Digital camera

This is perhaps the most common question I get, Duane--many readers are confused by conversations about the "size" of digital photos. Let me try to break it down for you.

Cameras are often characterized by "megapixels," or how many millions of pixels they can pack into a photo. A 10-megapixel camera takes pictures with 10 million pixels--and if you inspect the photo in Windows or in a photo editing program, you'll see that's some number of pixels width by height, such as 5000 by 2000 (5000 times 2000 equals 10 million).

You can also measure a photo by its file size--the number of megabytes it takes up on your memory card or hard disk.

These two numbers (megapixels and file size) have absolutely nothing to do with each other. A 10-megapixel photo might "weigh" about a megabyte on your hard drive, or it might be a 4-megabyte file. The file size depends on several factors, including the pixel size (megapixels), the file format (such as JPEG or RAW), and the amount of file compression used to save the photo (something your camera might describe as the quality setting).

Finally, there's the dots per inch (dpi) value, such as the 2700 and 72 numbers you referred to. These are red herrings, utterly meaningless most of the time. It might be useful to calculate the dpi of your photo when you plan to print it, but in general, I think that the dpi values presented by photo editing programs are misleading and should be ignored.

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