I really like my Pentax K20D DSLR, but when I first got it the autofocus was not accurate, particularly with my favourite FA 50mm 1.4 lens. Fortunately the camera has an autofocus adjustment in the standard menu, which fixed the problem, but I recently discovered that there is a hidden ‘debug’ menu that offers even more control over autofocus. However the procedure to turn on the debug menu is a little fiddly, so I thought I’d document it here.
[Note that these instructions only apply to the K20D. Other Pentax and Samsung cameras also have a debug mode, but the instructions are slightly different for each camera – see the end of the post for links.]
Step 1: Create a text file called MODSET.442 in the root of your SD card. The file should contain the following single line:
You can use any plain text editor, for example Windows Notepad. Make sure that the file is called MODSET.442 and not MODSET.442.txt
Step 2: Put the SD card into the camera but leave the card door open.
Step 3: Hold down the Menu button and turn on the camera. Keep holding down the Menu button until the debug menu appears.
Step 4: Close the SD card door.
Step 5: To enable debug mode, press the right arrow to change Debug Mode from DIS to EN, then press the OK button.
Step 6: Press the MENU button to get to the standard camera menu. (If nothing happens, make sure you have closed the SD card door. If necessary, turn the camera off and on again after closing the door.)
Press the right arrow twice to get to the Setup menu:
This is just the standard Setup menu, but some new items have been added to the bottom. Press the up arrow to quickly jump to the bottom to see them.
Select AF TEST then click the right arrow button.
You can now set the global focus correction. In this screenshot the correction is set to –90 and I am about to increase it by 20 (making it –70). Press the OK button and you are ready to take a test shot with your new AF setting. (Some reports suggest that the camera must be restarted after setting the correction value, but it seems to apply immediately on my camera.)
Step 7: Once you have finished playing with the AF correction, you will want to turn off the debug menu. To do so, just turn the camera on and when the debug menu appears, set DEBUG MODE to DIS and click OK. Your camera should now be totally back to normal.
You can leave the MODSET.442 on your SD card and then re-enable the debug menu at any time by starting from step 2. Or you can just delete the file if you prefer.
If some part of the instructions doesn’t seem to be working, check your SD card door. For some steps it must be open, and others it must be closed – follow the instructions exactly as above.
The debug menu is also available on several other Pentax and Samsung cameras – see the pentax-hack site for details.
Also there are detailed instructions for the Pentax K-x here, which I found very useful, even though the steps are slightly different than the K20D.
After many many test shots I think I’ve settled on –90 as my correction value. That allows me to run my FA 50mm 1.4 with no ‘standard’ correction, and my other lenses seem happy too.