When to change the ISO

The exposure is the combination of Aperture and Shutter speed at a given ISO. Now, what should be the given ISO? Lower is better as increasing the ISO leads to more noise in the photograph. At aperture priority, as you shoot keep an eye on the shutter speed so that it does not drop down lower than 1/60th of a second, the minimum shutter speed at which ideally you should be shooting handheld. In case the shutter speed drops lower, just open up the aperture and this will correspondingly let the photograph be shot at a higher shutter speed. If the subject is not moving, any shutter speed above 1/60th of a second does not influence the motion blur in the photograph as there is no motion in the subject. At slower shutter speeds it is the camera shake that gets registered on the sensor. On can certainly use a tripod for such situations but do keep in mind that the tripod does not take care of the subject movement but only the camera shake.

This is where ISO comes into play when you cannot use a tripod because you dont have one or need more flexibility while shooting or you cannot add a flash because you are not carrying one or you wish to be discreet while shooting.

With drastic improvements in technology, the image quality at higher ISO settings is also relatively very good compared to how it was in the early 2000s as the digital cameras were introduced. More professional cameras like the Canon Eos 5D Mark III is known for its image quality at higher ISO. This does not mean that one would use a higher ISO when not required. First exhaust the other options like opening up the aperture, using a tripod or flash (flash can always be used as a fill, bounced or off camera so that the images do not look flat). Even with a good camera, we do not really compare the same shot taken with two different ISO settings so it does not matter if the quality to lower as there is nothing else to compare with. In terms of comparison, the one with a lower ISO would definitely be better, something again you may not be able to notice on the small screen of your camera.

Good quality at higher ISO is a great boon for street photography and has opened up more shooting possibility over an extended period of time, one gets as the sky turns dark. Especially for us who have seen the Film era as well when it was not really possible to shoot handheld, in low light, increasing the ISO and shooting is a great change. A Canon camera is expected which can shoot in absolute dark (as of mid-2015) 

At times you may also shoot a bit under exposed and bring it up later on through the RAW converter in the computer.

In good light actually, it is a good idea to shoot at the lowest available ISO. Through the custom settings in many cameras, you can set the lowest ISO to 50 instead of 100.

Getting the shot at the right moment is more important than the noise at the higher ISO when shooting in uncontrolled situations.

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