Originally Posted by willie_901
Auto-ISO is convenient. This is auto-ISO's single advantage. However anytime ISO is higher than it really needs to be IQ will decrease.....
So, the trade-off is convenience vs. the best IQ possible. Sometimes convenience and spontaneity is more important... sometimes it's not.
This all depends upon how you look at it and what you are shooting. To me AutoISO is all about keeping the ISO as LOW
Say someone is shooting walking around a city (going in and out of shadow) trying for quick street shots. They decide they want a 1/500 of a second shutter speed to freeze both their own and their subjects motion. They could just set ISO at something fairly high to assure they can reach that shutter speed in all lighting conditions. Of course in doing that the cameras ISO is higher than it would need to be in all conditions. Leaving it at base an increasing when needed would result in missed shots.
If instead they set AutoISO the cameras ISO starts at base ISO and it only increases it when needed to hit the parameters the photographer already set up. If you are in full automatic mode the camera will open up the lens as much as it can before increasing ISO. Thereby keeping ISO as low as possible. If it hits the max ISO you set it will then start lowering the shutter speed below what you have it set at.
If you are in aperture priority mode the camera is going to try and keep the shutter speed you set (in auto ISO) and will increase ISO up to the max you set. Then shutter speed will drop off.
Shutter priority the camera will open the lens up as much as it can before increasing ISO. If you hit max ISO you will then start underexposing.
If you are in fully manual mode the camera is going to try and make your combination of aperture/shutter work. If it increases to the max ISO you set you and it still needs more light you are going to be underexposed. Open up the lens or slow down the shutter.
Obviously, the settings in AutoISO should reflect your shooting at that point in time. Shooting action would normally have different settings than if you are doing a landscape on a tripod. That is one of the nice features of the XP2... it has three different AutoISO settings to be able to quickly change between them. On the XP1 understanding how AutoISO works in shutter priority or full manual mode gets one a lot of the way there too.