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How Many Megapixels Do You Really Need?

How Many Megapixels Do You Really Need?
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The 1.5 x 1 inch sensor type is one of the largest in consumer cameras.The 1.5 x 1 inch sensor type is one of the largest in consumer cameras.Megapixels have become the ultimate measure of a camera. If your cousin has a phone with more megapixels than your year-old point-and-shoot camera, clearly he must have the better, higher-quality device. Right? 

Wrong.

Megapixels vs. pixel size

Like calories, megapixels are a measure of quantity, not quality. You need a certain number of megapixels depending on the way to want to share a photo. But just as the number of calories in a meal doesn't say much about how nutritious it is, the number of pixels in a camera doesn't say much about the quality of the image they can capture.

Quality is a complex issue based upon a camera’s optics, image sensor design, firmware, engineering, and yes, its pixels — but not its megapixel count. At the heart of your camera is the image sensor, which contains the array of pixels. These pixels are like buckets that collect photons (i.e., light).

MORE: Camera Buying Guide

Image sensors come in different sizes. The larger the image sensor, the larger the pixels can be, and the more photons each can collect. The result is a picture that is cleaner, with less image noise (graininess), and typically a finer differentiation and delineation of highlights and shadows. 

To get a rough idea of any camera’s sensor size, look at the diameter of the lens. An 8-megapixel smartphone camera packs 8 million pixels onto a minuscule sensor about the size of a baby aspirin tablet. However, an 8-megapixel compact camera has a significantly larger sensor, about the size of a pinky fingernail, so each individual bucket (pixel) is bigger and deeper. That allows it to capture more light without the light spilling over to adjacent pixels — which is a prime cause of noise (a grainy appearance) and ghosting (a double image).

(This may be changing, however. With the iPhone 5s, for example, Apple increased the sensor size but kept the same 8-megapixel resolution, resulting in pixels about the same size as those in many point-and-shoot cameras. And image quality went up.

Advanced compacts, mirrorless cameras and semipro and professional DSLRs come equipped with even larger sensors ranging in size from a postage stamp (known as APS) to a comparatively huge "full-frame" sensor of about 1.5 by 1 inches found in top-of-the-line DSLRs. So, all other things being equal, an 8-megapixel DSLR will produce far better images than an 8-megapixel compact camera, just like the 8-megapixel compact will capture better images than your 8-megapixel smartphone.

MORE: DSLR vs. Mirrorless Cameras: Which Is Better for You? 

Where megapixels do matter is the size you want your final picture to be. You need to have a camera or smartphone whose megapixel count matches how you plan to use your photographs. This is particularly important if you plan to print your pictures, because print quality is very dependent upon having enough data (pixels) to define the picture. (Please see tables for guidelines on how many megapixels you need for different size prints and for sharing on various social networks.)

Can you have too many megapixels?

Having more pixels than you really need can actually hurt image quality. That’s because when you upload an overly large picture to social media, output it to a printer or send it to a photo book producer, your image will be downsized automatically. In other words, the software or upload process will randomly delete pixels without the smarts to understand what might be critical in the picture, such as the sparkle in a child’s eye or the razor-sharp edge of a leaf.

Photos with too many megapixels also take much longer to upload and might even fail partway through. And if you're uploading on the go, you're eating into your wireless data cap more than you need to. 

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Apps such as Aviary let you select the number of megapixels.Apps such as Aviary let you select the number of megapixels.What’s more, even in this era of remarkably inexpensive hard drives and memory cards, extra- large photo files will quickly fill up storage space with unnecessary data that you probably won’t need, use or want.

Of course with the continuing megapixel inflation by camera makers, you may have no choice but to buy a camera with far more resolution than you need. Fortunately, traditional digital cameras, as well as camera apps for smartphones, allow you to adjust the resolution down, which is one option. 

Another is to downsize the photos after you shoot them but before you upload them to a printing service or social networking site. Even basic, free photo editing apps allow you to select the output resolution. This option will fill up your memory card faster, but it has the benefit of giving you more flexibility, for example if you decide later that the quick snapshot you took is actually worthy of a full-size print.

How many megapixels do you need for prints? 

For most people, the highest resolution files they will need will be for the occasional print or photo book. Here’s how you calculate the number of megapixels you’ll need for a printed photograph:

Determine the physical size of your print, such as 4 x 6 inches, 8 x 10 inches, etc. Then, multiply the width by 300, and the height by 300, which will give you the size in terms of pixels. (300 ppi — pixels per inch — is recommended for good-quality prints.) Therefore, an 8 x 10 inch print would be 2,400 x 3,000 pixels. 

