What was Ansel Adams Contribution to photography?

Answer 1

Ansel Adams invented the Zone System.

The Zone System allows the photographer to precisely expose the most important element in his photograph. You use a spot meter and measure every important thing in your image, bearing in mind white, black and gray objects all reflect different amounts of light but the meter thinks they're all gray--because that's what a meter is designed to do. After doing all the math, you dial in an aperture and shutter speed that will allow you to capture the scene as close to how it appears as you possibly can.

This worked for Ansel Adams, and it will work for anyone who's really familiar with relative reflectance and isn't shooting something that moves or changes. You can't shoot a Basketball game with the Zone System, for instance, and you can't shoot sunsets because you don't have time to meter them.