What is ISO 

ISO is the level of sensitivity of your camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. The component within your camera that can change sensitivity is called “image sensor.” With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash. But higher sensitivity poses a problem as it it adds grain or “noise” to the pictures.

The difference in this image is clear – the image on the right hand side at ISO 3200 has a lot more noise in it, than the one on the left at ISO 200.

Low ISO Speed

High ISO speed