Why am I getting poor AF results with my Pentax K10D?

Question

I've been using Pentax K10D for a while and I'm a bit frustrated with performance of auto focus on that camera. Very often the images are not sharp enough despite taken in with decent lens (e.g. Pentax DA Limited a.k.a. "pancake") in great outdoor bright conditions. I experience that in different modes (P, Av, Tv) and with various auto focus modes (auto, selective, center).

Can anyone explain to me why I'm getting images out of focus on my camera? Am I doing something wrong?

To focus (sic!) the question a bit please have a look at the picture of my 18 months old toddler boy I took yesterday. A couple of facts about that photo shoot:

• the little lad was still (that is very rare in his case),
• light was fantastic (summer evening, sun was quite high, clear sky),
• I used Pentax DA 70mm Limited lens,
• camera was in Av mode and I set aperture to 2.4 (max value for the lens),
• expose time was 1/1000 sec
• AF was in selective mode and pointed to focus on my son (right, slightly below center axis).

As you can see the resulting image is out of focus. Any ideas what could went wrong?

Answer

In your sample picture, one can see that the grass behind the child is in focus. There's several reasons this might happen. In this specific case, I think these are the most likely:

• The AF system may be out of alignment with this lens. That's fairly common (for any camera and lens) and you'll find a lot of fussing about it online. Unfortunately the K10D, unlike newer Pentax models in that range, has no easy way for you to adjust this yourself. Pentax will do it for free (once) under warranty, although it's unlikely that a K10D is still covered. (If the lens is new, maybe give them a call and find out.) There's also a secret debug menu which lets you make adjustments, but only on older K10D firmwares, and only overall, not per-lens.
• The AF sensor may cover more than you think it does. All AF sensors are larger than the indicator dots in the viewfinder, sometimes by a surprising amount. And they're not always centered on the red dots, either. (This is true of all systems, not just Pentax.) You can experiment with this with a focus target with a clear, contrasty edge. Put this close, and put the camera on AF-C and move slowly across the edge. This should help find where the edge is. In my experience with the K10D, if the focus sensor covers something near and far, it will always grab the back one. That can be really frustrating. (The AF system in my newer K-7 doesn't seem to do the same, thank goodness.)

Things you can do about it:

• When you have good light like this, use a smaller aperture for more depth of field. Less dramatic bokeh, but easier to be sure your subject is sharp.
• Manual focus. It's a bit hard when you're using a viewfinder screen meant for AF, though, and hard with a speedy kid.
• Try moving the composition a little bit so you make sure there's no way the focus point could also encompass the background. Lock focus and recompose.
• Use the quickshift feature of your lens to pull in the focus. The behavior is probably relatively consistent. The Pentax DA Limited lenses feature a clever clutch system where you can AF and then tweak the results manually. Since in situations like this you can reasonably guess that the camera grabbed the background to focus on, AF and then adjust slightly nearer.

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