, but the
    difference in size between the front element glass and the Scope VF glass
    would make this almost unworkable.

    Removing the scope VF and the DSLR lens will allow the Scope to bring its
    image to focus on the Sensor, and the adaptors just ensure that extraneous
    light is kept out, keep the Camera steady on the scope, and the adjusters
    allow the scope to focus.

    You are trying to re-invent the wheel. The magnification is determined by
    the front element of the scope. That is why they need to build such
    enormous scopes for professional astronomy.

    Roy G
    Roy G, Jun 11, 2007
  6. The large aperture is to capture light from faint objects..
    Magnification is secondary.T.
    Tony Gartshore, Jun 11, 2007
  7. Olin K. McDaniel

    Mike Russell Guest

    The aperture size determines resolution as well as light intensity.
    Magnification is an analog equivalent of digital zoom. It does not increase
    the amount of information available, but it is important because it does
    affect the number of pixels you can put on a given object.

    The upshot is that the coolpix 995 is a favorite among digiscopers for a
    reason - it captures a lot of detail using a conventional eyepiece. A
    T-mount will theoretically make better use of the telescope objective
    optics, but may, as the OP points out, not have enough magnification to use
    the entire width of the sensor.

    BTW - I recall that a Russian experimenter removed the optics of the cp995,
    and imaged directly on to the sensor without an eyepiece, basically
    recreating the T-mount concept, only with a tiny sensor. The person who did
    this was able to capture a good image of the Orion nebula, even with just a
    moderately long lens. I don't have a link, unfortunately, but can look
    around some more if you are interested.
    Mike Russell, Jun 11, 2007
  8. Wrong! Completely wrong, unless you are using a Galilean telescope that
    uses a negative lens for an eyepiece, and there haven't been many of
    those around in the last 400 years (I believe some really cheap
    binoculars still use this design). Of course an eyepiece can project a
    real image. I've projected the image of the sun onto a screen many
    times. The objective lens or primary mirror forms a real image in front
    of the eyepiece, and the eyepiece then can form a second, real image of
    that image somewhere beyond. Just think of the eyepiece as a simple,
    positive lens, with the telescope primary image on one side and the real
    image formed by that simple lens on the other side, outside the
    telescope. Though what this has to to with the subject at hand is
    another question. There is absolutely no reason a DSLR camera cannot be
    used in the same way as a small P&S, with the lens remaining in place.
    The problem isn't with the diamter of the lens, but its long focal
    length and the subsequent magnification you will get. As others have
    said, you could be much better off removing the lens (and maybe the
    telescope eyepiece if it is a long focal-length telescope) from the camera.

    Sorry to jump on this, but I've been reading a lot of questionable
    statements in this thread, some clearly wrong. This was going too far
    for me, especially after saying someone should know better.

    Joseph Miller, Jun 11, 2007
  9. Wrong! Completely wrong, unless you are using a Galilean telescope that
    uses a negative lens for an eyepiece, and there haven't been many of
    those around in the last 400 years (I believe some really cheap
    binoculars still use this design).[/QUOTE]

    Actually he isn't wrong at all, just describing the situation of the
    telescope in use as set up for normal viewing. When correctly focussed,
    the telescope and eyepiece produce parallel rays, (even on a Galilean
    telescope) with the image focussed at infinity. This allows you to look
    through the eyepiece and see an image with your eye relaxed, just as you
    normally would when viewing a distant object, where the rays entering
    your eye from that object are parallel, to be converged onto your retina
    by the lens in your eye.
    Yes, you can but you will notice that the distance to your "screen" is
    significantly greater than the distance between the camera mount and the
    sensor. Also, if you then look through the telescope the eyepiece will
    require adjustment to bring the image into focus for direct view (or at
    least it will if you have normal or corrected vision). All you are
    doing to focus on a screen is changing the separation of the primary
    lens or mirror and the eyepiece so that the rays coming out of the
    eyepiece converge to a point at a finite distance rather than infinity
    as parallel rays do.

    You can do exactly the same thing with a Galilean telescope too!

    Any telescope can produce convergent, divergent or parallel rays
    depending on the instrument focus.
    Pot, meet kettle - you are both black, so you can be buddies. ;-)
    Kennedy McEwen, Jun 12, 2007
  10. Olin K. McDaniel

    M-M Guest

    Yes, but magnification will be limited.

    I rigged up a T-mount for my DSLR that fit over the eyepiece of my
    Fieldscope 82.

    You CAN use a DSLR and an eyepiece on a spotting scope. However
    everything has to be manual.

