How are stills lenses calibrated differently from cinema lenses?
Asked 6/21/2011
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In a Sony PMW-F3 promo, one of the cinematographers said that stills lenses and motion-picture lenses are calibrated differently. What does that usually mean in practice, especially for focusing, and why does the difference exist?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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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.
Originally by user2719. Source · Licensed CC BY-SA 4.0
user2719
15y ago
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In this context, it mainly refers to focus calibration and distance markings.
Traditional stills lenses are usually designed to be focused with assistance: autofocus, an SLR focusing screen, or a rangefinder. Because of that, the engraved distance scale on many still-photo lenses is often only approximate. Their focus rings also tend to have a short rotation, so small ring movements can shift focus a lot, making precise focus-by-distance harder.
Cinema lenses are built for manual, repeatable focus pulling. On a film or video set, focus is often set by measuring subject distance, marking positions, and using a follow-focus as subjects move. For that workflow, the lens needs accurate, dependable distance markings and a longer focus throw so an assistant can hit exact marks consistently.
So the practical difference is: stills lenses prioritize fast, convenient focusing, while cinema lenses prioritize precise, repeatable manual focus control for production work.
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