Okay. APERTURE controls the amount of light coming in, and the amount of focus. SHUTTER SPEED controls the amount of time that light is let in. The third and final piece of the exposure puzzle (for our purposes, anyway) is the ISO*. ISO, or FILM SPEED, controls the amount of light the sensor needs to give you the proper exposure.
Here is a shot of my wives relatives. Keep in mind that all three photos in this post were shot with the same aperture and shutter. I wanted my aperture set at f-1.8, and my shutter set at 1/10th of a second. My ISO was then set at 100, meaning the camera needed a ton of light to get a good exposure. With only one lamp, at night, that just wasn't enough light, and here is the result:
So. Increase the ISO to 1600, make the sensor more....um.... sensitive... and voila':
Now, with this you can decide to make a more moody image, like the first one, or what is considered a correct exposure, like the second. Or, if you want to get crazy, set your ISO to 3200, and take a look:
Now, a word of warning. You may be asking why you wouldn't just keep your ISO at 800, 1600 or 3200 all the time, so you don't need as much light, meaning a faster shutter to catch that hummingbirds wings. And well, you may ask. Just keep in mind, there is a drawback to higher ISOs. DIGITAL NOISE, or graininess. Anything above 400 could easily make an exposure so full of noise that it would be unacceptable.
So tomorrow, we will learn how to make all three of the puzzle pieces work together to get exactly what you want in your image. Until then, today's homework: Try a shrimp egg roll.
*ISO = International Standards Organization. You will never need to know this until well beyond your dying day.