Here are some reasons why designed for film lenses may not work to their full potential on digital sensors:
"Each individual photosite (pixel) in a digital sensor literally sits at the bottom of a well with a tiny micro-lens on top of it. As light passes through a lens, it crosses over and begins to spread out again in order to focus on the sensor plane. In the center of the image frame, the light therefore strikes the sensor head on, but as we get further out from the center of the sensor the angle of incidence becomes more oblique. This means that light doesn't have a straight shot at a photosite near the edge of the frame, being partly blocked by the walls surrounding it. This results in diffraction at the edges of the photosite, which in turn produces light falloff and a form of chromatic aberration that appears as "purple fringing" along high contrast edges. Light strikes film at an angle near the edges too, but since film is a flat medium rather than a three-dimensional structure as is a digital sensor, the effect is rarely noticeable.
Fringing and light falloff are worse with full frame sensors since the edges are naturally further from the center. They are also much more common with wide angle lenses due to their large angle of view. Some lenses marked as digital have optical designs that help to minimize this problem by adding lens elements to straighten the light path out to a degree as it emerges from the rear element." from
Just What is a "Digital Lens"? - Photo Tips @ Earthbound Light