Originally posted by Christine Tham The understanding that I have (based on discussion in that other long thread) is that the AF has tendency for a very *slight* FF in low light.
So shooting at f4 masks out any inaccuracy in the AF, not that f4 by itself improves the accuracy of the AF. Of course, lenses are generally sharper when stopped, so that is also a factor.
Hope I have made that clear.
I didn't know about this and I am glad I learned about it. It makes sense. Assuming your focus point starts in front of your subject, at F1.8 or F2.8 you can imagine a thin depth of field wall moving toward your target as your camera is focusing. It stops at the first sign of an object to focus on.
Have you tried keeping your camera on AF.S (I have a K-5) and then manually focus your lens just before you take the shot? I don't know if this will work or not.