Originally posted by Marc Sabatella As other have said, no, you can't change ISO after the fact, any more than you can change shutter speed or aperture, or turn on SR.
But also, you should be aware if the camera was choosing a high ISO when set on Auto ISO, it probably needed to. Most likely, light was low, and shutter speeds would have been too slow had the camera not chosen a high ISO. If you had set the auto ISO range to 100-400, most likely you'd have simply ended up with blurry shots at ISO 400.
Marc is absolutely right.
Perhaps a little more explanation will help you better understand your camera and raw files.
Basically there are 3 variables you can manipulate to change your camera's exposure:
1. Shutter speed (slower = more blur), can be on purpose for showing motion.
For instance when you pan your camera while following a moving object.
Otherwise people want the shutterspeed to be such that pictures are not blurred by camera movement.
2. Aperture, the more closed (= higher number = smaller opening) the more of your picture will be in focus.
The wider the aperture is open, the less "depth of field" you will have.
Try chaning you aperture while using a close by object and do a DOF preview before storing the picture with various values, it is easy to see the difference.
I use digital preview myself (it is an user setting in the K10D, and not the default)
3. ISO value. The higher the ISO value, the more the sensor signal is amplified before it is digitized.
The issue here is that not only the sensor's signal generated by captured light is amplified, but also the sensor's signal noise.
It is like the background noise you hear when turning up the volume of an audio amplifier when listing to soft sounds.
There are different kinds of noise, that especially can be seen in the darker areas of your photo (dark = less light = worse s/n ratio)
In general one stop (change of aperture number) = doubling / halving shutter speed = doubling halving ISO value, if you want to maintain the same exposure.
So, if your exposure is right at f=5.6, speed=1/125, ISO=200, the exposure will be right at f=4, s=1/250, I=400. Or: f=5.6, s=1/64, I=200. Etc.
Your camera program must have decided that it should not have slowed down shutter speed any more to avoid blur and to open the aperture anymore to avoid to many objects out of focus (shallow DOF), hence it could only increase ISO.
Since in RAW the "raw" sensor's data is stored, the ISO value (the one that defines the amount of noise captured from the sensor) cannot be changed anymore.
You will have captured the entire digitized signal including the digitized noise.
The only way to attack is to use noise reduction. However, noise reduction in general will smear pixels causing loss of picture detail.
Hope this helps you understand...
- Bert