I was watching this promo video about the new Sony PMW-F3 and one of the DOPs mentioned that stills and motion picture lenses are calibrated differently. This is around the 4 minute mark in the video. Does anyone know what the differences are and why they exist?
Cheers
In that particular instance, the speaker is talking about the manual focus-by-distance capability. Stills cameras have always tended to be focus-assisted (whether by SLR screens, rangefinders, or autofocus), while professional cinema (and, by extension, high-end video) has been done my measuring tape, marks on the set, and follow-focus for moving subjects.
The focus distance markings on stills lenses have traditionally been "close enough" at best -- mostly because the focus ring doesn't rotate very far, so there's a distinct difference in focus point between the beginning of the scale marking, the middle, and the end. If you have any lenses hanging about with a focus scale on them, try focusing to 4 feet 8 inches without using a focus aide (autofocus or your SLRs focusing screen). Then smoothly move the focus to, say, nine feet six inches, following a walking subject and keeping the subject in focus.
Cine lenses (particularly the PL mount lenses mentioned in the video) tend to have a very long focus ring travel, and the focus gearing is such that it's more linear than a typical still camera lens. (The focus cam spiral is tighter at one end of the lens's travel than at the other.) Because of that, the distance markings can be more precise (the width of the line doesn't cover an appreciable focus distance) and the markings can indicate much smaller differences in distance. That means that a focus puller (the cine equivalent of autofocus -- it's a person rather than a machine) can accurately and smoothly track moving subjects, or accurately shift focus from one subject to another in the frame.
You can get PL mount lenses adapted for a stills camera, but they'll drive you batty. Manual focus with a cine lens is accurate, but getting from infinity to the close focus point takes nearly a complete turn of the focus ring. Similarly, trying to use a still lens for video/cine means either finicky, unreliable focusing or and additional down-gearing apparatus for follow-focus that's subject to slippage and loss of registration on any preset witness marks you make on the dial during a dry run.
