Originally posted by GUB I know there is a good argument upcoming but I am with steephill here. There is only two variables to input.
EDIT that is presuming you are using a modern Pentax (iso-invariant).
You are correct but...
...how people use their camera makes a big difference to whether they can do the same amplification post-production as increasing ISO does in camera. Shoot RAW in M mode, and only aperture and shutter speed matter.
Shoot jpg and use any mode where the camera is allowed to set parameters automatically there can be issues, as the camera will adjust shutter speed or aperture or both according to each other and the ISO setting which may not give the desired result, and in any mode, if you shoot an underexposed jpg, the camera discards a lot of raw image information so shadow information is lost.
With a RAW file, all the sensor information is there, so it doesn't matter whether you amplify it in camera or in post processing, but jpg discards a lot.
At 8 bit per channel jpg can only represent 256 brightness levels, whereas current Pentax models K-70 and up are 14 bits per channel, or 16,384 brightness levels, so what are actually 64 shades of grey in a raw file get compressed to just one shade of black in a jpg.
The problem doesn't mean lack of ISO invariance in the raw data, it's just how the camera compresses tonal range down when it converts to jpg, as it's effectively doing the post processing and discarding unused data before you get to it.
I've seen this effect with some of my images. RAW files have enabled recovery of usable images from what would have been total rejects as jpg, by increasing exposure in post processing.
I guess ISO on a modern camera is a bit like the notion of 'centrifugal force' which doesn't exist. There is no force flinging a spinning object outwards. It is actually just trying to continue in a straight line, and there is actually a force pulling inward preventing it from doing so, but the concept of 'centrifugal force' is convenient.
In the same way the concept of ISO increasing sensitivity isn't true in a physical sense but is a convenient concept, especially when it comes to automatic metering when producing jpg images.