Well just wanted to give my butter on the subject.
Lets look at what the different steps are and why ISO on digital cameras do make sense and that some types do not. Here is a simplified way of object to data. And a quick overview of the factors that thereof influence the image quality in 3 Steps:
Object ---> Medium(usually air) ------------------------------> Lens -------------------> filter on sensor for IR/UV ---> micro lenses --->Bayer Filter-------------------------------------------> Sensor --------->
-------------|Dust, Humidity, Temperature differences… ---| Optical errors, etc. ---| Filters out unwanted light ----|------------------- | Lowers luminosity and sharpness for colour--- |
---> Amplifier on each pixel(Mostly CMOS) --------------------------> Amplifier after processor -------------------------- ----------------------------------> Analogue processing and conversion to digital signal converter--------->
-----|First Factor for A-ISO, k-1 ISO happens here I assume ----- | This is the K-1 mark II ISO improvements happen I assume -----------------| Creates NOISE!!!!!
|||||| This ISO Amplification makes sense since it is before new Noise gets generated |||||
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--> Real RAW data ---> Magic Processing -------------------------------------------------------------------------------------------------- ----------------> RAW FILE -------------> Conversion to JPEG
-----------------------------| Here a lot of magic proprietary stuff and the ISO multiplication that can be done theoretically in post happens--|---------------------------- | Loss in DATA over and over in each save.
So why is the PENTAX almost ISO invariant, well digital cameras have improved a lot in these years and the Noise generated post sensor, has become relatively low. Also programs like photoshop apply noise filters in background that hides noise, that is why you will get similar results results shooting at iso 100. But ISO Analogue amplification still make a lot of sense as it avoids a lot of later noise.
I from experience in experiments outside of fotography know how important such early amplification of low signals can help with a lot better cleaner results.
Please also look at this article:
Is ISO Fake? | DSLRBodies | Thom Hogan