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Forum: Pentax DSLR Discussion
09-12-2007, 04:11 AM
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As others have said, both film and digital lenses will act the same way at the same focal length on the same camera as far as the focal length multiplier is concerned. The focal length multiplier is a factor of the camera; the distance from the lens to the internal sensor or film plain. Therefore, to see a difference, you would need to place one lens on a film camera and the other on a digital camera, set both to the same focal length number, and then compare the results.
However, disregarding that (and keeping this very simple so as to not overwhelm you with too much info), there are slight differences between a digital and film lens when it comes to how well they focus on that sensor or film plain. Film lenses were designed to focus on a point where film is located in a film camera, which several millimeters behind the point where digital sensors are located today in digital cameras. Hyprid lenses (those supposedly optimized for both film and digital) split the difference and focus at a point midway between a digital sensor and the old film location. The new digital lenses give up on film entirely and focus solely on the location of digital sensors today.
Of course, you're not going to see any of this in your images because it's only a matter of millimeters, with any minor focusing differences well within the depth of field of a particular lens and camera combination. Someone running tests with a focusing chart using a very narrow depth of field might notice it, but it very seldom matters in the real world.
However, a digital lens often does benefit a digital camera since this combination can often focus more quickly and more accurately. This is caused by the fact that the len's focus point is directly on the digital sensor (clearest and sharpest at that point, hence the camera can find it quicker) and the internal components inside a smaller lens usually travel less distances to reach the focus point. A side benefit to you is that a digital lens can be built shorter and lose some of the excess glass once used for the larger 35mm film. In other words, shorter, smaller, lighter, lenses which tend to focus slightly quicker on a digital camera.
stewart
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