    I prefer my Coolpx 990. It can meter and even focus. And there is no
    mirror shake.
    M-M, Jun 12, 2007
  11. Not at all, you should be able to use aperture priority auto exposure
    mode without any problem. Obviously you can't use any other auto mode
    because the scope only has one aperture and no aperture control link
    with the camera.
    Kennedy McEwen, Jun 12, 2007
  12. Olin K. McDaniel

    M-M Guest

    Not at all, you should be able to use aperture priority auto exposure
    mode without any problem. Obviously you can't use any other auto mode
    because the scope only has one aperture and no aperture control link
    with the camera.[/QUOTE]

    If using a T adapter, there is no electrical connection to the camera so
    the exposure needs to be set manually. There can be no auto-exposure.

    However when removing the eyepiece and using Nikon's FSA-L1 adapter on
    their Fieldscope, you can have auto-exposure.
    M-M, Jun 12, 2007
  13. Olin K. McDaniel

    John Bean Guest

    There can certainly be AE with *any* lens and/or adaptor on
    all the cameras I use. I don't use Nikon though.
    John Bean, Jun 12, 2007
  14. Olin K. McDaniel

    M-M Guest

    Which DSLR cameras do you use?
    M-M, Jun 12, 2007
  15. Olin K. McDaniel

    John Bean Guest

    Olympus E-1 and a Pentax *istDS. The Sigma SD9 I had years
    ago could also meter with any old lens/adaptor fitted.

    I always assumed any dSLR could do this but it seems that
    some can't.
    John Bean, Jun 12, 2007
  16. You're talking about two substantially different ways of acquiring an
    image from a telescope.

    With the CoolPix, you leave the telescope eyepiece in place, and the
    telescope provides a magnified image projected to infinity. Then you
    place the camera's lens where your eye would normally be, and the camera
    focuses on the image and captures it. It works, but there is *a lot* of
    glass in that optical path, which reduces contrast and may add
    distortion. Also, to work well in this scheme, the camera you're using
    needs to be physically mountable on the telescope eyepiece, with a lens
    that has an entrance pupil near the front so it can capture most of the
    light coming from the eyepiece when set to an appropriate field of view.
    Eyepieces with a long eye relief will work with more cameras than one
    with a short eye relief.

    You *can* use a DSLR with its lens in place with your telescope. But
    the DSLR is bigger and heavier and harder to mount than the CoolPix,
    and the lens is a longer focal length and so probably has the entrance
    pupil deeper inside it, so you may get a lot of vignetting off-centre.

    Besides, that just isn't the best way to couple a camera to a good
    telescope. The best way is to remove the camera lens and the telescope
    eyepiece (thus removing all that glass) and using the telescope
    objective directly as the camera lens. You'll get better image quality,
    and probably a physically more robust assembly as well. This is called
    prime focus photography, because the sensor is at the prime focus of the
    telescope objective.

    So, you don't *have* to remove the eyepiece to use a DSLR with a
    telescope, but it's generally better to do so. With a P&S camera, you
    can't remove the camera lens, so you can't do prime focus photography at
    all - you're stuck shooting through the eyepiece.

    Dave Martindale, Jun 12, 2007
  17. I'm sorry, but much of what you say is wrong or confused as well, but I
    see no point in getting into an argument. His statement that the
    eyepieve ONLY creates a virtual image inside the telescope is nonsense.
    That was my point. Enough of this.

    Joseph Miller, Jun 12, 2007
  18. Olin K. McDaniel

    Mike Russell Guest

    Kennedy McEwen wrote:
    Thanks, Kennedy, for a concise, correct, and courteous post. Much
    Mike Russell, Jun 12, 2007

  19. If using a T adapter, there is no electrical connection to the camera so
    the exposure needs to be set manually. There can be no auto-exposure.
    Wrong. Even without electrical connection between the lens and the
    camera you can ALWAYS operate with aperture priority automatic exposure.


    That has been the key advantage of aperture priority over shutter
    priority since automatic exposure was introduced almost 35 years ago!
    Kennedy McEwen, Jun 13, 2007
  20. Olin K. McDaniel

    dj_nme Guest

    If using a T adapter, there is no electrical connection to the camera so
    the exposure needs to be set manually. There can be no auto-exposure.[/QUOTE]

    That simply isn't true with all DSLR cameras.
    Canon EOS will AE in (at least) aperture priority mode, Pentax DSLR
    cameras will AE in Auto, aperture priority and manual (using the
    AEL/green button) modes and I would be extremely surprised if Nikon DSLR
    cameras could not do so as well.
    Is that an equivalent to a mechanical adapter (like a T-mount) for a
    Nikon DSLR or an adapter for a coolpix?
    dj_nme, Jun 13, 2007
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