The Benefits of and How to Use Auto ISO
Auto ISO is a feature that allows the camera to add more sensitivity to the sensor if you have reached the limit of your aperture and shutter speed.
Say you are taking a photo at dusk of a church, the light is fading and you have no flash. You open the aperture to the widest it will go on your lens (F2.8 for example), you have the slowest shutter speed you can go to without creating camera shake and a blurred image (60th/sec for example) and it is still too dark.
With Auto ISO enabled, your camera will, once you have reached the limits, increase the sensitivity of the sensor from say 100 ISO (not very sensitive) up to 400, 800, 1600, 3200 (super sensitive) or beyond (depending on your camera) giving you that extra stop* or two of light to enable you to get the shot…magic! I use this a lot at weddings.
Note: *A “stop” is the term given to a one stop shift or increase/decrease of light for your exposure. For example, changing your aperture from F2.8 to F5.6 is a one stop decrease in light as F5.6 lets in less light than F2.8. Whereas F5.6 to F8 is another one stop decrease.
F5.6 to F6.7 is an approximate half stop decrease in light. F3.5 to F2.8 is an approximate half stop increase in light.
Using the same principle with regards to the shutter speed, going from 125th/sec to 60th/sec is a one stop increase in light as you are allowing the shutter to stay open for longer. Going from 500th/sec to 1000th/sec is a one stop decrease in light as you are speeding the shutter up.
60th/sec to 100th/sec is in effect a half stop decrease in light and 250th/sec to 180th/sec is a half stop increase in light…make sense? Read it again.
Note: Make sure that your camera, if it has Auto ISO, can use it in Av (Aperture Priority) and Tv (Shutter Priority) mode and possibly Manual (some older DSLR's don’t allow it to be used in manual. The Canon EOS 5D Mark II for example, just switches the ISO to 400 and leaves it at that regardless…not very reliable at all).
However, at the end of the day this is all good and well but as far as I know at the moment, no-one has managed to alter the principles of light and aesthetics of composition.
So, broken down and removing all complications, the basic and most important fundamentals of photography are…
…nothing else. It is up to you to manipulate all three factors to produce a decent image using your own skills, patience and passion. Getting to know and how to use your camera’s functions (Auto or Manual) plays a big part in this.