Photography is about compromises and ISO is no different, however the last camera generations have improved the odds in our favour dramatically.
Firstly, what is ISO? In short and without going into the technical details, it is the measurement related to the sensitivity of the sensor to light. The higher the ISO, the more sensitive to light the sensor becomes and that is good. However, there is a price, as we raise the ISO, the image gets noisier or, in film terms, grainier and that is not good.
In the film day’s, the baseline ISO is 100 and is generally the accepted best quality image standard, as of ISO 800 many older generation cameras get noisy and rarely usable beyond ISO 1600. This caused people to avoid it, even ignore it as a tool, however the last few years saw leaps and bounces in this department, ISO 12800 is now commonplace and has become usable. This allows us to shoot in darker places with fewer worries about image quality.
The most efficient way to use this is to find out what the maximum acceptable ISO setting is for your camera is, to do that I suggest that you shoot a few photos in a relatively dark place and decide for yourself what is acceptable to you. Once that has been done, as light vanishes you can raise your ISO until you reach your maximum or simply set it to your maximum when you know that you will be in dark places. This will allow you to shoot at faster shutter speed in darker situations, this is important to get sharper images.
Additionally, your camera may have an automatic ISO setting, I set mine to automatic with the maximum limit set to my maximum acceptable ISO, this allows me to focus on the other more important elements of photography such as depth of field, shutter speed and composition. It is also very handy in conditions where the light changes often and without warning, like tracking wildlife in changing ground condition, shade to light and back. It ensures that the photograph is as intended as opposed to photos that are too dark or too light and cannot be corrected.
In tandem with image stabilization, a higher ISO setting may allow you to shot a scene at night without the need of a tripod or a low light shot without the need of a flash. Noise at high ISO is inevitable, however modern software may come to the rescue. Software such as Abobe Camera RAW (ACR), Adobe Photoshop Lightroom and even Canon’s own software will help to reduce the noise without sacrificing image quality.
ISO is no longer something to fear but rather a tool to use and sometimes abuse.