Below is Pentax's answer to my question, which is included under their reply.
"Dear Sir/Madam,
Thank you for contacting Pentax.
If you suspect a particular lens is showing front or back focus, it is a good idea (if you haven’t already) to download a focus test chart (
http://focustestchart.com/focus21.pdf) and shoot a few tests before sending the lens in to be checked out. Also, if you do decide to send the lens in to our shop for evaluation, it is very important to include the camera body as well (since any lens-specific focus adjustment information is stored in the camera body).
The following focus adjustment instructions are based on input from the Pentax engineering department, and our own experiences in house.
1. Use a focus test chart such as Tim Jackson’s “focustest” chart. Do not use batteries or film canisters set up at an angle. The curved surfaces don't allow for accuracy.
2. Make sure the chart is flat and that you are shooting the chart at a 45-degree angle (or close to it).
3. Use back-lit daylight or tungsten light for the best results.
4. Use a tripod. This will prevent any error due to camera movement.
5. Position the camera approximately 1.5 meters (5 feet) from the test chart. (This may vary of course depending on the lens focal length and minimum focus distance.
6. Set the camera to aperture priority or manual exposure mode and set the lens to its maximum aperture.
7.Shoot the first test shot using manual focus. This is important to establish a baseline for judging back or front focus in regards to the normal depth-of-field of the tested lens at a particular focus distance and f/stop.
****IMPORTANT: Lenses will typically have more depth of field behind the focus point than in front. This is true for any lens from any manufacturer, and is the basis of "hyper focal" charts used in landscape and macro photography. The first shot manually focused will establish a baseline for the particular lens in use****
8. For zoom lenses, set the lens at a middle zoom position or at the zoom position where front or back focus is suspected. Note: it is not possible to save multiple custom focus adjustments for different focal lengths of a zoom lens. Only 1 adjustment per lens can be saved.
In response to the question about what distance to shoot at, really anything over 5 feet is fine but you do not want to be in closer then that.
If you are in need of further assistance, please respond to this email or call our technical support center at 800-877-0155.
If you would like to participate in our new Pentax Survey, please call 1-800-350-3891.
Sincerely,
Pentax Imaging Technical Support
Your Email:
Inconsistent AF on K20D.
Only the center AF point is used in AF-S.
The camera seems to generally tend to "front-focus" when using a new lens that does not have any sort of adjusment in the camera stored for it.
The camera allows micro AF adjustments, here is where your info comes in.
To assist in adjusting the focus on my lenses i use focus test charts, you shoot a target at a 45 deg angle to be able to judge the amount of front or back focus.
Once a satisfactory adjustment seems to be found, i take the camera on a shoot and the actual shots seem to be either still front focusing or even back focused.
The amount of adjustment needed seems to vary.
I need to know if the use of a focus test chart at rather close distances is the recommended method to adjust focusing on the K20D with Pentax
lenses.
Or if you would prefer that adjustments are made at distant objects at infinity focus.
Further on a zoom lens, do you recommend making adjustments at the
"wide end", the "middle" or the "long end"?
Thank you in advance for your assistance, i am hoping the info you provide can help me achieve more consistent results, without the need to send in the camera with lenses for factory adjustments.
The lenses used are/were DA* 16-50 f/2.8, DA* 50-135 f/2.8, DA 12-24 f/4.
Thank you in advance for your time,
Daniel Goller"