Everything You Want Know About Megapixels, Megabytes, and DPI

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.

DPI Measures the Density of the Pixels

Finally, there's the thing that Sue actually asked about: dots per inch, or dpi. Dpi has no inherent value of its own when describing the size of a photo. The only thing that dpi does is help you to understand how large a photo can be printed or displayed, and--here's the key thing--it refers to the display medium, not to the photo itself.

What am I talking about? Suppose you take the 10-megapixel photo I mentioned at the beginning of this article and display it on a computer screen. Computer screens tend to have a resolution of around 72 dpi, which means the screen has about 72 pixels per linear inch. If you show the photo at its "full size" (so every pixel in the photo uses a pixel on the screen) then you'd divide 3872 by 72 and find that the photo would be about 53 inches across. But send that same photo to a 300-dpi inkjet printer, and you would expect that you could make a high-quality print that's about 12 inches across (3872/300).

In Summary

So what does all this teach us? A few things:

Megapixels are a general guide to the size of the photo, as measured by the number of pixels it includes. Megapixels is a strong indicator of quality in the sense that it helps you know how large the photo can be safely printed--more megapixels equals a larger print--but it doesn't really tell you anything about the quality of the camera's sensor or its lens. And other factors, like a high level of JPEG compression, can affect the quality as well.

Megabytes tells you how much space a photo takes up on your hard drive, and has nothing to do with your camera's megapixels. The same photo, saved at different JPEG quality levels, will yield wildly different file sizes in megabytes.

Dots per inch (dpi) is just plain meaningless most of the time. You can use this number, along with the photo's resolution, to find how large it can be printed or displayed on a particular device. But to be useful, you need to know the dpi of the device in question--for example, most inkjet printers give good results at no more than about 300 dpi.

Here's the annoying thing, and what is no doubt tripping up Sue: A dpi value is usually stored as metadata with your digital photo. That's really misleading, especially when a program resets the dpi value for some mysterious reason. As a general rule, you should ignore the dpi value and pay attention to the photo's resolution in pixels. That's the real indication of a photo's size.