DPI
The term DPI simply means dots per inch. A dot is also often called a pixel. So sometimes the term PPI (Pixels Per Inch) is also used. Actually the term pixel is only correct when referring to digital files and not when referring to printed media. But we'll come to that later. It is not important right now.

One very important thing you have to remember is that DPI is absolutely not a measure of image size. This is probably the most often made mistake. People will say: " my file is 72dpi and I have enlarged it to 300dpi". But changing dpi has nothing to do with the size of the file ! If the file you started with was 1000 by 2000 pixels, it will be exactly 1000 by 2000 pixels after changing the dpi setting. Regardless of the dpi setting, whether 10dpi or 10000dpi, the file will remain exactly 1000 by 2000 pixels. So the number of pixels is a measure of size for a digital image, dpi is not. DPI is a measure for resolution, Dots Per Inch. Once you realize this, things become much simpler. It's easy to see that DPI is a measure of resolution. The more dots per inch your (printed) image has, the sharper it will appear.

Suppose you want a print sized 6 by 9 inches in photo quality. What steps to calculate the required scan dpi?
1. A print of 6x9 inches in 300dpi requires a file of 6x300 by 9x300 pixels, or 1800 by 2700 pixels.
2. To get 1800 pixels out of 0.945 inches (or 2700 out of 1.417 inches), you would need to scan at 1905 pixels per inch. Divide 1800 by 0.945, giving 1905 pixels. Or divide 2700 by 1.417, giving the same result of 1905.
3. So scan your entire slide at 1905 dpi scan resolution.

DPI не применим к размерам картинки, он применим для разрешения.

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