DPI Versus PPI

[Todd Caswell]

Could someone please explaine the difference between pixels per inch and dots per inch?

If a publisher is asking for files at 72 dpi or 300 dpi what are they asking for? 72 is low res and 300 is high res but if I go into my elements and try to resize all I find is Pixels per inch.

Also when I save a photo onto my drive straight from my camera is it a 300 dpi image? I feel really stupid asking this, so be nice

[Dbl]

Your print shop/magazine has asked for a digital photo at 300 DPI. Here is what they mean by that, I think!

They are really asking for a photo that will print at a certain paper dimension in inches at 300 pixels per inch (PPI). The term DPI is a holdover from when this setting in a digital photo would set the paper output quality (resolution) of a printed image (number of printer dots per inch). This is no longer the case. So DPI is a print term. DPI is a measure of how an image is printed on paper. Many software programs call DPI a measure of "resolution" which leads to more confusion since it is the resolution of the printed output, not anything to do with the "resolution" of the digital image.

If your publisher is looking for a digital photo to print at 10 inches by 8 inches, at 300 PPI, then they are really looking for a digital image with a resolution of 3000 pixels by 2400 pixels (regardless of the DPI setting of that image).

The concept that 300 PPI = photographic quality is also a holdover from the quality of printing equipment years ago. Current printers will output a good quality digital photo, with "photographic quality" at 200 PPI - so the requirements for a 10 inch by 8 inch paper photo become a good quality digital image with pixel dimensions of 2000 pixels by 1600 pixels.

What they really need is a file that is xxx amount of pixels. PPI is a term that equates to the amount of pixels per square inch or what your Canon camera (I think you shoot Canon) properly refers to digital resolution as the pixel dimensions of the photo. Neither the DPI or PPI setting in a digital photo changes the digital quality of that photo. In simple terms the resolution of a digital photo is its pixel dimensions. So when you save your file from your camera you are saving a file in PPI.

I found this example which illustrates the problem on a commercial photo web site.

Quote:

Here's the scenario - a print shop/graphics designer/magazine asks a client for a photo at 300 dpi. They wish to print it out at 5" x 7". The client already has a beautiful digital photo with pixel dimensions of 2048 x 1536. The client notices that the photo editing software is showing that the photo is set to 72 dpi. So, following orders, the client types in 300 to reset the dpi to 300. In doing so the image is resampled and is enlarged over 4 times to pixel dimensions of 8533 x 6400. The client sends this enlarged 300 dpi photo. The print shop/graphics designer/magazine reject it (too grainy, too colour blotched). The client is crushed. The sad thing is that the client already had the perfect photo (2048 x 1536 @ 72 dpi) which would have been beautifully printed at 5" x 7" (at 292.6 PPI). The print shop/graphics designer/magazine didn't know what they really wanted.

So here is the deal, don't just change the DPI size because some programs re-size your photo PPI. Be careful or you end up with the above example. To change the DPI without changing the pixel size of the photo you should click off "resample image" which I believe Elements offers.

[Dbl]

Just to follow up, I am working on a number of print projects this week and again I ran into the DPI vs PPI issue. My print shop loves DPI and doesn't understand how photos play into that equation at all! It is a struggle to help them understand our requirements along with speaking the same language! Some day maybe! Bottom line make sure everyone is on the same page and wants the same thing!