asahi pentax tele-takumar 5.6/400mm
A heavy, black lens that comes in
a chunky rigid leather case, the Tele-Takumar is not a wildlife photographer's
dream, unless the wildlife happens to be asleep or in rigormortis.
I bought it for my old Pentax
Super A, but it was hardly ever used. It is a Pentax/Pentacon M42
screw mount lens, that came with a Pentax K bayonet mount adapter when
I purchased it from a 2nd hand store. It has a built-in lens hood which slides down the body when not in use. These lenses in good condition typically sell for around 450 US dollars or so, according to eBay statistics.
Recently, I obtained an M42 - Canon EOS adapter ring,
which makes it possible for me to use this lens on my D60. The main issue here is obtaining an accurate focus; the D60 has no manual focusing aids, nor any means of replacing the ground glass with one more suitable to focusing manual lenses. Nevertheless, one can manage it with care. Here's a
sample of what this combination can do:

Canon D60, Asahi Pentax Tele-Takumar 400mm, f/5.6,
ISO 400, hand-held at 1/2000
Here is a 100% crop from the above image, with no post-processing
(complete with ISO 400 noise present). Note that this cactus plant is
about 150 feet away. The needles are reasonably well resolved, but I
don't expect to be counting the hairs on a rhino's snout with it.

Canon D60, Asahi Pentax Tele-Takumar 400mm, f/5.6,
ISO 400, 1/2000
More Sample Pictures
More examples of photographs from this lens can be
found in the Tele-Takumar 400mm
Samples folder.