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There used to be a time where people had to shoot with tripods during daytime, when ISO 50, 25 or even below were common values for film speeds. But of course, digital changed everything. You no longer had to change a whole roll of film to change your film speed (aka ISO sensitivity), since the sensor itself could adjust its sensitivity all by itself. And of course, each generation brought up better low-light capabilities, with today’s (as of 2014) cameras such as the Sony A7 and its variants or the Fuji X-Trans series being capable of delivering relatively clean and usable images up to ISO 6400 and upwards. Maybe you don’t have the newest gear out there, but nearly all cameras from 2010 and later on should be able to handle ISO 800 and 1600 without too much trouble. Most cameras have a base ISO of 100, with a few sitting out there at 200. If your camera can produce relatively clean images from, say, ISO 100 to 3200, that gives a whopping 6 stops (100, 200, 400, 800, 1600, 3200, 6400) of sensitivity to work with! With such a great amplitude, it often startles me why not many people uses auto ISO. In fact, there are a few cases where auto ISO should be absolutely useful.
If you, for any reason, need to shoot from a vehicle, auto ISO should be your best bet (especially if your auto ISO is configurable in your camera’s settings). Since a fast shutter speed is paramount, by setting a high minimal shutter speed using either your auto ISO’s settings and/or by using speed priority (Tv for canon users), you can essentially assure a clear shot every time without the hassle of a fully manual exposure.
Auto ISO is also great for general walkaround photography. Transitions between sunny outdoors, shady areas and indoor lighting is often too great of an amplitude. I know that some people swear by manual ISO control and like to set it to a good compromise value, such as ISO 400 or 800. However, in these cases, using auto ISO would be far superior than trying to manually fiddle with your sensitivity every time you enter a new room, possibly ruining a few shots.
Many people do not like auto ISO because they feel that manual control over ISO makes them somehow a better photographer, myself included before. But unless you are shooting in manual all the time and swear by it, if you use aperture priority 90% of the time like most of us, your camera’s meter will do the heavy lifting and decide your final exposure, whether it controls 1 variable (shutter speed) or two variables (ISO and shutter speed).