Originally posted by bdavis Also, if a lens is made for digital, aside from the adjustments in focal lengths I just mentioned (18 vs. 28 kits, 14 vs. 21), the lens won't have an aperture ring, and there is one other difference. Imagine a square (rectangle) circumscribed within a circle. The rectangle is your frame of film, or sensor inside your digital camera. The circle is the circle of light that can be thrown upon that frame. The digital 'square' is smaller than the film 'square', and the circle that encompasses the film square is big enough to cover both the digital and film squares, but not vice versa. Digital only lenses only have a 'circle' big enough to cover the digital frame, so theoretically they can be made smaller and lighter, and are unsuitable for film frames because they can't throw enough light to cover them completely.
This isn't always true. The D-FA Macro lenses are 'made for digital' but have aperture rings and can be used on 35mm film cameras. The DA40/2.8 can be used on 35mm film.
Many Sigma and Tamron 'made for digital' lenses likewise have aperture rings and can be used on film without vignetting.
It is a disturbing trend in general that lenses are losing aperture rings. I really think that soon no lenses will have aperture rings, mainly because even the lowest camera models have automation to control it. This will come to pass for 'full-frame' and cropped sensors equally.
To the OP - focal length is focal length, period. If a lens is listed as 50mm, it doesn't matter if it is made for medium format, 35mm film, or digital - it will give you the same view
on your camera.