by Dave Johnson, PCWorld   Jul 17, 2012 6:30 pm

To take good photos, you don’t really need to know a lot of technical details about digital photo files any more than you need to know how a car engine works to drive to work every day. But sometimes it can help, especially when it comes to sharing and publishing your photos. Take the seemingly simple concept of resolution. Do you know what size a photo should be in order to turn your photo into a high-quality 8-by-10-inch print? What if you want to email it? What do megapixels and megabytes have to do with one another?

To help answer those questions—and others—here is a primer on everything you ever wanted to know about photo files, from megapixels to megabytes, with a little dpi thrown in for good measure.

Understanding Megapixels

Every digital photo is composed of pixels—millions of them. Pixels are an easy thing to measure, so many people buy a camera that captures a lot of them, assuming that more pixels equals better quality. As any camera advertisement will reveal, cameras are typically rated by the megapixel, which describes how many millions of pixels are embodied in a photo. A 1-megapixel camera takes photos with a million pixels in them; a 20-megapixel camera captures 20 million pixel photos. Think of it like a grid that tells you how wide and tall the photo is.

Consider the Nikon D7000, a 16-megapixel camera. It takes photos that are 4928-by-3264 pixels. Multiply those two numbers, and you get about 16 million. Compare that to the iPhone 4S, which takes photos that are 3264-by-2448 pixels: 3264 times 2448 is about 8 million, or 8 megapixels.

The number of megapixels gives us an indication of the resolution of the photo. Imagine zooming in to a photo until you can see every pixel on the screen, like the image below. The resolution tells you how large your monitor would have to be in order to see the entire photo.

So what do megapixels buy you? In a word, detail. More pixels can capture more fine detail so you can crop away unwanted parts of the photo and still make a high-quality print. Consider this: If you crop away half of a 12-megapixel photo, you’ll still end up with a 6-megapixel image, which should have a lot of rich detail.

In general, if you’re comparing two cameras with similar megapixel counts, the one with the larger sensor will generally take better photos. That’s why some photographers pay a premium for digital SLRs with full-frame sensors.

Measuring Megabytes

While the megapixel rating tells you how many pixels are in a photo, there is another important thing you sometimes need to know about your photo: the file size of an image that the camera produces. This directly affects how much storage space it takes up on your hard drive, its size as a file attachment in email, and how long it takes to traverse the Internet. You might think of this as how “heavy” the file is, as if you were weighing photos on a scale.

Alas, there’s no direct way to correlate pixel size and file size. 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.

You can use a lower quality setting when saving a JPEG to significantly reduce the file size. Of course, if you try this, always save the smaller photo as a new file, so you don’t damage your high-quality original.
If you’ve ever noticed that the file size of a photo changed significantly when you saved it in a photo editing program, it’s usually because the photo has been saved with a different quality level—and hence compression level—than it started with. This is handy if you need to “shrink” a photo to send it in email. You can reduce the resolution by shrinking the number of pixels.

Using DPI and PPI to Print

Finally, one last concept you might encounter is dots per inch (dpi) and pixels per inch (ppi). These are similar terms, and both measure the density of the pixels in whatever medium you are looking at the photo. Dpi usually refers to printed photos, while ppi measures images on a computer screen.

Dpi and ppi have no inherent meaning of their own, so you can’t say that a particular digital photo is, for example, 72 dpi or 300 dpi. Dpi does, however, help you to understand how large a photo can be printed or displayed. Here’s the tricky part: it refers to the display medium, not to the photo itself.

Imagine that you want to print a photo on a 300-dpi inkjet printer. You might take your iPhone photo, which measures 3264-by-2448 pixels, and divide those dimensions by 300. That tells you that you can get a fairly good quality 8-by-10-inch print.

If you open the Image Size dialog box in Adobe Photoshop Elements, it tells you that the image is 5184-by-3456 pixels, and this assumes the photo is displaying at 72 ppi. If you change the resolution field to 7.2 pixels per inch, then Photoshop Elements will resize the photo to 518-by-346 pixels.
There’s more to the story than megapixels and megabytes, though, which is why you shouldn’t buy a camera based on specifications alone. Raw pixels aren’t going to help a lot if the camera has poor optics, high digital noise at low ISO settings, or other limitations that keep it from taking great photos. In other words, resolution and megapixels tell you about the photo’s dimensions and how it will print (or display), but not a lot about the quality. That’s why that you should read camera reviews, like those here at Macworld.

Editor’s Note: The  article is reprinted from MACWorld.com.