K5 - ISO 80

You knew what I meant. Speaking of amplifiers, as we are, I always understood that the noise and distortion of a transistor amplifier was constant. hence a disproportionate problem at low levels. This appears as hiss in audio and seems to be the case.

By comparison, a valve amplifier will distort more as the volume increases. I don't see a Pentax DSLR with valves though, interesting as the concept might be. We'd probably have to call it Full Frame Plus and a cart to carry it provided.

1. Turning the "volume down" does not work since once the sensels have reached their full-well capacity you cannot save anything anymore by using less than a unity-gain in A/D conversion.

Any ISO settings that are lower than the sensor's native ISO are achieved by overexposing the image during the shot and downscaling it digitally later. The tell-tale sign of doing so is a loss of dynamic range (due to the downscaling).

2. There is no such thing as a common "native ISO" for all Pentax cameras. The native ISO is determined by the sensor used.

The K-5's Sony sensor probably has a native ISO of ~100 (if not 80), but certainly not ISO 200. Otherwise, you would not see an increase of dynamic range up until ISO 80. Pentax may have employed some special shadow noise reduction to boost measured DR figures for the ISO 80 setting (that isn't used by neither Sony nor Nikon) but a) that is just a speculation unless proven, and b) it seems impossible to achieve the K-5's ISO->DR graph (see DxOMark) with a sensor that only has a native ISO of 200.