Originally Posted by FieldingMellish
The short answer is: yes, but.... You can use an old Nikon lens on a D-SLR but you have to factor in the fact that your D-SLR's sensor will be physically smaller than the piece of film (roughly 35 mm wide) that film cameras used. So, there is a 'cropping' which means, among other things, that an old Nikon wide-angle 20mm lens will not be so wide angle on your D-SLR. Depending on the 'crop factor' (ie how much smaller your sensor is than full 35mm), the focal length ofthat lens could, when used with your Nikon D40, be something more like 35mm. Of course, you could get round this by buying a 'full frame D-SLR' such as the Nikon D3, but these cost serious money, like 2.5K. This will change with time of course, and probably one day kids wil marvel at our fogeyish tales of less-than-full-frame sensor sizes.
Another major factor is teh risk of dust getting onto your sensor when you change lenses. This can be a real pain and unless you have one of the newer models, with inbuilt mechanisms to combat dust, you can change lenses a few times (even being careful) and find yourself getting any number of nasty black spots in the same places in every single photo. Not nice.
Not all lenses will work fully with all bodies, though. For example, with my Nikon if I coupled a lens that was fairly old and without the electronic feedback, the body won't be able to do proper metering and may not be able to autofocus.
I took photos of a cheetah in Africa and for some reason, no matter how carefully I cleaned my gear, the spots were always in the same place. The same thing happens on the leopards, too.