FAQ - Resolution, Megapixels, dpi
Image resolution can be expressed in different ways. Any digital image is made up of pixels, square blocks that create the image. The first image above is 20 pixels wide and 20 pixels high. Multiply these two numbers and you get the total number of pixels in the image - 400.
The more pixels there are, the sharper an image will appear. An image that is 1,000 pixels high and 1,000 pixels wide would have 1,000,000 pixels also known as 1 megapixel. Our scanners create images that range from 3 megapixels to 40 megapixels.
DPI (dots per inch) is a measure of how many pixels (dots) can fit into one inch. This can be misleading because it does not tell us anything about the number of pixels in the image. One digital image can be 400 dpi and actually have more pixels than an image that is 800 dpi. When we use the term dpi - dots per inch - we also must know how many inches there are in the image before we can know anything about how many pixels the image has. One image can be 1 inch wide and 1 inch tall at 800 dpi and another image can be 4 inches by 4 inches at 400dpi. In this case the image that was 400 dpi would be the larger, sharper image. For purposes of scanning dpi is an acceptable way to compare scans of the same slide type because the size of the film area will not vary from one scan to the next. If, however, someone has told you that you need an image at some specified dpi (72, 150, 300dpi, e.g.) for some submission or application that you may be using what they probably mean is that it should be that resolution at its full print size - so if you intended your slide to be made into an 8x10 and someone wants it at 100 dpi your final image needs to be 800x1000.