Originally posted by asahi man the sensorquality is at the highest point at 100 iso
First of all it is great seeing you post at pentaxforums!
Dpreview's forum is too bad for me to go there often.
It is not true, though, that "sensor quality" is always best at ISO 100. It depends on the sensor what its minimum ISO is, i.e., where it delivers maximum DR (dynamic range).
The ISO figures Ogl quoted have been measured by DxO Labs. They are not fake manufacturer EXIF figures or the result of "software ISO"; they actually indicate an increase in DR.
Having a (real) ISO setting lower than ISO 100 can be helpful to work around the speed sync limitation when using standard flash. An ISO setting of 50 would be equivalent to raising the shutter speed from 1/180s to 1/360s in terms of exposure. This can be useful at times when one cannot go faster than 1/180s (= the sync speed).