How many megapixels does a digital camera need?

This magnified area of a digital photograph shows
how pixels are used to make up the entire image.

When talking about any digital camera, one of the first things people ask about is the number of megapixels. Most seem to take for granted that having more megapixels somehow equates to a better camera, but is this always the case? Before we can answer that, it’s important to know what the term “megapixels” really means when discussing digital cameras and photographs.

You can really think of a photograph (or any other raster image) as a collection of thousands, or even millions, of tiny different colored squares. These tiny squares are called pixels. The image to the right is a portion of a digital photo that has been magnified several times so that you can see the actual pixels that make up the image.

By extension, a megapixel is simply 1 million pixels. So, a 10 megapixel (MP) digital camera is one that takes photographs that contain 10 million pixels. In this sense, the number of megapixels is important since a higher MP value potentially gives you the ability to print larger copies of the photo without losing clarity.

But how many megapixels do you need?

First, we have to go back and think about how big each pixel is. Actually, that’s a bit of a misleading statement since a pixel really has no defined size. Instead, when describing pixel size, we talk about the number of pixels that make up an inch. This is called “pixels per inch” and is often abbreviated ppi. Many people will also refer to this value as dots per inch (dpi). This isn’t exactly correct terminology since dpi really refers to the number of dots per inch that a printer can print. Despite this, you’ll often see the two terms used interchangeably.

What’s the best ppi?

In general, a higher ppi should result in images with more clarity since there are more pixels per inch. However, this value is subject to a number of constraints. For instance, since most computer monitors use resolutions that equate to 72-96 ppi, there’s really no reason to go much higher than 96 ppi if you only plan to use your photos as web graphics. On the other hand, if you plan to print your photos, you’ll probably want to target the 150-300 ppi range depending on your printer’s capabilities and what you intend to do with the finished prints.

Some examples

Let’s look at some examples to see what all this really means. Suppose you want to take a photograph that can be displayed on a web page or sent as email at a size of around 5 x 7 inches. Then you would need a digital camera capable of taking photos that are (5*96)*(7*96) = 322,560 pixels or 0.3 megapixels. That’s no problem – any digital camera can do that! Now suppose that you want to print the same photo with a 200 ppi resolution. Then you’ll need (5*200)*(7*200) = 1,400,000 pixels or 1.4 megapixels. That’s still not going to be a problem for any modern digital camera, but it may be beyond the capabilities of certain other devices like cell phones that have very basic built-in cameras.

Looking at it from another angle, if you never plan to print anything bigger than an 8 x 10 inch photo with 300 or less ppi, then a 10 MP digital camera is more than enough to do the job (assuming you don’t plan to crop and only want to print the original photos). And, if you only plan to use your photos for web graphics, email, and the like, you can get away with a whole lot less.

Keep this in mind when you’re out shopping for a digital camera. It may help you save some money by foregoing a feature that you would never use anyway.