Are "old lenses" that predate digital cameras different in some way?
One issue relates to angle of incidence
- the angle at which rays strike the sensor.
Film responds to oblique rays (those that do not strike at right angles) proportionately to the cosine of the angle of incidence (which is measured with respect to arrival at right angles). This is one ingredient in the fall-off of effective exposure for off-axis parts of the scene (natural vignetting
), as well as a loss of effective exposure even for on-axis scene points in the case of a large-aperture lens..
With a typical digital sensor, the fall-off in response may be faster
than as the cosine of the angle, thus exacerbating the fall-off in effective exposure. The problem is most pronounced in wide-angle lenses.
To mitigate this, in lenses intended to be usable in digital cameras, design schemes are often used than decrease the maximum obliquity of the rays. These include the so-called quasi-telecentric
designs, in which the exit pupil of the lens is quite far forward.
The bottom line is that wide-angle lenses (in particular) not specially designed for use with digital cameras may exhibit more off-axis fall-off, and more "shortfall" of effective exposure at large apertures, than we might wish.
Another issue is that digital cameras may be more susceptible to internal reflections caused by rays bouncing between the sensor and various lens surfaces. For that reason, in lenses specifically intended to be usable with digital cameras, special attention may be given to anti-reflective coating and the curvature of critical lens surfaces.
For that reason, lenses not having that design attention, when used on a digital camera, may exhibit more loss of contrast (through "flare") or even overt "sun dogs" than when used on a film camera.