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michaelw91155592
Participating Frequently
March 14, 2018
Answered

Why doesn't PPI and Image dimensions directly correlate?

  • March 14, 2018
  • 3 replies
  • 1381 views

I've done a lot of research on ppi and image dimensions and find unclear answers all over the place. My question is, if you have an image with a set size, for example, 3000 x 2000 pixels, shouldn't the ppi be determined by the size of that image? I don't understand why when I export an image from lightroom or  photoshop, regardless of the image dimensions, they all export at 240 ppi. If an image is exported at 3000 x 2000 pixels, there should be a set ppi density at that size, and the ppi should change for every different dimension, am I not correct? The main reason why I am asking is because I have a large flatbed printer I am about to blow up some prints on, and I want to make sure I get the best resolution out of these images. This particular printer prints up to 2400 dpi and I want to make sure I'm getting the most out of it. I just want to know if it will make a difference when printing an image from a file that reads 240 dpi or 2400 dpi, even though I feel like the dpi should always stay constant at a set resolution. 

This topic has been closed for replies.
Correct answer gener7

To get a visually  interactive idea, make a 3000 x 2000 pixel image and then go to Image > Image Size. Uncheck Resample.

Then starting at 200 and going to 360 ppi resolution, you will see the image size on paper shrink. PPI is the instructions to the printer at what size on paper you want that image to be. Although depending on the printer, 200-360 PPI is the reasonable range for a good printout. Pretty simple math, 3000 px/200 PPI = 15 inches.  PPI is flexible for your needs. It is not a constant number.

3 replies

gener7
Community Expert
gener7Community ExpertCorrect answer
Community Expert
March 14, 2018

To get a visually  interactive idea, make a 3000 x 2000 pixel image and then go to Image > Image Size. Uncheck Resample.

Then starting at 200 and going to 360 ppi resolution, you will see the image size on paper shrink. PPI is the instructions to the printer at what size on paper you want that image to be. Although depending on the printer, 200-360 PPI is the reasonable range for a good printout. Pretty simple math, 3000 px/200 PPI = 15 inches.  PPI is flexible for your needs. It is not a constant number.

Derek Cross
Community Expert
Derek CrossCommunity Expert
Community Expert
March 14, 2018

Your 3000 x 2000 px at 240 PPI will print optimally at around 12 x 8 inches.

Are you sure your printer is printing at 2400 DPI, it sounds more like an image setter.

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michaelw91155592Author
Participating Frequently
March 15, 2018

These prints will be coming out much larger than that, some 4x6 feet.

barbara_a7746676
Community Expert
barbara_a7746676Community Expert
Community Expert
March 15, 2018

Viewing distance is another factor that comes into play.

barbara_a7746676
Community Expert
barbara_a7746676Community Expert
Community Expert
March 14, 2018

PPI and DPI are two different things. Pixels do not have an inherent size. If you a use a 300 pixels per inch image for your 2400 dots per inch printer, you should be fine. Even a 240 ppi would most likely be enough for most images to be output on your 2400 dots per inch printer.

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