Advantages of a lens with adjustable back focus?
 

Hi all,

Like some of you, I have been burned on an HD project by a back focus problem. I didn't realize how frequently it has to be adjusted.

So here is my question: what are the ADVANTAGES of having a lens with a back focus adjustment versus a fixed lens? After all, the fixed lenses never have back focus problems.

thanks much.

Actually they do have back focus adjustments and back focus problems (don't ask me how I know this). For a lot of the cams this is set in a service menu and is difficult to do in the field. The blessing and curse of the HD100 etc is that you can easily set the back focus.... and you will probably have to be setting it a lot.

I'd always assumed it was a consequence of having a removable lens.

And you can use it also to film objects very close to the lens with a great DOF effect.

All camera lens can have recognizable back-focus problems. The purpose of a back-focus adjustment on a lens is to compensate for manufacturer tolerance error. The distance from the back element of a lens to the CCD or film plane is critical and the back-focus adjustment allows one to "zero" in on the exact distance necessary to maintain focus throughout the focus or zoom transition of the lens. Google LENS BACKFOCUS. There are a number of good explanations and instruction for proper alignment. Once set, re-back-focusing should not be necessary.
Ken

Back focusing is necessary if the lens and camera configuration has been subjected to a change in temperature, bumped in any way or otherwise affected by any action that could change the very precise distance between the lens elements and the imager.

Errata: Of course, subsequent adjustments SHOULD be very small compared to initial setup.