ISO…

The acronym ISO stands for “International Organization for Standardization”. However camera ISO doe not directly refer to the organization that creates various technology and product standards. Ever since two film standards called ASA and DIN were combined into ISO standards in 1974, they were referred to as one word “ISO” from that point on. Although ISO initially defined only film sensitivity, it was later adopted by digital camera manufacturers with the purpose of maintaining similar brightness levels as film.

Quite simply, when you double your ISO speed, you are doubling the brightness of the photo. So, a photo at ISO 400 willbe twice brighter than ISO 200, which will eb twice brighter than ISO 100.