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

On Wed, 20 Jun 2007 20:53:15 +0000 (UTC), davem@xxxxxxxxx (Dave
Martindale) wrote:


Thanks again Dave for a most comprehensive and coherent set of
answers. (Not the norm for news groups, HI.)

And once again I got involved in some community activities I couldn't
avoid, so am late in responding, but will add my comments and further
questions below your appropriate answers.

omcdaniel.abcd@xxxxxxxxxxxxxx writes:

I will admit to not fully comprehending lots of the technical issues
involved on this subject, like - what exactly is a T-Mount and what
does it do? Also why do some combos require removal of both the DSLR
camera's lens AS WELL as the scopes eyepiece, while others use one and
not the other.

You asked about two different cases, so I explained two. But that's not
exhaustive. There are probably at least four ways to couple a camera to
a telescope or a microscope:

1. Prime focus: remove eyepiece and camera lens, place camera focal
plane so telescope or microscope objective forms an image directly on
camera sensor.

2. Afocal, "digiscoping": leave eyepiece and camera lens in place, have
camera lens look into eyepiece

3. Eyepiece projection: remove camera lens, leave eyepiece in place,
mount camera a few inches out from the eyepiece. Then readjust focus on
the telescope so that instead of getting a virtual image at infinity,
the eyepiece produces a real image on the camera sensor. Good for very
high magnification (e.g. photos of planets) but uses the eyepiece in a
way it's not designed for.

4. Use special relay optics designed to couple the scope to the camera,
generally by replacing eyepiece and camera lens.

So, based on these 4 procedures, I therefore believe the following
tests that I ran fit your No. 4. Is that correct?

Today, just to do a little experimenting on a subject I really don't
have a good grasp on, I took an old 35mm SLR camera (Exakta V1000 to
be specific) , and used a set of lens extender tubes, like taking a
macro shot. I attached a standard, but old, 50mm f2.0 lens to the end
of the extender tubes. This placed the lens 2.5 inches further out
from the camera than normal. Then I removed the eyepiece from my
Swarovski AT-80-HD spotting scope, and fitted a 2 inch long plastic
cylinder from the front of the lens to the scope. (This was simply
hand held for brief visual testing.) Thus the camera lens was now
roughly midway between the camera body and the scope body. By
refocusing the scope slightly, I got a good image in the Exakta's
viewfinder of an object about 75 feet away, and it filled
approximately 50% of the focusing screen thru the camera's viewfinder.

Ok. By adding the extension tubes to the 50 mm lens, you made it
operate as a macro lens with more than 1:1 magnification. Then you
positioned it to focus on the real image inside the scope barrel, where
the eyepiece would normally be.

Yep, I knew it was a "macro" application, that's what the extension
tubes were bought for many years ago with the Exakta camera. But I
will admit to not fully understanding the statement "to focus on the
real imabe inside the scope barrel". That confuses me, but let's let
it ride - it's only a matter of my incomprehension, not important to
what I'm trying to achieve, hopefully.

What puzzles me though, is this essentially how the "adapters" offered
by Swarovski and others work? Do they contain internally a nominal
lens? At the price they charge, they surely must. I've not been able
to find a satisfactory description (to me at least) of what that
Swarovski adapter does. At their B&H price of $460, I would like to
know what's inside it.

The simplest adapters, often found for astronomical telescopes, do
nothing more than mount the camera to a tube of the correct diameter to
fit in the focuser mount on the telescope. If the focuser has enough
adjustment range, this allows prime focus photography. Since the only
glass used is the telescope objective, quality is high. But spotting
scopes are generally designed to provide a real image that is somewhere
inside the eyepiece barrel, and you can't put the camera focal plane
there. For $460, the Swarovski probably has some sort of relay lens in
it equivalent to a low-power eyepiece in focal length, but specifically
designed to project a real image into a camera.

Is not that essentially what I was attempting to replicate with the
extender tubes and the Exakta's 50mm lens?

But I'm just guessing.

Dave

Your guessing makes a lot of sense! Now if I could only get someone
who works for Swarovski to admit to what's in their adapter, and what
magnification (or focal length) it provides, it might be worth
considering. Trouble is - they are almost impossible to make contact
with anyone in the know.

Once again, thanks for taking the time and giving such clear, and
reasonable explanations.

Olin

.

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