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 1

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 — see this answer or the Pentax hacks site if you want to go that route.
• 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.

Answer 2

If you Google "K10D back focus" then you'll find quite a few hits to it. It looks to me that the camera actually focussed behind, so that would indicate that problem and with an f/2.8 aperture, you're not giving a lot of depth to help compensate.

In any case, make sure your K10D is up to date on firmware and have a look at this thread on fixing the back focus on a K10D. It does require you entering into debug mode on the camera and then making some adjustments. Don't do that without some focus chart that can help you get it right. Normally, I would make dire warnings about warrantee at this point, but yours will probably be expired, either way do at your own risk. If all else fails, you may need to have it serviced and calibrated.

Answer 3

The comments about backfocus are right on several points. There are several roadblocks to being able to get good results with the adjustments.

First, tackle the issue with AF. Then you can handle other steps that may be necessary. I've been through these steps several times with my two K10D bodies.

You'll need a controlled environment with which to test AF. You'll also need your fastest auto focus lens you can get -- the 50mm f1.4 works very well for these tests. Then get a surface that has a texture that allows you to understand when you can try to focus on something and compare results. These are what the focus charts are useful to do. You can also use a ruler set at an angle. Also, you'll need to know the color temperature of the light you are using and make sure that you repeat this setting. It does make a difference if you are under incandescent light vs sunlight.

Next, you'll need to access the debug menu for your K10D. There are several documented ways to get to this feature. If you have the original firmware (1.0) then there are basic key presses to turn it on. There are ways of putting a file on an SD card and thus by editing the text in this file you can toggle it on and off. I personally use a piece of windows software that allows me to turn on and off the debug setting when I connect via USB. The software that I use has instructions available http://photographyrulez.blogspot.com/2008/06/debug-mode-with-latest-firmware-yeah.html>here. If you want to know about the file on an SD card method, http://www.pentax-hack.info/documents/debug.html>visit the Pentax Hack website. For what it's worth, you do not have to update your firmware to the latest version before doing this. In fact, if you have any SDM lenses in your collection, you'll want to not update past 1.30 so that you can flash back to a pre-SDM firmware to allow you to use these lenses with in-body AF motors (... this is another can of worms).

Once you've enabled the debug menu, start taking pictures of your focus chart. You'll use the debug settings to move the focus adjustment for the software. For me, I've found that under daylight (color temp > 4500 K) I use a setting of +140. For incandescent lighting (anything less than 3300K) I use a setting of +90. As I tend to shoot in different environments, I leave the debug menu on all the time, even though it will use the battery faster (Debug always shows on the LCD and thus it never goes blank).

Note that to really see if the AF adjustment is moving, you have to view the results on a full size screen. The camera LCD really doesn't have a good enough view to tell, especially when you're down to the last little bits.

As has been noted, these adjustments are for the body and will change the AF behavior for all lenses. If you have lenses that require a different setting, then you may have to remember this and change as necessary. I have been lucky in that all my lenses are able to use the same settings.

Once you've been able to get your AF working well, check under different lighting as noted above and learn what you'll want to set for when under that lighting. Note that if you use the flash-assist light from either the popup flash or a hotshoe flash, those will count as "daylight" temperatures. Otherwise, if you have dim environments that you'll be using flash, most likely you'll need the incandescent setting.

The reason why you'll need to have the different setting for the two types of lighting is that the AF software in most DSLR cameras is sensitive to light wavelengths in different ways. The good thing is that if you test properly, you'll know what to change to accommodate the light rather than be bound to just one setting.

Lastly, once you have your AF working well, take the time to learn what a scene looks like in the viewfinder once the camera thinks it's focused. If you see a scene that definitely shows your subject out of focus, then you may have to re-shim your focus screen. These shims are metal spacers that you can buy from Pentax that are in different thicknesses. By changing them, you move the focus screen slightly and thus what you see with your eyes you can then trust is actually in focus.

One final bit of information about the focus screen. The stock screen for these cameras (and many other DSLR cameras) show a DOF that is much bigger than what is really available. This is especially true when using lenses faster than f2.8. That can be the source of a lot of frustration when you believe your eyes and the camera shows otherwise. The solution for this is to get a different focus screen that is able to handle these lenses. These are available from many different suppliers. The main vendor for the high-quality ones is Katz Eye Optics. They can be a little expensive, but they make the difference!

Hope that this helps you in your efforts and congrats on owning a great camera! I have been shooting with mine since 2007 and still find new things to do with it.

Answer 4

Do the following:

• Create a text doc and name it modset
• Put the following in it [OPEN_DEBUG_MENU]
• Copy this to the root of your sd card
• Put the card in your camera
• Leave the card door open and turn on your camera
• When it says debug, switch to and close the door. You are now in debugmode.
• Go into menu setup and push the up key on the 4 way and you will see aftest

Play around with the plus or minus to see if this helps.

To exit debug just do the reverse.

Answer 5

I can't explain the particular problem but can affirm that I used to have the very same issue with my Pentax K100D. It always focused farther out then the subject. I found the problem was worse when I was using the Pentax prime 50mm f1.4. I eventually started to focus manually if I wanted a good shot and relied on the optics not the indicator light.

I wonder since Pentax AF uses use in-body motor whether there is some type of deterioration over time because I don't remember this being an issue when my Pentax body was new. Perhaps bumped it somewhere and that damaged the AF.

I have since sold all my Pentax gear and went Canon.