“DPI” creates more confusion than any other topic on photo.net. In an effort to clarify this, photo.net is publishing this FAQ that should answer many questions before they are posted to the forums. </P
What does DPI stand for: Dots Per Inch. Note that DPI specifies a relationship rather than an absolute value. It is the number of dots that are in an inch of something. This is important to understand, as we will come back to it.
What is the difference between DPI and PPI: This is problematic. Many people use them interchangeably, and many image software programs use the term DPI when they mean PPI. PPI means pixels per inch and is the relationship between pixels in an image and inches in something. Technically, DPI has to do with how a printer lays down ink on a page. Fortunately, Adobe has been moving towards PPI, see Fig 1.
My scanner requires setting DPI:DPI matters for scanning because you are translating something in inches (the physical dimensions of your film) into pixels. In this case, you need to set the DPI for the largest size you will ever require. For example, scanning a 35mm for maximum resolution with a 4000DPI scanner, if you set DPI to 4000, you will get an image that is 4000×6000 pixels (4000 × 1″ and 4000 × 1.5″). After the image is scanned, the DPI setting is not important until you print.
What PPI do I need for my images: It doesn’t matter. Until you print, PPI is irrelevant. Even when you print, PPI may be irrelevant. What you need is to know how many pixels you need for either screen display or printing. For screen display, the DPI setting is irrelevant. It can be set to 1 or 1,000,000 and the image will be displayed exactly the same. If you want to check this, make an image with different settings in the dpi field and you will see they are identical. Fig 2 and Fig 3 show the same image, processed identically except for the setting in the PPI box in Photoshop. The file sizes are identical.
I thought screen images had to be 72PPI: No. This is a myth and it is propagated by the default settings for camera output and most image processing software. One more time, the DPI/PPI setting is irrelevant to screen display. Actual resolution of the screen is not a factor. You are still displaying pixels.
My camera sets DPI to (something) in the images: This is also irrelevant. It doesn’t matter what the camera sets dpi to as the file has no inches. It’s just a number at this point. If your camera has a low number, it doesn’t matter.
If DPI/PPI don’t matter, what is my image resolution: Your image resolution is the number of pixels in the width direction times the number of pixels in the horizontal direction.
What about printing: Well now we get to the part where the PPI really matters. Sometimes. Many printer drivers and most online or walk-in businesses offering printing can do the interpolation to print at the proper resolution. What this means is that you don’t have to set the DPPI to anything specific, the printer will take care of it. However, that means you live with their interpolation (enlarging or shrinking) your image to fit the size. Home printing is typically between 240PPI and 300PPI, but outsourced print businesses may not specify the DPI, so you should assume 300DPI if you want to do your own final sizing.
So now you know what you need to know. Summarizing: For anything but scanning or printing, ignore the DPI or PPI setting.