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I've wondered about this myself.
I have a Canon 400 mm f5.6 lens - basically a 71.5 mm telescope. I have actually made up a eyepiece holder out of a rear lens cap and used the lens as a telescope.
As far as I can see, the main differences are the camera lens extra elements to reduce the actual physical length of the lens - it's only 256 mm long - and so that the focussing is all internal so the total length doesn't change when focussing.
My ED80 is about twice the length - and also was about 1/3 the cost.
I've been meaning to do some tests for CA and so one to compare the two - I'll see if I can do the in the next few daze...
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