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Everything you want to know about megapixels, megabytes

Dave Johnson | Oct. 4, 2011
I bet there are still some things about photo files you don't know.

You kids today have it so easy. Back in the old days, using technology like digital cameras and photo editing programs was difficult. My first book on digital photography came out around 1998 and was filled with page after page of arcane troubleshooting tips, like how to get your camera connected to a PC's serial port (this was before USB) and how to get your software to read TIFF files. But whether you're just starting out and looking for tips or you're a veteran who has been reading this column for years, I bet there are still some things about photo files you don't know.

This week, I've put together a primer on everything you ever wanted to know about photo files--megapixels, megabytes, dpi, and more. This is sure to help you better understand digital photography.

It All Starts With a Question

Digital photo jargon can be perplexing, especially when so many terms sound so similar. I frequently get questions from people who confuse megapixel and megabyte, for example. And this recent question, from reader Sue Scott of Phoenix, Arizona, addresses one of the most confusing issues of all: How on earth do you interpret the "dpi" value associated with your photos?

Sue's question: "When I e-mail a photo that's 300 dpi, it gets changed to 72 dpi. Why does it do that? How do I send it so it maintains its resolution?"

Let's Start with Megapixels (How Big)

First the good news: Sue is facing a false dilemma, and things aren't nearly as bad as they seem. But to explain why that's the case, I need to take you on a journey through some fascinating trivia about digital photography related to the size of digital photos. Ready to go?

Cameras are most often characterized by the term megapixel, or how many millions of pixels their sensors can pack into a photo. A 10-megapixel camera takes pictures with 10 million pixels. For example, my Nikon D200 shoots photos that are 3872 pixels wide by 2592 pixels high. Multiply those two numbers together and you get 10 million pixels.

So megapixels defines the size of the photo a camera can take, as measured by how many pixels it contains.

Next Up: Megabytes (How Heavy)

It's also important to be able to measure a photo by its file size, or the number of megabytes it takes up on your memory card or hard disk. I like to think of this as how "heavy" a file is, as if you were weighing it on a scale.

Megapixels and file size have virtually no relation to each other. A 10-megapixel photo might "weigh" less than a megabyte on your hard drive. Or it might "weigh" as much as 6 megabytes. The file size depends on several factors, including the number of megapixels, the file format you're using (such as JPEG or RAW), and the amount of file compression used to save the photo, which is sometimes referred to as the quality setting.


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