Nikon lenses on Canon EOS

It is possible to mount Nikon lenses on Canon EOS bodies with a simple adapter ring that can be found online. All Nikon lenses become mechanically compatible. Only Nikon G lenses are less practical to use as they have no diaphragm ring (not to mention Nikon S rangefinder lenses). Apart from that all Nikon lenses can be mount bot those with F ring (uncut) and AI/AIS/AF/AFD.

Nikkor S auto 50mm f1.4 on Canon EOS 50D

Of course, you have to focus manually and contrary to Nikon bodies, Canon does not provide focus confirmation. However it provides metering. Lenses focus beyond infinity. So overall, this require a bit of attention and is not as carefree as leaving the AF kit zoom all the time. Results may vary in terms of quality of the images produced depending on the lens used.

With 50D, being a cropped sensor camera, the lenses that are most interesting to use are Macros or fixed tele (200 f4). On 5D, being a full frame camera, one can use nikon wides, 50/1.4, vintage 85/1.4, etc. One interesting feature of EOS 50D/5DmkII is the hability to use live view function for fine tuning the focus (not for action photography). Another approach is to use hyperfocal and rely on auto-ISO function given the good high ISO performances of the cameras.