Originally posted by flomat
It would be interesting to have dragonlord’s profound analysis reviewed and supplemented with regard to the latest AF developments as seen in the in K-3/II. At least the size of the AF points was obviously overhauled.
As you wish...
The Pentax K-3 introduces a new autofocus system SAFOX 11, bringing a new AF array to the Pentax K system for the first time in more than a decade. A quick-and-dirty comparison, not to scale, is attached. (For SAFOX 11, the bold lines in the center indicate f/2.8 high-precision points. For the EOS 5D and 5D Mark II, the lighter lines and open circles represent Assist AF points that are invisible to the user and used only with AI Servo AF.)
However, the new AF sensor is not the only major change. The K-3 also introduces the Pentax Real-Time Scene Analysis System (RTSAS), which combines the PRIME III image processor and a new 86k pixel RGB metering sensor to perform intelligent scene analysis, metering, and subject tracking. This system provides functionality similar to Canon's EOS iTR AF and therefore has the potential to dramatically increase subject tracking performance.
In what would initially seem to be a regression from previous autofocus systems, only one point will light up in the viewfinder at a time when focusing. However, when AF.C with Expanded Area AF is in use, the AF sensor will collect distance information from other points around the selected point and the camera will use this information in tandem with the image from the 86k pixel RGB metering sensor. The result is a significant improvement in subject tracking performance.
The K-3 II has further AF algorithm improvements providing a measurable increase in tracking accuracy and reliability with subjects moving directly towards the camera. While the behavior of the AF system is still less than ideal because it still only confirms focus on one point, the K-3 II (and to a lesser degree, the K-3) is much more tenacious in locking onto and tracking fast-moving subjects than cameras with SAFOX X and earlier AF systems.
Introduced in the K-30 and its SAFOX IXi+ AF system is Hold AF Status, which allows the camera to ignore obstructions when tracking a moving target. This feature is of limited effectiveness in cameras without RTSAS as it can only use distance information from a limited set of AF points. However, in the K-3 and K-3 II, Hold AF Status is much more effective, as the camera now has distance information from a much denser grid of AF points as well as an image from the metering sensor.
Ricoh has yet to leverage the full potential of the new AF and metering hardware. They still need to expand the AF grid to cover a wider area, add more AF points, and further tweak the AF algorithms. SAFOX 11 and the Real-Time Scene Analysis System nonetheless represent a significant step in the right direction, and my (limited) experience with the autofocus system on the K-3 II does indicate a dramatic improvement over the K-5.
—DragonLord