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digital_davem

Camera lens as telescope objective

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Just a bit of fun and satisfying some curiosity...

Looking at entry level refractors like the SW ST80, many of them seem to be relatively short focal length.  Which makes me wonder why a telephoto lens can't make a good telescope.  Even a fixed focal length lene will have 4 or 5 elements and ought to be well corrected.  So, I had a look through my gear cupboard....

First up, an old M42 Chinon 200mm f3.5.  I cobbled together an optical back from old tubes and eyepiece holder and lots of duct tape.  It works! With a 25mm EP I have a 50mm f/3.5 200mm refractor with 8x magnification or 16x with a 12.5mm EP.

Next up, something more ambitious:  Sigma 100-300 F4 constant aperture zoom.  The 100-300 is a big, heavy lens with it's own tripod collar built in so it's almost a telescope in waiting. It has a 75mm front element. I added a 2x teleconverter to turn it into a 3"/75mm 600mm f/8 APO.  With my eyepieces this provides 24x or 48x.  

So, question is, how come a dedicated telescope with a humble 2 element objective is better than a multi-element APO arrangement camera lens capable of covering the full 35mm frame to very high quality.  Optically, what stops this being an effective alternative to traditional refractors? Like I said, just a bit of fun...

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Good question although scopes like the Star 71 with its 5 elements or a short ED with an additional 3 elements in the FF/FR is getting close to a camera lens design. The one issue with lenses is the lack of backfocus so you cant fit a diagonal what would be nice though would be a Canon scope.

Alan

Edited by Alien 13

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Hmmmm.... found a few on ebay but can only get them from Japan  I'm going to ask Santa for one... :)  Approx 65x mag on the back of the 600.... and a 150mm objective... Might even give the Swarovski crowd a run for the money... :evil::grin:

A lot cheaper on Amazon...

http://www.amazon.com/Kenko-Scope-Eyepiese-Canon-KF-SCE-CEF/dp/B00FGLDE1A

Peter...

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I have made a few adaptors using a spare  rear lens cap and 35mm film canister in the past... I am considering one of these to use behind my 600/f4L ....

http://www.kenkoglobal.com/optics/miltol/miltol_scope_eyepiece_kit.html

Peter...

It looks good and for such a great lens too. Any idea how much and where from? I bet it will be a lot more than my duct tape but functional version

post-47707-0-90386100-1447090919_thumb.j

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I like it.... :) I'm a big DIY Fan...

I do a lot of wildlife/birdwatching and photography, the adaptor is £100 from Amazon  from Japan (about a month to get here though) and has optics to give a corrected image...

I normally have the 600/f4 on me and carry a Nikon Spotting scope (ED 50 with Nikon  Zoom and WA EP's)  - I have talked about getting one of the adaptors for a long time your post spurned me on....

so it's 60-180X with the BP 8-24 MKIII  zoom... or 36X with the TV 40mm...

Peter...

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The designers of photographic lenses primary brief is to provide good colour and wide field correction, this they do very well, very high resolution and high powers associated with telescopes would be wasted photographically for the purpose for which they are intended. Unless the user pays a high premium, photographic lenses don't make good high power telescopes and telescopes don't perform as well for very wide fields. For low powers, telephoto lenses can give very pleasing views with an eyepiece.   :smiley: 

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I'm hoping the "great off white" is premium enough :)

Peter...

But perhaps not the best 'value' spotting scope. ;)

Looks like a handy gadget.

Rich

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But perhaps not the best 'value' spotting scope. ;)

Looks like a handy gadget.

Rich

LOL...

There are a few different types about.... if you have a dig around... Should work well on the 300/2.8 as well sensible mag and plenty of aperture...

Peter...

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I have an eyepiece adapter I made for old M42 lenses not too difficult to cobble off the shelf bits together and it does make a very good astro scope but that correct image one that has been linked to looks great.

Alan

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I suppose a fixed aperture would be o.k. but don't lenses stop down to their minimum aperture by default. How do you keep the iris open?

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I suppose a fixed aperture would be o.k. but don't lenses stop down to their minimum aperture by default. How do you keep the iris open?

Most lenses are open by default to allow for AF and metering so not a problem the only issue would be the fly by wire ones like my new nifty fifty that cant focus without a camera attached.

Alan

Edited by Alien 13
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I suppose a fixed aperture would be o.k. but don't lenses stop down to their minimum aperture by default. How do you keep the iris open?

A lot of my lenses still have aperture rings...

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Picked the Miltol adaptor/EP  up from Parcelforce this morning after paying them £23:20 for paying £9.20 VAT on my behalf... There was no Duty just VAT... So all in cost £121 delivered in 10 days

Tried it on the Sigma 300/2.8 and it's awesome.... Infinity focus is a bit tricky to nail but at the distances I will be using it is surprisingly easy...

Peter...

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I finally got to try the Miltol  on the 600/f4L and wasn't disappointed....

I'm fortunate with my Canon Glass collection and already have a Nikon ED50 Spotter  and a couple of matched EP's which is superb in  half decent light small and compact and punches well above it's lowly 50mm aperture...

I will now likely carry the 100-400 and Miltol instead and have a bit more flexibility...

Peter...

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