8 Replies Latest reply on Mar 3, 2010 3:37 PM by Frigglesworth

    72 pixels per inch?

    Frigglesworth

      i have read that images for the web should be 72 pixels per inch...

       

      i have also read that images used for print should be 300 pixels per inch...

       

      does this mean if I have an image that has resolution of 300 pixels per inch I have to somehow change it to 72 pixels per inch to use it for the web?

       

      this is confusing me because when I resample an image to the needed pixel dimensions it doesn't change the pixels per inch and it stays at 300 pixels per inch...

        • 1. Re: 72 pixels per inch?
          Level 5

          Frigglesworth wrote:

           

          i have read that images for the web should be 72 pixels per inch...

          You read incorrectly or you were reading the writings of someone without a clue (most likely the latter case).

           

          Web images are measured by the number of pixels tall by wide. There is no other resolution data needed.

           

          '72' is a default value that Photoshop uses when there is no resolution defined. It means nothing.

           

          Google for the '72 dpi myth'

          • 2. Re: 72 pixels per inch?
            whytefly Level 1

            You do not have to change the images pixel count but it will reduce your file size which in turn reduces the upload time for the viewer.

             

            You can do this by:

            Image>Image Size - then change the resolution.

             

            Also use the save for web option: File>Save for web & devices...

             

            This helps with file size as well.

            • 3. Re: 72 pixels per inch?
              Frigglesworth Level 1

              thanks for the responses everyone cleared up a lot...

               

              so I was reading that the human eye can't distinguish past 300 pixels per inch for printed images... so using 400 pixels per inch for print would be uselesss...

               

              does the SAME CONCEPT apply to web images at 72 pixels per inch? say I have an image with 300 pixels per inch I am using for the web... am I just making load times slower when I should make it 72 pixels per inch? is this a common practice to degrade images like this for the web?

              • 4. Re: 72 pixels per inch?
                Frigglesworth Level 1

                hmmmm... i see when I do a save for web and devices that it saves it at 72 pixels per inch... i guess this is the optimal way to do things for web images in internet browsers???

                 

                but my file only went from roughly 1.6mb to 1.2 mb when changing from 300 pixels per inch to 72 pixels per inch... i dont think this would make much of a difference for load times even if i was using the image for JQUERY simple animations... what do you guys recommend?

                • 5. Re: 72 pixels per inch?
                  whytefly Level 1

                  as marian said the 72 dpi is a myth.

                   

                  http://www.rideau-info.com/photos/mythdpi.html

                   

                  Make a standard for your image quality and stick with it I say. If I have an image that does not look good to me on one of my sites, I make sure I fix it.

                  • 6. Re: 72 pixels per inch?
                    whytefly Level 1

                    1mb file? what are the dimensions of that thing?

                    • 7. Re: 72 pixels per inch?
                      John Woram Level 1

                      "hmmmm... i see when I do a save for web and devices that it saves it at  72 pixels per inch."

                      Nor exactly. The ppi setting in the Document Size area is nothing but an instruction to the printer to print 72 (or whatever) of your pixels to each inch of paper if you decide to print it. This info is meaningless on the web, so "Save for Web" strips it and other data from the file, and that helps make the file size as small as possible. If the file is re-opened later on, Photoshop can't find the missing ppi data, so it plugs 72 ppi back into the "Document Size" box.

                      • 8. Re: 72 pixels per inch?
                        Frigglesworth Level 1

                        cool alright thanks everyone... i will go on my own little PS mission trying to inform people of the 72 pixels/per/inch myth now... that is doesnt degrade rather just strips resolution info...

                         

                        and i will also "save for web and devices" everytime and do things correctly when preparing images for the web...

                         


                        thanks again everyone... it has been a very informative day ...