If you think of resolution only in terms of megapixels, you may not realize that any digital camera can make high-resolution photos. Resolution merely refers to the number of pixels per inch, which makes "high" and "low" terms that are relative, not only to the dimensions of the image, but to what the viewer believes is detailed enough. How you capture an image plays one part in determining its resolution and how you distribute those pixels over a given space plays another. Choosing the right settings on your camera is only the starting point to getting the best resolution possible out of your images.
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Megapixels and Resolution
Megapixels play a role in determining potential image resolution, but they don't indicate the actual resolution. Megapixels are the dimensions of an image, while resolution is the number of pixels per inch, a pixel being the smallest element of an image. The more megapixels a camera has, the larger you can make a picture of the same resolution. A camera sensor may have 8.6 megapixels in an array of 2,400 rows of pixels by 3,600 columns, representing the image dimensions. If you distribute those pixels over an 8-by-12-inch area, you have 300 pixels per inch, representing the resolution. If you redistribute the pixels of that image over a smaller space, say four-by-six inches, it becomes a smaller image with 600 pixels per inch, thus greater resolution. Unless you artificially enlarge an image, it contains a fixed number of pixels, which you can interpret along a grid that assigns more or fewer pixels to an inch. For example, at 300 pixels per inch, a five-by-seven-inch RGB file takes up 9.01MB. Reinterpreted as a 72-pixel-per-inch document, the same file measures 29.167-inches-by-20.833-inches, but it still contains the same number of pixels occupying the same amount of disk space.
Since megapixels determine the maximum potential resolution of a picture, it's best to use the most megapixels possible on your camera. Most cameras enable you to change the size in dimensions of the picture, usually from large to medium to small. Each size represents a percentage of the total number of pixels the camera can record. If you choose a size smaller than the maximum, not only are you wasting some of the pixels you paid for, you're reducing the total potential resolution. You can't simply enlarge the image in software and achieve the same crisp, fine details of a file originally created with more megapixels.
Along with size settings, digital cameras also include compression settings, sometimes referred to as quality levels. Whether it refers to them as high, fine, medium and low or applies arbitrary abbreviations such as "HQ" or "LQ," your camera uses these schemes to reduce the storage size, but not the dimensions, of the files it saves, and it does so with JPEG compression. Because you can choose a low quality preset in conjunction with a large image size, you may think that compression simply enables you to squeeze more photos onto a memory card. While file size compression doesn't change the resolution or dimensions, it affects detail nonetheless. JPEG algorithms evaluate image data and remove what looks like redundant detail. You have virtually no control over what information gets discarded and it can't be recovered in software. The results can show up as blocky areas of distortion, along with the obliteration of fine details, if the quality is set too low.
Unlike other image file formats, RAW files contain all of the uncompressed information your camera's sensor records at the time of capture. RAW files don't increase the number of pixels in an image, but they can help you bring out more detail in your images because, unlike JPEGs, they incorporate more color data, a wider range of tones per pixel, the ability to control sharpness and noise, and they accommodate color corrections that may produce unwanted artifacts if applied to compressed files. Camera manufacturers create their own proprietary RAW files, each with different file extensions, that may not open directly in the everyday image editing applications you use to view, retouch and edit photos. If you value high-resolution image data for the flexibility it gives you over the final product, choose your camera's RAW file format.