It's really easy to get tripped up over image file size, but once you get a few key concepts, you'll be off and running in no time.

WHY DOES FILE SIZE MATTER?
Imagine going to the supermarket and picking up a can of Coke and a can of seltzer water. Both come in 12 oz cans, but since I'm trying to lose a few pounds, the most important thing to me is the number of calories in each drink. It turns out that Coke has 110 calories per can, whereas the seltzer has zero. So the volume of the can (12 oz) has nothing to do with the amount of calories.

Photography has a similar ambiguity. When dealing with stock photography, we're concerned with the amount of picture data, and the uncompressed file size is the only accurate way to determine the amount of picture data in a given image.


MEGAPIXELS - Your Camera's Sensor
Your camera has a sensor that is used to capture an image. The sensor is composed of a whole bunch of photosites that collect light to create the larger image. Each one of these photosites represents a pixel in a picture. The number of total photosites on the sensor determine the camera's resolution, which is expressed in megapixels.

For example, the Nikon D300 has a sensor size of 4288 (width) x 2848 (height). If you multiply these two numbers together, you get the total pixel count.

4288 x 2848 = 12,212,224

If you divide this number by 1,000,000, you'll get the camera's resolution expressed in megapixels (mega from the Greek for "great", which is commonly used as a prefix for million).


12,212,224 / 1000 = 12.21 megapixels