speedlight question

Hi, just picked up an AF540, until now I've used a vivitar that did not support P-TTL. Always used manual. Now I've been experimenting with the 540 and P-TTL. With my K5 set to manual and if I have my shutter at 1/90th of a second and shoot a sequence of pictures changing my aperture setting say from 2.8 to 4.5 to 8.0 to 22 ETC shouldn't the 540 track this and increase the power so that all the shots are exposed consistently? It does not, once I hit 8.0 and up each picture is considerably more under exposed. I've tried changing the K5 from manual to AV mode but the results are the same. Also, a second question, when I am using the 540 in manual mode and I shoot a sequence of pictures each time decreasing the power of the 540 from 1/1 to 1/2 and so forth down to 1/64th my exposure is consistent and the guide always indicates a distance of 20 feet. Am I so lame as to not understand some basic principles? I have read booth manuals and a bunch of stuff online and still I am totally confused. Any suggestions would be appreciated. Thanks in advance

But what iso number are you using i set mine to 100 to 400 max, i think auto doesnt help.
setting x(flash x-sync mode) stop auto iso being used..

My iso is set at 200

In your first question, are you not assuming the 540 has sufficient power to cover the output required for the smaller apertures. Perhaps it does not?

You will find this helpful in understanding Pentax P-TTL flash.

Matt is an admin here and I'm sure he'll be along soon to explain all.

Barrie, yes I am assuming that it has the power because in my second question, I am shooting the exact same scene, distance etc. but in manual and even at 1/64th power my scene is exposed very well.

Peter, you took the words right out of my keyboard

Just ordered Matt's P-TTL guide, hopefully it will answer my questions