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I have an image in photoshop I want to print at 200%, and right now it's 48x18 inches, and the resolution is 72 dpi. I read somewhere that changing it to 600dpi would allow me to scale it by 200% at time of printing without any degradation, but I want to make sure. Should I change the dpi to something larger like 600dpi? Or what would be good?

(I've also chosen Bicubic smoother in the image size dialogue box)

Thank you for any help!

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2 Answers 2

I think there is a bit of misunderstanding as to image resolution. The number of pixels is the important factor in determining the maximum high quality print size. Pixels is the absolute measure of information in the picture. You can print at whatever DPI you want, but the higher the DPI the better the image quality and the smaller the picture will be. Percents are only relevant to adjustments being made to DPI, canvas/image size or pixel count.

Increasing the number of pixels artificially to have a higher size and "higher DPI" will not increase the quality at all. It will actually decrease it unless it is an even multiple of the original number of pixels since the software will have to guess at the in between pixels through a lossy process called interpolation. You don't gain anything in terms of quality unless the printer is incapable of making a smooth interpolation.

To achieve a high quality print, the general requirement is 300 dpi which means that to get a 48 by 18 inch print, you would need about a 78 MP image. For a 16 by 20 image you need about 24 MP. The image you have is only 4.5 megapixels and is only going to be sufficient to get a high quality 11 by 4.

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Ah, I see. So if I'm sending this 48 x 18 inch to the printing service and telling them to print at 200%, I just need to set it to 300dpi and they should be able to just change it instead to 96 by 36? –  Gnuey Apr 13 '13 at 6:12
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DPI and ppi are two separate values. See photo.stackexchange.com/questions/1715/… The 300 pixel per inch guideline assumes a viewing distance of 12 inches. If the viewing distance was going to be more like 2 feet, then 150 ppi might work, but that would still require a 7200 X 2700 image. In the case of a 3456 X 1296 image, though, there's not a lot of room to work with. Even at 100 ppi, you would only get a 35 X 13 print, which would probably be too small to view at 3 feet. –  Michael Clark Apr 13 '13 at 6:23
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Keep in mind that the DPI number embedded in the JPEG (and the number in the resizing dialog) is just a suggestion. (See Does the dpi number reported by camera in JPG have any meaning?). The print service will be used to this and ignore it. If you need more pixels than you really have, pre-upscaling the image (possibly with special software) can have a slight impact, but mostly you just send the file and tell them the dimensions you want. –  mattdm Apr 13 '13 at 11:35

It depends solely what you are printing it on. I will assume you are using a large format inkjet. If this is the case, 100dpi will suffice as you will not be standing close enough to a 48inch print to notice any slight imperfections.

If however you are planning to have it litho printed, you will need 300dpi.

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