File Size Versus Picture Size
One of the most important characteristics of digital cameras is the size of images they produce. It is expressed in megapixels. For example, if the image size that camera makes is 3,000 x 2,000 pixels, the image is 6 megapixel large. It has little to do with the number of bytes or the actual file size that takes up space in your memory card. This is because you can use different compression rates: the more the image is compressed the worse is its quality, but the pixel dimensions remain the same. Now is the question: what image size do I need to produce a nice looking 6 x 4 inch photo? Without a loupe, a good eye can see about 5 line pairs per (one white and one black line is one pair) millimeter, that translates into 127 line pairs or, if the lines are shrunk to dots, 127 dot pairs or 254 dots per inch (dpi). To make up for even better eyes than average, industry assumed that an image with 300 dpi resolution is a good quality image. Traditionally, it has been also assumed that a standard monitor resolution is 72 pixels per inch (ppi), but recently it is about 100 pixels per inch. We can still see those tiny dots on the screen, but usually they do not bother us.
So here's the answer: for a good monitor screen (with 100 ppi) a 6" x 4" image needs 600 x 400 pixels, and for a HQ 6" x 4" print we need 1800 x 1200 pixel image (about 2 megapixels). We know from experience that a 2Mpix camera not always produces those nice-looking post cards. This is because the image is frequently not as sharp as we'd wish it to be and also we frequently need to crop the original, so that we end up with less pixels to play with.
Currently I am using a full frame (24x36mm) Canon 5D Mark II camera, that makes 21 megapixel images. How large I can make my pictures is explained on the figure below.
When we hold a photo in our hands (sometimes wearing glasses) a good rule is that a resolution of 300 ppi is sufficient. When we go into 3 x 2 feet posters, on the other hand, we don't need this high resolution because we look at them from a distance, so sufficient is the 'monitor' resolution or about 100 dpi. Therefore, the sizes that you see on the figure above still hold for poster size prints.
Now a slightly different subject. How really large is a gigapixel photo? I've made some panoramas that are about a third of a gigapixel. The figure below shows the dimensions of such a panorama.
Now imagine a 3x taller picture. It would be one gigapixel in size.
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