digi camera ccd is smaller than 35mm ... and because it's mounted at a different distance the focal plane is out
this means that if it fits - the focus ring is wrong ... but more important the f stops are out - ok in auto mode ... but difficult if you want manual aperture
the ccd size difference might also cause distortion because the back lens isn't at the right distance.
AC, you are incorrect. The ccd is at the same distance but as it is smaller it only sees the central portion of the image through a 'normal' lens. Some manufacturers ccds are same size as film (e.g. professional Nikon).