Advantages of a lens with adjustable back focus? at DVinfo.net

Jay Rosenstein · December 31st, 2009, 01:10 PM

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.

Burk Webb · December 31st, 2009, 01:25 PM

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.

Brian Luce · January 3rd, 2010, 03:38 AM

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

Marc Colemont · January 4th, 2010, 03:27 AM

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

Kennedy Maxwell · January 4th, 2010, 04:44 PM

Quote:

Originally Posted by Jay Rosenstein

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.

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

Shaun Roemich · January 4th, 2010, 10:07 PM

Quote:

Originally Posted by Kennedy Maxwell

Once set, re-back-focusing should not be necessary.

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.

December 31st, 2009, 01:10 PM	#1
Jay Rosenstein Tourist Join Date: Jan 2008 Location: Champaign, IL Posts: 3	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.

January 4th, 2010, 03:27 AM	#4
Marc Colemont Major Player Join Date: Nov 2005 Location: Belgium Posts: 695	And you can use it also to film objects very close to the lens with a great DOF effect. __________________ Marc Colemont - Belgium - http://www.mc-productions.be JVC GY-HM850's, HM890, HM600

December 31st, 2009, 01:25 PM	#2
Burk Webb Major Player Join Date: Mar 2005 Location: Portland, Oregon Posts: 249	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.

January 3rd, 2010, 03:38 AM	#3
Brian Luce Inner Circle Join Date: Jan 2006 Posts: 2,290	I'd always assumed it was a consequence of having a removable lens.