Musings on a few topics ...
The K5 has an ISO 80 setting. What does that really mean? Just my opinion but I would think it goes something like this. ISO is an abbreviation for International Standards Organization. In olden days we used to talk about ASA which I think stood for American Standards Association. Now some cameras have sensitivity settings like L1, L2, H1, H2, but the K5 has ISO 80. Perhaps that is because Pentax is able to use the ISO trademark since the K5 does comply with the complete definition of ISO 80. That means, within some tolerance, it is equivalent to an ISO standard for film grain and sensitivity documented as standard number 80. Other manufacturers use things like L1, L2 simply because these sensitivities do not comply with an ISO standard, therefore they are unable to use the ISO trademark.
Noise: Our camera club had a discussion about "native ISO". To me this makes no sense. Now my electronics education was when the vacuum tube was king, but pretty well in any amplification system, the more gain, the more noise. With semi-conductor sensors I expect the least noise occurs when the amplification gain is lowest and in the K5 that is at ISO 80. If that is what is meant by native, OK.
This discussion went on to claim what I think is another misconception; that mid ISO settings are interpolated and it is better to shoot 1/3 stop lower than an integer multiplier of native than it is to shoot 1/3 stop higher. In other words. since the native ISO of the K5 is 80 and the next integer multiplier is 160, it less noisy to shoot at ISO 125 than ISO 100 etc. This rumor may have come from some video/Technicolor issue, but IMO I call it Kazunga when it comes to still shot cameras. So I did a test...
Completely dark room, lens and viewfinder caps on, and the camera covered by a black lightproof blanket. Let's say about -8ev (who knows, it was too dark to see). The process is shoot, increase sensitivity by 1/3 stop, shoot again. Then take all those files (in DNG format) and import them into photoshop as layers. Apply a threshold adjustment layer (set at level 1) at the top of the layer stack, and make 4 101x101 pixel samples around the middle of the frame. Then average the RGB levels from all 4 samples and plot the graph.
This is what I got. A couple of "shelves" in the curve, but nothing other than what I would call a sample anomaly. The results I got, are pretty much what I expected.
Thoughts? Comments? By the way, although I don't have the curve to show, a friend did the same thing with his Nikon D7100. The K5 was less noisy by far.