Backfocus?

Paulo Eduardo Uchoa · October 1, 2009

I was wondering if someone could explain why there is back focus on HD but not on Film.

Thanks

Chris Keth · October 1, 2009

I was wondering if someone could explain why there is back focus on HD but not on Film.

Thanks

My understanding is that it's the materials used, combined with construction methods.

Video cameras- the sensor itself even- are made of fairly light weight materials that expand and contract quite a lot by optical standards with weather changes. A heavier lens mount goes a long way; the panavised f900s require much less frequent backfocus resets than the stock cameras do.

The equivalent parts of film cameras are heavy chunks of solid steel or aluminum which don't expand or contract enough to require constant recalibration. Film itself doesn't significantly enough change with weather, either.

Satsuki Murashige · October 1, 2009

Chris pretty much summed it up.

Remember also that electronic cameras generate a lot of heat, which causes the metal in the lens mounts to expand. There's not as much heat generated in a film camera if the camera is not actually running because the insides are mostly mechanical and are usually at rest.

Also, there's more variety in build quality with modern HD cameras (and there are many more of them) as compared to modern film cameras. You can bet that of the hundreds of modern Aatons, Arris, and Panaflexes out there that each mount has been built with the best materials available, with great precision, and with a high level of quality control. It's not as likely that the lens mount of every one of the thousands of HD cameras out there was put together with the same level of care. Most of them cost a lot less than an Arri or Aaton.

On the other hand - one time during a camera checkout, I was pulled aside by a rental house owner who had just become the proud owner of an F35. He excitedly showed me the heavy steel plate (Anvar!) holding the lens mount, saying it was designed to withstand the torque of an unsupported 25lb. lens at rest. So presumably on an F35, once the mount is collimated, it's solid. So not all HD cameras are equal.

John Sprung · October 1, 2009

Scale is another factor. The super-35 image is 2.5 times wider and higher than a 2/3" image. So, the tolerances can be 2.5 times looser with the same result.

-- J.S.