Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
04-23-2014, 08:27 AM   #16
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 40,231
QuoteOriginally posted by monochrome Quote
Learn something every day. I have always been mystified by autofocus query/complaint threads since I rarely seem to have problems. Now I suppose I know why.
There is a very good article on the Web that describes the hows and whys in very understandable detail. Unfortunately, I always forget to bookmark it, but found a link on the Web this morning (thank you RiceHigh...oops! Did I say that?). It also addresses the similar limitations to focus screen microprism and split-image focus aides. It is enough to say that the inclusion of the three f/2.8 center point sensors on the K-3 is a big deal for people using faster AF lenses.

It might be asked why adaptation of the f/2.8 sensors has been slow. Good question. The linked article is almost a decade old and the last paragraph indicates that Canon had a minimal implementation on the 20D at that time. I can just imagine that there are technical issues.


04-26-2014, 10:59 AM   #17
Veteran Member

Join Date: Mar 2007
Location: U.K.
Posts: 685
Lenses don't always focus the same at all apertures due to aberrations, and yet AF is always done wide open, so there's another problem to add to f5.6 AF sensors inaccuracy with bright lenses.
04-26-2014, 12:14 PM   #18
Veteran Member
awaldram's Avatar

Join Date: May 2008
Location: Hampshire
Photos: Gallery
Posts: 732
QuoteOriginally posted by stevebrot Quote
The short answer is yes.

The long answer is complicated and has to do with how phase-detect AF (PDAF) systems work. Most of the AF sensor sites for most dSLR cameras have a working aperture of f/5.6. I say most because some cameras (the K-3 being the sole Pentax example) have at least some sensors optimized for f/2.8. The result is relatively poor precision when used with faster lenses at wider apertures.

Contrast-detect (CDAF) systems such as used in live view do not have this limitation and generally provide both greater precision and greater accuracy.

The k5ii series has a single f2.8 basined sensor the the K3 has 3 ... at least the canon 1ds has multiple f2.8 sensors.

As to contrast detect it a double edged sword.... because it works on contrast not phase the camera has no idea which way to focus so has to guess this means performance is so so and in moving targets generally no-no unless the lens is designed for contrast detect (no Pentax lens are).

Also as light drop contrast falls off faster than phase so under low light level CD will fail.

Both PD and CD and be fooled into false locks as they find a peak that is no 'in focus'

As they are both using differential between two points both can only be as accurate as the baseline in use. I have no idea what the base line is for CD off sensor but have not noticed any improved accuracy with Fast lens over PD once PD is calibrated.
04-30-2014, 12:27 AM   #19
Senior Member
jkomp316's Avatar

Join Date: Feb 2013
Photos: Gallery | Albums
Posts: 227
Collimation and divergence are a good read. Factors also change depending upon the temperature of the light your capturing. Manufacturing tolerances affect things as well. Crisp shots at F/1.4 with point and shoot operation is quite a feat. Its much easier when the lens is permanently attached to the camera...


  • 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, af systems struggle, answer, bit, camera, cameras, d7100, dslr, f1.8, focus, k30, photography, post, precision, shots, sony, steve, system, systems
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Apple iPhone the world's most popular camera? stormtech Photographic Industry and Professionals 89 05-29-2013 03:14 PM
The Worlds 11 Most Expensive Camera Lenses jogiba Non-Pentax Cameras: Canon, Nikon, etc. 4 08-23-2012 05:24 AM
Which AF mode do you use the most? FrancisK7 Pentax K-5 & K-5 II 13 06-15-2012 06:03 AM
Autofocus systems may be about to get extremely FAST! like 10 miliseconds fast photoleet Photographic Technique 26 01-26-2012 03:15 PM
Who's shooting with multiple systems? leeak Pentax DSLR Discussion 9 12-03-2008 07:53 PM

All times are GMT -7. The time now is 07:45 PM. | See also:, 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?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]