Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
02-12-2009, 12:41 AM   #1
Veteran Member




Join Date: Jan 2009
Location: Frozen white North
Photos: Albums
Posts: 845
Front focus/back focus

My undersdanding of front and back focusing is that the lens focuses at a point slightly in front or behind the intended subject.

I was always under the impression that the autofocus is controlled by the camera, so how is front/back focusing even a "problem"?

02-12-2009, 08:27 AM   #2
Senior Member




Join Date: Oct 2006
Location: Montreal
Photos: Gallery
Posts: 179
It's not a problem if both the camera and the lens are perfect. Focusing problems are by definition the result of an engineering error or mechanical defect.

But even if there is no actual defect, engineering tolerances are sometimes wider than we want them to be. A tiny fraction of a millimetre too far this way or that way, and you have a machine that will be consistently just a little off.
03-22-2009, 11:06 PM   #3
Veteran Member




Join Date: Jan 2009
Location: Frozen white North
Photos: Albums
Posts: 845
Original Poster
Sorry, I'm still a bit confused by this.

The camera uses some sort of digital logic to determine when the subject is in focus. If it is adjusting the lens until it is in focus, how can it end up being front/back focused?

Sorry for the newbishness
03-23-2009, 11:23 AM   #4
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
The camera might think the subject is in focus but it really isn't - that would happen if the AF sensor (the thing that detects whether things are in focus or not) is not exactly the same distance from the lens that the main sensor is. On most P&S cameras, the main sensor *is* the AF sensor, so this doesn't happen, but in a DSLR, the main sensor is at the back of the camera and the AF sensor is at the bottom, with the mirror used to take some of the light that otherwise goes up into the viewfinder an redirect it down to the AF sensor.

There are other possible causes too, but that's the easiest to explain.

03-23-2009, 12:03 PM   #5
Veteran Member




Join Date: Jan 2009
Location: Frozen white North
Photos: Albums
Posts: 845
Original Poster
QuoteOriginally posted by Marc Sabatella Quote
The camera might think the subject is in focus but it really isn't - that would happen if the AF sensor (the thing that detects whether things are in focus or not) is not exactly the same distance from the lens that the main sensor is. On most P&S cameras, the main sensor *is* the AF sensor, so this doesn't happen, but in a DSLR, the main sensor is at the back of the camera and the AF sensor is at the bottom, with the mirror used to take some of the light that otherwise goes up into the viewfinder an redirect it down to the AF sensor.

There are other possible causes too, but that's the easiest to explain.
Thanks for clarifying.. so it is the body that is more likely to cause front/back focus issues than the lens?
03-23-2009, 03:41 PM   #6
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
The scenario I jus described is definitely a body issue. But there is also the issue of commuinicatio between the body and lens, where the body tells the lens how far to turn to focus itself, and if the lens over or undershoots, that could be a lens problem. of course the camera is "supposed" to double check and be sure the image really is in focus after the lens focuses itself, but it seems it doesn't necessarily check that well.

The whole subject is pretty complex, really, and most people (myself included) who have attempted to test for focus problems end up doing little more than confusing themselves.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, front, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Front focus and back focus K-5 allantois Pentax K-5 48 03-31-2012 09:21 PM
Trying to understand front focus/back focus thing 41ants Troubleshooting and Beginner Help 16 08-31-2010 08:42 AM
Front/back focus or focus shift? fulcrumx29 Pentax SLR Lens Discussion 0 08-17-2009 04:56 PM
Front focus/ back focus on Manual lens. Possible? WangJianWei Troubleshooting and Beginner Help 6 04-20-2009 07:50 PM
How to check Back Focus/Front Focus grishazzz Troubleshooting and Beginner Help 14 03-18-2009 11:35 AM



All times are GMT -7. The time now is 03:51 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top