Megapixels (MP) have been the primary marketing ploy of camera manufacturers since the introduction of the first digital camera, but many questions arise when one tries to dig deeper on the mysterious megapixel. How many megapixels do you really need? Is a 24 MP picture twice as sharp as a 12 MP picture? Is a 13 MP cell phone camera better than a 10 MP point-and-shoot camera? Can you have too many megapixels? For answers to the mysteries behind the megapixel, please read on.
In the post “Reading 256 Shades of Gray”, we learned that a pixel (short for a picture element) is the smallest element of a digital image and that each pixel is represented by an individual photosite on the camera’s image sensor. A 12 MP camera, therefore, has approximately 12,000,000 photosites on its image sensor.
How many megapixels do you need?
The answer to that question depends on how you want to share your pictures. The quality of the image is a function of the number of pixels available and the density required for your sharing medium. If all you do is share pictures on social media or the web, the required density is 75 Pixels per inch (DPI), since that is the normal display resolution of monitors. For example, Facebook limits your cover photo to a size of 851 pixels x 315 pixels, so 851 x 315 = 268,065 or .27 megapixels. Yes, that is point two seven MP. Below is a table illustrating the MP requirements for several social media sites:
As you can see, you do not need a camera capable of taking large megapixel photos if all you do is share your pictures online. If you plan to print your photos, however, your megapixel requirements start to increase. For printing, the required density is between 150 and 300 DPI depending on the combination of ink quality and paper type used. Assuming 300 DPI prints, the formula for calculating the MPs that you need is (300 x width in inches) x (300 x height in inches) / 1 million. Since I know how much you love math, I have included a table below that lists the MP requirements for several common print sizes:
Are 24 MP photos sharper than 12 MP photos?
With all things being equal, the answer to that question is a resounding “No.” Megapixels have nothing to do with picture quality, but instead are a measure of the picture’s size. Much more important to the “sharpness” or quality of the picture is the size of the image sensor. The image sensor in most cell phone cameras is about the size of a baby aspirin, while the image sensor on a full frame professional camera is the size of a large postage stamp. If both image sensors have the same number of photosites, the tiny photosites in the cell phone camera will produce more picture noise and capture less color and detail than the considerably larger photosites on the full frame camera. In fact, when Apple released the iPhone 5 series, they did not increase the number of pixels, but instead increased the image sensor size, thereby improving picture quality.
Is a 13 MP cell phone camera better than a 10 MP point-and-shoot camera?
You can probably guess where this is going, but wait for it . . . . . . . . . . . . “No.” There are actually three reasons for this. First, the point-and-shoot camera has an image sensor that is about the size of your pinky nail, making room for much larger photosites that capture sharper photos. Second, outside of sensor size, the next most important factor in image quality is the lens. That tiny piece of glass on a cell phone camera is not quite on par with most cameras, and may be stretching its technical limits to resolve anything past 12 MP. Third, the cell phone camera has digital zoom as opposed to the optical zoom found on most small cameras. Digital zoom works by cropping the outside edges and enlarging the center of the image. This also enlarges the pixels and reduces image quality.
Can you have too many megapixels?
The answer to this is “Sometimes. “ When you take a large picture and reduce the size for either web display or small-scale printing, the software downsizes the photo by deleting random pixels of its choosing, which can hurt your image. Facebook is rather notorious for this, which is why it is better to downsize your images with Lightroom or Photoshop (with more intelligent pixel deletion) prior to uploading them. This also allows for faster uploads. Additionally, taking pictures that are larger than what you need will increase your file size and storage requirements. This may be especially troublesome with the limited storage available for your cell phone camera.
BONUS QUESTION: Is there anything positive about having more Megapixels?
In February, Canon announced the Canon EOS 5DS featuring a 50.6 MP sensor. Up until that announcement, Nikon had been ahead in the megapixel race with the 36 MP D810. If you were not printing poster sized prints, why would you want to deal with these huge files? For one, a landscape or fashion photographer my well be printing poster sized prints that require an incredible amount of detail. Additionally, large images allow wildlife photographers to aggressively crop a scene and still end up with a fair size picture. Finally, using good software, you can downsize a high-resolution image taken in dim light to reduce the noise that is inherent in images taken with a high ISO setting.
The dirty little secret of camera manufacturers is that more megapixels does not necessarily translate into better photos. A large image sensor and a good lens will have far greater effect on image quality. For the 1% of photographers that require high-resolution images, and that can afford the lenses capable of resolving that resolution, having more megapixels may make sense. For the rest of us, a 12 MP camera is more than enough to take fantastic photographs, share them on the web, and make beautiful 8 x 10 prints.