[006]

I installed smc-pentax-k 50/1.4 in my k-5 body. how can I force the popup flash to cooperate with this one?

I used to use m42 lenses with my older camera - canon eos550d. there is a special button, that makes test flash to evaluate the reasonable value of next flash according to lens aperture that can not be measured other way (because it's manual), but there is no such button in k-5.

is there any way to make popup flash work fine when using manual aperture lens in k-5?

[sterretje]

Yes, just calculate the aperture to use, based on guide number, iso and subject distance.

See page 185 of the manual.

[eddie1960]

on manual lenses the Flash is full power, no auto setting, so you need to set the iso/aperture appropriately as mentioned above