So what is a good exposure? Simply put, it's an image that is an accurate and artistically satisfying recording of what you saw when you took the picture.
Every pixel that lives on your camera's image sensor records either red, green or blue, each measured in increments of 0-255. 0 = black and 255 = white with every shade inbetween. It's those numbers that make up a digital photograph.
In fact, your images are natively recorded in a color mode that your computer understands as RGB, referring to - you guessed it - red, green and blue. This will be important later when you bring your images into your computer... but for now, let's get back to exposure.
If too much light is let into your camera and allowed to land on the pixels that make up your image sensor, then you will get pictures that are a sea of white! Those tiny little pixels were slammed with more light than they could handle, so they recorded 255 (all white!) and BOOM - you lose important color information. This is called an overexposed photo - your image sensor was over exposed to light!
Anyone wanna play spot the child in this overexposed picture on the left?
Clearly, so much light was let into the camera that the pixels were all "blown out" (that's cool speak for pushed to 255 or all white). Not all examples are this extreme - think of when a blue sky appears white instead of blue, or when just part of your little one's nose or cheek gets lost in a sea of white. There was too much light let into the camera for the sensor to give you an accurate representation of what your eye saw so the actual color information is lost!
Compare that photo to one taken just a few seconds later on the right - this time with different camera adjustments.
It's your job (and sometimes your camera's) to determine how much light is allowed in. This is done by adjusting your aperture, shutter and image sensor settings to prevent images like this from happening. But before we get to how we determine what those settings should be, we need to look at what happens when not enough light gets into your image sensor.
So what happens when too little light is let in? The pixels lose out on information too - just to the opposite end of the spectrum. All the important information for your image gets lost in a sea of black! Your picture becomes underexposed - your image sensor was under exposed to the amount of light it needed to make the picture!
We weren't going for a silhouette of a rocking chair in this photo! There wasn't enough light let in the camera to accurately record these sweetie-pies' faces (or much else). So most of the pixels on the camera's image sensor recorded 0 - or black.
How then, do you control how much light hits the sensor? By finding the right combination of settings for the three most important parts of your camera. Again - aperture, shutter and image sensor.
Sounds simple right? Well, it's not. There are an infinite number of combinations and finding the right one for your situation can be really daunting at first.