Originally posted by Eric Seavey Falk, Interesting theory about the AF aperture, but it wouldn't explain why it would change from bright to dim conditions. LV would focus accurately actually, but would not be useful trying to focus on dancing people. Also I always though the camera focuses with aperture wide open, at least that is what it seems to be doing. When I press the DOF preview, the viewfinder always gets darker unless I am trying to shoot wide open.
I think that's all still in line. In dim conditions, you shoot at wider apertures and phase AF has its own effective aperture which is about f/7 or so, allowing a f/5.6 lens to focus at the near distance.
Originally posted by Eric Seavey In very dim conditions (0 to 3EV), the camera behaves like, it is "lazy"; it focuses, locks and "says: good enough" while being not focused accurately, while the K10d and K7 would fine tune the focus a little.
That sounds odd. So I did some more tests myself, using an FA*31/1.8 which is Pentax made.
I can confirm your findings. At the edge where the AF assist light would come on (sometimes it did, sometimes not) and where it didn't, focus was missed. With LV and AF assist light, it was
much better (a difference like: can read the text easily to cannot even say it is text!).
Obviously, the K-5 firmware uses an a priory estimate of what minimal contrast(*) should be to have aquired acceptable focus which takes the amount of availabe light into account.
And obviously, this estimate is wrong. As seen by not even activating the AF assist light. The AF module "thinks" to see well while it is almost blind.
So, I withdraw my spherical aberration theory.
My preliminary assessment: A
firmware bug with wrong a priori thresholds or a failure to correct for lens aperture when assessing scene luminance. So, the AF module uses a too short AF module exposure time and disables AF assist light.
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(*) phase AF uses the correlation function between two halfs of the linear AF sensor data. This is a one dimensional contrast measure. The phase is the shifting distance between both halfs where this contrast is maximized. In low light, this contrast remains small if AF exposure time isn't increased (the AF "thinking" time).