Using an M42 lens with a digital camera is likely to present many problems. The most obvious one is that an M42 lens is designed to focus an image onto a 35mm film frame, whereas digital cameras normally require a lens to focus the image onto a far smaller CCD array. Additionally you'd lose the metering functions of the camera, so that the manual settings would have to be used, together with a light meter.
See here:
http://www.techdot.eu...arison-by-dslr-brand/
and 'Use on modern cameras' here:
http://en.wikipedia.org/wiki/M42_lens_mount
Chris