Dots Per Inch is a useful measure of relative resolution. But if you don't know the image size in inches or some other measure of size, then the amount of dots per inch doesn't mean much. That's why DPI is used in conjunction with scanner Specifications, because you know that the size is going to be 3.8 by 1.7 inches!
Different resolutions are used for different purposes. 72 or 75 DPI for screen viewing; 250 for digital photography and 300 dpi for printing. See how this is a nice comparison, yes, you can get away with 50 less pixels per inch in your photographs than would be required for A4 printing, this is because of the physical size of the photo!
Dots Per Inch specifies how much information is resolved in a picture, whereas the number of megapixels is usually used to describe the total output size (in pixels) of an image.
Example: I want to reproduce the standard photograph size. This would be 6 by 4 inches. Digital photography requires a minimum of only about 250dpi, So: 6 * 250 = 1500 and 4 * 250 = 1000, so you need an image sized about 1500 * 1000 pixels (about 1.5 megapixels). Thus you can get a perfectly good standard print from a 1.5 megapixel camera!
If you take a photo twice, one at 5 and the other at 1.5 megapixels, than ask your self which one looks "better"? They will both look reasonable because your computer generates colours such that the image looks better than it really is; if you were to print both as a 6 by 4 image, they will look identical, because your printer can't generate more than 300 dpi so the "better" image is effectively reduced in quality anyway. This example demonstrates how it is possible to literally waste memory, ie for everyday use the additional memory required for the other 4 megapixcels gives you no real benefit!
This is quite good because a 36 Megabyte media card can store 50 to 65 images at 1.3 megapixcels but only 8 to 10 at 5 megapixcels, so unless you are planning on getting your images printed larger than 6 by 4 there's no real advantage to the full 5 megapixcels.
Now we have established that a 1.5 megapixel camera will produce a 6 by 4 inch standard photograph, that is, it will generate a resolution such that each pixel is simply reproduced "as is", we come to the more interesting issue of enlargement.
If you wanted to print your 6 by 4 inch image as a 10 by 8 photo (which would normally need the full 5 megapixels), there would not be a sufficient number of pixels for the additional surface area, so we now need to scale up the existing pixels by "resampling" the existing pixels, a process which estimates how the "missing" pixels should appear, and fills them with the appropriate "colour". A 25% resample
I am the website administrator of the Wandle industrial museum (http://www.wandle.org). Established in 1983 by local people to ensure that the history of the valley was no longer neglected but enhanced awareness its heritage for the use and benefits of the community.