LR5b smart preview
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For those that like there information graphically, Design 215 has a great megapixels chart. (also shown below)
Finally here is a great little megapixel calculator that will back calculate a lot of useful information, given your image size in pixels. It will calculate such measures as aspect ratio, file size in various formats, approx. number of photos that will fit on your memory card at that resolution and suitable print sizes. It is worth noting that the 300dpi requirement so often claimed by otherwise knowledgeable photo sites, is a little misleading. The dpi (dots per inch) is actually a measure that dates back to traditional printing whereas modern ink jet are more truly measured in ppi (pixels per inch). Each pixel normally being produced using 4 tiny dots of CMYK colour, Cyan, Magenta, Yellow, Black, in a 2 by 2 dot pattern. Frequently these dots overlaps on purpose. So at the simplest level 300ppi is equivalent to 150dpi. A lot of common ink jet "photo" printers normally only have a standard print density equivalent to 120 or 190 dpi, but with good results. Some of the more expensive ones may offer a High Res, setting that will be equivalent to 200-300dpi. So if you are a keen home printer you might be able to "push" the maximum size a little.
Remember that megapixels are not a measure of the true picture quality, which relies on a number of other factors, exposure, focus, composition, points of interests, colour balance etc. Megapixel is purely just a measure of size.