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We often hear cameras described by how many megapixels they are; when a camera like the Canon T5i is described as an 18 megapixel camera, this means that it records image files that are 5184 x 3456 pixels in size, which gives us an area of 17,915,904 total pixels when we multiply the horizontal pixels by the vertical pixels, which gets rounded to 18 million (mega) pixels.
This tells us the camera’s resolution, which is the measurement of image detail, usually calculated by the number of pixels the sensor records information from.
You might also see the specs of a camera ad reference the actual and effective pixels. For example, Canon T5i‘s specs state that it has an 18.5 megapixel actual resolution and an 18 megapixel effective resolution. If you’re wondering where those half a million pixels wandered off to, some of the sensor photodiodes are used by the camera for other purposes.
Most everyone knows that more pixels means more detail captured, but before you run out and buy a 42.4 megapixel Sony a7R II or 50.6 megapixel Canon 5DS, keep in mind that a low resolution of only 1 or 2 megapixels is plenty for displaying on most computer screens or on social media, which — if we’re being completely honest here, is what most people do with their photos. For photographers who do plan on printing their images or cropping them to focus on a specific part of the photo, having more megapixels can be extremely useful.
For printing nice looking photos, 180 to 300 pixels should be printed on each inch of paper (that’s where the term PPI comes in – pixels per inch). You’ll often see people refer to PPI as DPI (dots per inch), and while they aren’t technically the same thing, most people use these terms interchangeably. Although many printing companies using professional grade photo printers prefer all images to be at least 300 PPI, 240PPI is usually enough to ensure an image that looks very good, particularly if you’re using a home printer.
How Large Can You Print an Image?
Image height in pixels ÷ desired pixels per inch (PPI) = Maximum print height in inches
Canon EOS 7D II
By making use of this easy formula, an 24MP image printed at 300 PPI could produce a very sharp 13.3 x 20 inch print with no noticeable loss in quality. To print the same image bigger than this size, you can print using a lower PPI setting — the quality of the print will decrease somewhat as your PPI does, but depending upon the subject matter, the type of paper you’re printing on, the type of printer you’re printing with, and the distance you’ll be viewing the print from you might be able to produce a significantly bigger print without much noticeable loss of quality. To clearly illustrate this concept, think of the images you see on billboards, which appear sharp thanks to the long viewing distance.
Besides decreasing the PPI to print a photo larger, you can also resample your images using image editing programs like Adobe Photoshop and Lightroom or free options like GIMP. Resampling actually changes the number of pixels in your image, using interpolation algorithms which assign colors to new pixels based on the colors of existing pixels. To give you an idea of how this works, imagine an artist trying to restore an old painting. The painting is missing a small speck of paint, and to decide what color to add to this blank spot, the restorer looks at the colors around it and makes an educated guess. Resampling does the same thing, and it can often do a pretty good job of increasing the resolution of an image significantly without much obvious loss in quality.
When it comes to photo quality, it’s not just the megapixels that matter, it’s the actual size of the camera’s sensor too since the larger the sensor, the more numerous and bigger the photosites containing the sensor’s photodiodes (pixels) are. Even though compact digital point and shoot cameras may have as many megapixels as their DSLR counterparts, their sensors are usually much smaller, typically producing images which can’t match the higher quality recorded by their bigger DSLR cousins. Some modern cell phones feature built-in cameras with more megapixels than many professional digital cameras but the quality of these images is generally much lower, thanks to their smaller sensor size and lower quality optics.
Marketing for cameras and phones might focus largely on high megapixel counts, but as an educated consumer, keep in mind that it’s not just the megapixels that matter when it comes to photo quality!