Taking a look at Auto ISO
Auto ISO was introduced into digital cameras several years ago to help photographers maintain desired camera settings to arrive at a correct exposure. Turning on that feature allows the camera to push the ISO up when it calculates the settings (for shutter speed and aperture) are getting too low for a good picture. Almost all new Digital cameras (including new DSLRs and mirrorless cameras) have an Auto ISO setting.
Is Auto ISO Handy?
We control exposure by using Shutter speed, Aperture, and ISO. The usual method is to set the ISO, select what shooting mode you want to use and then set the shutter speed and aperture to achieve the correct exposure.
When manufacturers introduced Auto ISO a few years ago, many photographers didn’t like the idea of leaving it to the camera to set the ISO. And I would say they were right!…to a degree.
You usually don’t want to shoot at a high ISO because your photos can get grainy (noisy), but with the new sensors, you can quickly shoot images at 1600 ISO or even higher, with no noticeable noise!
Does this mean I can shoot all of my photos at higher ISO settings?
No. It is not a good idea and for several excellent reasons. The most important reason is at a higher ISO, you reduce the Dynamic Range of the sensor, which means the camera can not capture as many details in shadow and highlight areas. Other disadvantages of higher ISO are lack of sharpness, chances of getting artifacts in the image (Pixilated edges), etc.
For the above reasons, photographers try to use the minimum ISO to achieve the correct exposure. And Auto ISO provides the minimum ISO automatically.