A few nice tele lens images I found:
Look, a bird

Image by 55Laney69
Cute little bird seen at my balcony. Taken with this:
My Olympus E-PL1 with an old 70-200 exacta Tele Lens.
www.flickr.com/photos/hansel5569/6928000206/
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Quick comparison: Tele Vue-60, Sigma 120-400

Image by markkilner
Having come into temporary possession of a Sigma 120-400, I thought I’d do a quick comparison against my lens of choice, the Tele Vue TV60. This is by no means a scientific test – I wasn’t really far enough away from from the target (a calendar) for one thing – so I wouldn’t use this to make any statements about the finer qualities of either lens, but a couple of things are obvious when inspecting the full-size version:
1) The RPF-2804 field flattener (which wasn’t specifically designed for the TV60) makes a big improvement to the TV60′s edge-of-field performance.
2) When shooting wide open at 400mm, the Sigma produces a very soft image. The performance at f6.3 (not shown here) is much closer to the sharpness at f7.1 than it is to f5.6, with gradual improvements at each stop up to a peak at around f9.0.
All images were taken with a Canon 40D DSLR in Av (Aperture-Priority) mode at ISO 640, with mirror lock-up on and using Live View at 10x to focus manually. Both lenses were mounted on a tripod and the Sigma’s OS was turned off. The photos were converted directly from RAW and cropped in PS, with no additional post-processing. Centre crops are on the left; corner crops (top right of the frame) are on the right.
2000mm lens (a.k.a. telescope)

Image by Alberto Jaspe
Cool! A friend lend me his scope (and his help!), a Celestron NextStar 8i, and using the appropiate T-adapter and ring, here we have a 2000mm lens 
If you like photography, it’ll be so interesting for you to see the amazing and extreme values… We have a 250mm fixed aperture, and the focus range it’s enought to spend twenty seconds or so turning the mechanism. Of course, the depth of field is just zero. Although, the minimun focus distance is about 5 meters, so you can do macro-photography 10m away from the pretty bug!
Mola! Un amigo me prestó su telescopio (y su ayuda!), un Celestron NextStar 8i, y usando el adaptador T apropiado, conseguimos un tele de 2000mm 
Si te gusta la fotografía, te gustará conocer un par de datos extremos y chulísimos: una apertura fija de 250mm, un enfoque manual que te puedes tirar 2 horas dándole a la rosca para conseguir algo. Por supuesto, una profundidad de campo nula. Y, aun así, una distancia mínima de enfoque bastante considerable (unos 5m). Puedes fotografiar un bicho a 10m !!! xD
Photo Cameras & co.