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bavd · 01-24-2015, 01:39 PM

When using focus adjust for a front focussing tamron lens, I stumbled acros the the fact that although I can perfectly correct the lens using focus adjust,
it only needs to be done when focussing via the view finder. ie when the camera is using phase detect .
If I use live view the camera and lens focus perfect .
Could anyone explain? I am aware that the 2 focusing systems differ but I was under the impression that front or back focussing was a lens/body maladjustment that should
exist no mater the focus system used?

Thx in advance.

Adam · 01-24-2015, 01:42 PM

Front and back focus only applies to viewfinder AF. Here's an explanation why:
Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles

bavd · 01-24-2015, 01:44 PM

Originally posted by Adam

Front and back focus only applies to viewfinder AF. Here's an explanation why:
Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles

Wow that is quick. Thank you!

aleonx3 · 01-24-2015, 03:31 PM

As far as I know, Live view focusing use contrast detect AF system (CDAF) which is different from PDAF using viewfinder. So, it is conceivable that you may have FF or BF issue using PDAF.

jawsy · 01-24-2015, 03:41 PM

The viewfinder prism might need to be shimmed. If you're using a split prism, I'd put money on it. There are plenty of threads you can look up describing the issue and fix if this happens to be the case.

DeadJohn · 01-24-2015, 06:09 PM

CDAF uses the imaging sensor to focus. There's no front- or back-focus issues because of that.

Phase detect (or manual focusing in the viewfinder) follows a different optical path and calibration can be off.

01-24-2015, 01:39 PM	#1
bavd Forum Member Join Date: Jan 2015 Location: Almere (NL) Posts: 96	diffrence between focussing in lv and phase detect When using focus adjust for a front focussing tamron lens, I stumbled acros the the fact that although I can perfectly correct the lens using focus adjust, it only needs to be done when focussing via the view finder. ie when the camera is using phase detect . If I use live view the camera and lens focus perfect . Could anyone explain? I am aware that the 2 focusing systems differ but I was under the impression that front or back focussing was a lens/body maladjustment that should exist no mater the focus system used? Thx in advance.

01-24-2015, 01:42 PM	#2
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,608	Front and back focus only applies to viewfinder AF. Here's an explanation why: Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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01-24-2015, 01:44 PM	#3
bavd Forum Member Join Date: Jan 2015 Location: Almere (NL) Posts: 96 Original Poster	Originally posted by Adam Front and back focus only applies to viewfinder AF. Here's an explanation why: Fixing Front and Back Focus - Causes of Front- and Back-Focus - In-Depth Articles Wow that is quick. Thank you!

01-24-2015, 03:31 PM	#4
aleonx3 Veteran Member Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,996	As far as I know, Live view focusing use contrast detect AF system (CDAF) which is different from PDAF using viewfinder. So, it is conceivable that you may have FF or BF issue using PDAF.

01-24-2015, 03:41 PM	#5
jawsy Veteran Member Join Date: Mar 2009 Location: Australia Photos: Gallery \| Albums Posts: 422	The viewfinder prism might need to be shimmed. If you're using a split prism, I'd put money on it. There are plenty of threads you can look up describing the issue and fix if this happens to be the case.

01-24-2015, 06:09 PM	#6
DeadJohn Pentaxian Join Date: Jan 2011 Location: New York Posts: 4,834	CDAF uses the imaging sensor to focus. There's no front- or back-focus issues because of that. Phase detect (or manual focusing in the viewfinder) follows a different optical path and calibration can be off.

diffrence between focussing in lv and phase detect

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