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Tutorial: What is ISO?


There is bound to be someone who is going to point out something that isn’t technically accurate with this post, be it with my definitions or diagrams. So, before we start, I would like to state that my sole intention is to simplify, as far as possible, some of the fundamentals of photography. My intention is not to swamp the reader with technicalities but rather to outline the basic principles.

What is ISO?

Simply put, ISO is how sensitive your camera's sensor or film is to light.

  • A Slow (Low) ISO number  = Less light sensitive camera sensor or film. Therefore suitable for bright lighting conditions.
  • A Fast (High) ISO number = More light sensitive camera sensor or film. Therefore suitable for low light conditions.

However, there is a trade-off. Higher ISO’s also reveal what’s known as noise or grain in images. This noise, on digital cameras, is caused by the sensor picking up the electromagnetic interference caused by the other electronics in your camera. In film, the grain is caused by the size of the silver halide grains in the emulsion.

The ISO numbers increase with incremental value and geometric progression, using the sequence is 100, 200, 400, 800, 1600, 3200, 6400, etc.
As the ISO number doubles the camera's sensor becomes twice as sensitive to light.

Congratulations, You've completed my 'Basics of Photography' Tutorials. I hope that I have been able to answer some of your questions and that you have found it useful. If I haven't - please contact me and let me know, I'd love to hear from you!