Using the keyboard to control screen brightness and speaker volume is a no-brainer on a Mac. However, sometimes it would be nice if you could make it just a little brighter or not quite so loud. By using a keyboard combination, you can gain more precise control over these functions and get the result you desire.
On the Mac I use, the following keys are used to control sound and brightness:
F1: Decrease brightness
F2: Increase brightness
F4: Decrease volume
F5: Increase volume
Each time you press F1, F2, F4, or F5, you increase or decrease the brightness or volume by one unit.
If you combine Option+Shift with F1, F2, F4, or F5, the increase or decrease is equal to 1/4 of a unit. It will break up each of the normal increments into 4 sections.
This is great for fine tuning the brightness and sound to place them at the optimum levels for you.