are lenses interchangeable from the old 35mil. cameras

Olympus digital cameras use ' Four Third ' lens mount which is smaller than the lens mount of the old 35mm cameras.

If you have old Olympus lenses and want to use it, you will need to buy an adapter as shown at this site

If you stick with the same manufacturer there is often a lens mount adapter, if the lens mount is not the same. There is a further consideration in the usage of the lens. Most digital SLR's use a sensor that is smaller than those on 35mm SLR's, this means that they will only capture a percentage of the image cast by the lens, effectively increasing the focal length of the lens. Depending on the size of the sensor used, the focal length could be increased by a factor of 1.5 - 2.0x it's 35mm equivalent, so a 50mm prime lens may equate to 75mm or 100mm on your new body. There are a few high end cameras that use a "full frame" sensor, the same size as 35mm film, such as the Nikon D3 and D700 or the Canon EOS 1D and 5D, but for the majority you will have to factor in a lens mulitplier to determine the new focal length. This is not necessarily a bad thing, as the edges of images are the areas more prone to problems such as distortion, vignetting, softness, etc., and because the smaller sensor will only capture the center of the image, you will avoid most such problems. It also has an advantage at the telephoto end of the focal range, because what may have been a 400mm lens on a 35mm body may be effectively a 600mm plus lens on your new body, thus saving on the greater expense of longer focal length telephoto lenses. The disadvantage is obviously at the wide angle end, where your once wide angle 20mm lens, becomes a more standard 30-40mm.

Regards,

Corey
http://captivecameras.com

In case you are thinking of using dSLR lenses for old 35mm SLRs, you will find that the light circle will not cover the whole area of the 35mm film.

The end result is the corners of the film will be dark. Moreover, there might be difficulties in finding an adapter for old SLRs to use lenses meant for digital SLRs.