Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
08-17-2009, 04:56 PM   #1
Inactive Account




Join Date: Aug 2009
Location: Hawaii
Photos: Gallery
Posts: 117
Front/back focus or focus shift?

Hi All,

These days I experienced much focus issues with my new K-7 and FA 50 1.4 and DA 16-45. I searched the forum and saw others also have the same problem with these two lenses. Here after some research I got some hints to get sharp images with them.

Observations:

1. For AF tune -10 I still have front focus with these two lenses when shooting wide open;
2. But when stopping down to f2.8 for 50 and f5.6 for 16-45, I have the focus spot on. This was confirmed both by chart and real life shooting;
3. Stopping down more, they will back focus but it is OK if you have your object not so far away since the DOF is thick enough.

Indications:

1. If you google focus shift or spherical abbreviation, you may find some articles discussing focus backward shift when stopping down. This is exactly what's happening to K-7 and the two lenses mentioned above;
2. The problem is the image sensor is not consistent with the AF sensor. Image sensor seems to be a little further from the lens than it should be. This gives us a good focus when stopping down. Let's call it a threshold aperture for best focus. So if I have AF tune weaker than -10, I need to stop down more to get a good focus. A different case is my Nikon D90 with AF 50 1.4d. The similar focus shift exists but the solution is different that it get best focus at f1.4 and all back focus when stopping down. I don't know if Pentax did this on purpose to have best focus on best apertures, but it would be nice if we can tune the AF spot on when wide open and choose our best aperture. It just need a little offset! Maybe -5 is enough. Or if we can have a larger range of focus adj in next firmware I will be absolutely happy!

Pls test yours and see if you agree!

Thanks,
Tony

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!
af, aperture, focus, front/back focus, k-mount, lenses, pentax lens, sensor, shift, slr lens, spot, tune
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 focus, back focus or other? imfinetoday Pentax SLR Lens Discussion 23 07-08-2010 11:49 AM
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 02:51 AM. | See also: NikonForums.com, CanonForums.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