Since this comes up a lot and recently in the shout I figured I'd set the record straight and remove one of the biggest myth's in the coverworld as it seems most think that 300dpi is some kind of "image quality indicator", this is far from the truth.
DPI is the --dots per inch-- "print resolution" so it's a "printer setting" and it has nothing to do with "viewing" quality at all.
The resolution of an image is determined by the number of pixels it has, so a typical Blu-ray cover would be 3173 X 1762 pixels, we set them to 300dpi so they will print to the correct "physical" size and for no other reason.
You can change that number to 72, 96, 150, 300, 600, 1200 whatever you want and as long as the pixels are not changed and they remain 3173 X 1762 the image will remain exactly the same quality wise.
Screens can only display 72 or 96 dpi no matter what size the image is or what the DPI is set to.
Viewed images are always seen at 72 or 96 DPI, printers however can print the dots closer or farther apart depending on the DPI setting, this is why it matters and why it needs to be set right.
300dpi is considered a pretty standard resolution for printed items although most Posters are printed at 150dpi which is why a lot of studio images come at 150 but I've seen them all over the place, fact is it don't matter unless you actually print it, then you need to know exactly what that is -- else you will not know what size it will "print to" you see.
Back to the Blu-ray cover 3173 X 1762 pixels @ 300dpi will print to a finished size of 10.5 X 5.8 inches due to the 300dpi setting, in other words it will take the 3173 divide it into 300 pixels per inch and print it --- so 3173 / 300 = 10.576666666in for 3173 pixels where one pixel equals one dot printed.
This is why if the DPI is not 300 then a cover will print to the wrong size.
Let's say the image was 3173 X 1762 @ 72dpi for example it would still be the same "quality" as the 300dpi one because there are still the same number of total pixels in the image 3173 X 1762 so the image has not changed what will change it the number of dots per inch if printed.
So @ 72dpi those 3173 pixels would get spread out more on the paper, making the final printed width go from the 10.5in it was @ 300dpi to almost four times the "physical size" or 44.06 inches wide and 24.47 inches tall for the same image.
Short version DPI only matters to the printer, it affects nothing else but the final printed size.
If one were to make that number larger say 600dpi but keep the same number of total pixels at 3173 X 1762, well as you would suspect the image will be printed way too small to fit a standard case as it will drop in half "physical printed size". Or 5.28 inches wide, see the problem in order to print more dots per inch and not change the final print size then the overall pixel size would need to be increased as well -- so to make a 10.5in X 5.8in @ 600dpi image --- the total pixels would need increased to 6300px X 3840px which would be a very large file size for very little noticeable change in such a small printed image, which is why 300dpi is considered a good standard for size vs quality when printed.
Since this comes up a lot and recently in the shout I figured I'd set the record straight and remove one of the biggest myth's in the coverworld as it seems most think that 300dpi is some kind of "image quality indicator", this is far from the truth.
DPI is the --dots per inch-- "print resolution" so it's a "printer setting" and it has nothing to do with "viewing" quality at all.
The resolution of an image is determined by the number of pixels it has, so a typical Blu-ray cover would be 3173 X 1762 pixels, we set them to 300dpi so they will print to the correct "physical" size and for no other reason.
You can change that number to 72, 96, 150, 300, 600, 1200 whatever you want and as long as the pixels are not changed and they remain 3173 X 1762 the image will remain exactly the same quality wise.
Screens can only display 72 or 96 dpi no matter what size the image is or what the DPI is set to.
Viewed images are always seen at 72 or 96 DPI, printers however can print the dots closer or farther apart depending on the DPI setting, this is why it matters and why it needs to be set right.
300dpi is considered a pretty standard resolution for printed items although most Posters are printed at 150dpi which is why a lot of studio images come at 150 but I've seen them all over the place, fact is it don't matter unless you actually print it, then you need to know exactly what that is -- else you will not know what size it will "print to" you see.
Back to the Blu-ray cover 3173 X 1762 pixels @ 300dpi will print to a finished size of 10.5 X 5.8 inches due to the 300dpi setting, in other words it will take the 3173 divide it into 300 pixels per inch and print it --- so 3173 / 300 = 10.576666666in for 3173 pixels where one pixel equals one dot printed.
This is why if the DPI is not 300 then a cover will print to the wrong size.
Let's say the image was 3173 X 1762 @ 72dpi for example it would still be the same "quality" as the 300dpi one because there are still the same number of total pixels in the image 3173 X 1762 so the image has not changed what will change it the number of dots per inch if printed.
So @ 72dpi those 3173 pixels would get spread out more on the paper, making the final printed width go from the 10.5in it was @ 300dpi to almost four times the "physical size" or 44.06 inches wide and 24.47 inches tall for the same image.
Short version DPI only matters to the printer, it affects nothing else but the final printed size.
:)
If one were to make that number larger say 600dpi but keep the same number of total pixels at 3173 X 1762, well as you would suspect the image will be printed way too small to fit a standard case as it will drop in half "physical printed size". Or 5.28 inches wide, see the problem in order to print more dots per inch and not change the final print size then the overall pixel size would need to be increased as well -- so to make a 10.5in X 5.8in @ 600dpi image --- the total pixels would need increased to 6300px X 3840px which would be a very large file size for very little noticeable change in such a small printed image, which is why 300dpi is considered a good standard for size vs quality when printed.