ISO is the final piece of the exposure triangle, we discussed shutter speed and aperture all that is left is ISO. ISO is simply the camera’s sensitivity to light. A lower ISO makes the camera less sensitive to light while a higher ISO makes the camera more sensitive to light.
The more sensitive the camera is to light the more grainy the image will be. The way that ISO works is pretty complicated but I am going to attempt to explain it as simply as I can here.
Your camera has a sensor. On that sensor are photon receptors. The photon receptors convert the light received into electrons that the camera can then process and use to create an image. Each photon receptor (pixel) can receive a predetermined amount of light. Which in turn means that it can transmit a predetermined amount of electrons. When the ISO is at it’s lowest setting all of the photon receptor is used to emit electrons. As you increase the ISO you increase the gain of the sensor and decrease the sensitivity of the photon receptors. The step from ISO 100 to ISO 200 means you lose 1/2 of your sensitivity. By the time you get to ISO 1600 you are down to about 1/16 of the beginning sensitivity.
Increasing ISO causes grain. I talked about the importance of sensor size here. You can see example images of the degrading effects of higher ISO below. These images were shot in good lighting with a Canon 5d Mark 3 which has exceptional ISO performance. I cropped the images to better show the effect of the grain and the degradation of the image quality. Even the jump from 100 – 200 is noticeable but as you can see quality decreases more and more with each jump. These images are unedited aside from the crop. I screwed up when taking the images and forgot to set a constant aperture. Therefore the aperture throughout the range of photos changes and more of the image comes into focus as you go along, this has nothing to do with the ISO performance. The graininess and loss of detail is what is important to note in these photos.
This article was extremely simplified, if you want an in depth and very technical explanation check out my source article here.
Have a great day!