Steve's Digicams Forums

Steve's Digicams Forums (
-   Newbie Help (
-   -   Lens focusing (

conor May 7, 2008 9:29 PM

Alright, I've seen lenses that say "rear focusing" and I've seen lenses that say "internally focusing", and I'm guessing that there's more.

I did a bit of reading, and I think I understand the phsycial/mechanical differences, but I'm curious about the performance differences.

Anyone have a handle on an answer (or a forum/page that might answer my question)?

Thanks once again! :-)

TCav May 8, 2008 3:48 AM

Actually, 'rear focusing' and 'internal focusing' are the same thing.

A conventional lens will get longer as it focuses on a subject that is close to the camera, and get shorter as it focuses at infinity. This can make the lens harder to handle. 'Internal focus', or ocassionally, 'rear focus', moves interior elements to adjust the focus. It will still affect the balance, but only very slightly, but does not change the length of the lens.

BillDrew May 8, 2008 7:23 AM

In addition, an internal focusing lens does not rotate the end of the lens as it focuses. That allows the use of "pedal" lens hoods which are esp usefull on short lenses. Also the non rotation makes using a polarizer much easier.

conor May 8, 2008 10:03 AM

Other than length,blanace, lens rotation and ease of use benefits, are there any optical or performance benefits to either one (internal/rear vs "conventional")?

VTphotog May 8, 2008 8:38 PM

Some zoom type IF lenses change the effective focal length when focussing. This is mostly noticeable at close distances, where the framing is tight, and you can see the difference in image size when you change focus. I have a Tamron 28-200 which does this, and it can be annoying under the above conditions.


All times are GMT -5. The time now is 4:28 AM.