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What is the difference between lenses with linear focus mechanism and lenses with non-linear focus mechanism?

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I guess linear would refer to lenses with a helical focus mechanism. This is your standard threaded brass or aluminum assembly that basically screws or unscrews to move the focusing elements forward and back. Pretty much like any manual focus stills lens. It is linear because you can't change the pitch of threads and therefore you can't change the focus throw of the lens or the spacing between the numbers unless you change the size of the lens barrel. Because of how optics work, the difference between far focusing distances would only require very minute movement and require progressively more movement for close focus. This can make manual focus pulling rather tricky.


Non-linear focus lenses don't use a helix, they use cams. This is basically a custom cut channel on one part, and a follower on the other part that moves though the channel. You can shape the channel so that the barrel requires more movement to travel a shorter distance for far focus and then ramps up in speed to travel a large distance with less movement for near focus. So the marks on the lens are more evenly spaced out while keeping the lens barrel relatively small if necessary. Cams are also smoother as there is less metal in direct contact so the focusing action of the lens requires less force. The most modern expensive cinema lenses generally use cams - Cooke S4, 5i, Anamorphic, Zeiss Master Primes, Master Anamorphics, Leica Summilux-C's, Angenieux Optimos, etc.

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