The dimensions of the rings are smooth and allow a gradual elongation. These extensions tubes for macro work perfectly. The loss of diaphragm is normal, you have to use a lens with an aperture ring. Inexperienced photographers who have not known non autofocus targets will be a little disoriented, but everything can be learned. Pentax I have for 30 years, and for the macro I take a 80mm lens with manual focus and manual iris. It is more accurate and provides greater satisfaction in the end. Pentax is (with nikon) one of the few brands to have retained the same lens bayonets, allowing continuous Usage requirements accessories. It's backward compatibility. There are also tube-extension tubes of any automatic Pentax brand, but the price is not the same. You should know what you want.