Multiply the width (in pixels) by the height (in pixels). So for that 8 x 10 inch print, it would be 2400 x 3000, which equals 7.2 million pixels.

Divide the result from step 2 by 1 million, and you have the number of megapixels you need to make a good print. In this case, the minimum resolution you’ll want your camera to have is 7.2 megapixels.

How many megapixels do you need for social network photos?

If, like many people, you never plan to print your pictures, your megapixel requirements will be far less. A Twitter photo, for example, measures just 375 x 375 pixels, which equals a mere 0.15 megapixels. A Facebook timeline photo, at 960 x 720, requires 0.69 megapixels. One of the largest social network images you can post, a Google + cover photo, requires 2.53 megapixels. 

MORE: Best Smartphone Cameras 2013

At the bottom of the page is a chart that gives optimum megapixel counts for photos that you plan to print or upload to Twitter, Facebook, Google+ and Pinterest. 

Adding extra megapixels for cropping

Professional photographers are usually careful about how they compose their pictures, making sure all the space in the screen or viewfinder is filled with what they want to capture before they click the shutter button. Most people, however, tend to include more visual real estate in photographs than they actually want or use, so they end up cropping out those extraneous areas. If you tend to crop your photos, look for a camera with about 50 percent to 75 percent more megapixels than the tables below recommend. Chances are, though, that even the cheapest cameras nowadays will have more than enough megapixels.

Sally Wiener Grotta (www.Grotta.net) is a fine art photographer, author and speaker who has practiced and written about digital photography from its beginning. Follow her  @SallyWGrotta, Google+.Follow us @tomsguide, on Facebook and on Google+.

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  • 0 Hide
    Cryio , December 9, 2013 6:20 AM
    I stopped giving a damn when I read "This may be changing, however. With the iPhone 5s, for example,". Yeah, the 5s expanded it's sensor by 5%. Whoop-de-freakin' do. Best example !
  • 0 Hide
    Parsian , December 9, 2013 7:30 AM
    forgot to mention that pixel size is a trade off between High Dynamic Range and Resolution. A future digital camera will utilizes technologies employed by Nokia's 1020 and Samsung ISOCELL. Nokia delivers better signal to noise ratio due to averaging mulitple pixel data, while ISOCELL allows for better dynamic range.

    So soon, you will get cameras that can maintain high resolution (avoid aliasing) and perform well in dynamic range.
  • 0 Hide
    chesteracorgi , December 9, 2013 9:43 AM
    The real revolution to come in imaging is when they get beyond digital graphics and start using vector graphics (used in the likes of Corel Draw and other high end programs), If they develop vector graphic cameras then the lines are defined by a mathematical formula, which, produces the same image whether at 1 cm X 1 cm or 1 m X 1 m.

    Nevertheless, cameras will not be consigned to the ash heap of history, because the optics involved in producing the image remain.
  • 0 Hide
    Parsian , December 9, 2013 4:01 PM
    you cannot generate infinite details out of absence of details. I am not sure what you mean.
  • 0 Hide
    chesteracorgi , December 9, 2013 6:00 PM
    Parsian,

    Digital graphics works by dividing an area into little sub areas (pixels) and reproducing an image by mapping the image and trying to get it duplicated in the pixels. The limitation is you can only be so accurate because of the limitation of the number of pixels. Hence the use of anti-aliasing logarithms in both computer display and photography. While there were a few others who championed vector graphics Corel was the major developer of describing shapes by mathematical vectors. This approach gives an "accurate" reproduction by using a mathematical vector rather than mapping pixels. If a curved line is adequately described by Y = X squared -3X + 2 it is mapped mathematically and pixilation or noise can be reduced to the limit of the digital reproduction. However, the beauty of this approach is that you can (with enough detail) create an exact analog reproduction. The mathematical formulae are infinitely more accurate than the digital maps. The problem is setting a common standard for describing the vectors and reproducing them. With contemporary computer processing power the use of vector graphics not only becomes possible but preferable to digital reproduction. You can spend all day tracing an S curve digitally and not get the accuracy or detail of the mathematical equivalent.
  • 0 Hide
    John Camaney , February 11, 2014 9:41 PM
    I like the infographic, but I think how big you can print depends on more than just the megapixel count. In fact, I'd argue that other aspects are more important, like a having a sharp and well exposed photo with no noise to begin with. See <a href="http://www.disturbancesinthewash.net/journal/how-many-megapixels-do-i-need">this article</a> for example.
  • 0 Hide
    John Camaney , February 11, 2014 9:42 PM
    Looks like I can't add a link in HTML to the comments. How about this http://www.disturbancesinthewash.net/journal/how-many-megapixels-do-i-need
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