Quote Originally Posted by zumbido View Post
This is not true of the more recent Pentax DSLRs, including the budget K-x. I just came inside from shooting origami boats in the park across the street with mine with the 50/2 SMC-Pentax.
Here is a Pentax K-x with its mount:
http://www.imaging-resource.com/PRODS/KX/ZURFRONT-L.JPG

Where is the coupling between the house and a K-lens aperture ring?

Compare to the picture attached below where i have added some red color to the coupling devices needed to be compatible with a K-mount lens.

Read also this statement at pentaximaging.com, that also supports what i am saying that Pentax K-mount only can be used in manual exposure mode using stop down metering. Only those lenses with an "A" on the aperture ring can be used in automatic modes. An ordinary K-lens has to be used in the same as an ancient M42 screw mount on new bodies.
http://support.pentaximaging.com/documents/908

Pentax is _not_ fully compatible with its old lenses and cannot use them in aperture priority as 30 year old cameras can.