The K100D, K110D, and K10D all don't have TTL flash, meaning that the on-board flash will, as roy said, fire at full power with M and K lenses.
Your options are to either play around with the shutter speed and aperture until you get the most desirable lighting (and that takes ages), to use an auto or manual external flash, or to get an *ist D, which has full TTL suppor).
For more information on TTL, visit this thread:
https://www.pentaxforums.com/forums/pentax-dslr-faq/2345-pentax-dslrs-support-ttl-flash.html Adam
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