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‹ Photo Notes · 23 června, 2016
What is the difference between a photo of 3000 x 2000 px/300 dpi and 3000 x 2000 px/72 dpi? Let’s get this straight.
For photographers this explanation will certainly be useful; for print professionals (or those who call themselves that) it is entirely elementary knowledge.
Yesterday I came across a debate about photo processing. At its core it was nothing but so-called „measuring contests“, transferred to computer and software performance. One of the debaters wrote how fast he processes photos with a longer side of 1200 px/72 dpi, another countered that he had worked with photos of 3000 px/300 dpi. The whole part after the slash is in this case a meaningless figure.
What is DPI?
The abbreviation DPI stands for dots per inch and means print resolution. Put even more simply, it is the number of dots that fit into one inch, that is, 2.54 cm. It is, however, purely a print figure and therefore matters only in the case of print, because that is where 1 inch is used as the unit of measure. You can happily convert it to centimetres; sometimes you may even meet the value DPCM (dots per centimeter), but it’s unnecessary. The value of 300 DPI (dots per inch) is the standard for professional printing of books, magazines, photographs and so on. This knowledge is enough for us.
The computer works with pixels
The basic unit of measure of an image, in the world of computers and cameras alike, is 1 pixel (dot). DPI plays no role here. Try, on purpose, resizing a photograph to 3000 x 2000 px/300 dpi and 3000 x 2000 px/30 dpi. The result will be two completely identical photographs of 3000 x 2000 px. Nothing more. As soon as we work with pixels, DPI doesn’t exist. The value 3000 x 2000 px does tell us something else, though, and that is the suitable print size.
Let’s try dividing. 3000 : 300 = 10, 2000 : 300 = 6.6. Once more, with the values I’m counting. 3000 (px) : 300 (dpi) = 10 (inch), 2000 (px) : 300 (dpi) = 6.6 (inch). The result tells me that a photograph of 3000 x 2000 px can be printed in professional quality (300 dpi) at a maximum size of 10 x 6.6 inches. Because we work with centimetres, I multiply both numbers by 2.54 (that’s how many centimetres fit into one inch). I get dimensions of 25.4 x 16.8 cm.
So if we wanted to print a photograph twice as large, it’s enough to change its size to 6000 x 4000 px and I don’t have to look at dpi. But when I want to print, say, 30 x 20 cm, the right moment comes for me to set in Photoshop (or anywhere else) a new size of 30 x 20 cm/300 dpi.
Here the difference will show. A photograph intended for printing at 30 x 20 cm will have (computer) dimensions of 3543 x 2362 px. At a print resolution of 300 dpi. If I set the same centimetres and a print resolution of, say, 72 dpi, its new size would be only 850 x 567 px. Such a photograph could be professionally printed only up to a size of 7.2 x 4.8 cm (again it is enough to divide the number of dots by the print resolution…).
Note: the article doesn’t address the change in photo quality when enlarging it; it doesn’t address printer or print resolution, where DPI values are also stated, but that is unrelated to our topic, those are different values; and it certainly doesn’t address, for example, the LPI value (lines per inch), which gives the density of the print screen; that too is no important information for a photographer.
Hopefully that’s clear enough.
Have a nice day