When discussing the resolution of digital images, far too often people get hung up focusing on DPI (or PPI) as a way to measure the size and quality of a digital image. This may sound completely wrong to you, but the DPI of an image has nothing to do with digital image quality. The reason? A digital image’s resolution is expressed as it’s pixel dimensions, either as pixels by pixels or the often heard keyword, megapixels. So why do people get hung up on DPI? The simple reason is that when it comes to printing, DPI actually IS the measure of quality. Confusing, right?
10 DPI vs 1,000 DPI
Click on each image below. Can you tell the difference?
You should be saying to yourself, “They look exactly the same.” Both are 600 x 900 pixels, both saved at the exact same 20% compression rate but they have vastly different DPI values. So why isn’t there a visual difference? As noted above, digital image quality is measured by the number of pixels in an image, either by the pixel dimensions or the megapixel value. In this case, they are both 0.5-megapixel images since they have the exact same pixels dimensions (600 x 900 pixels).
Now, if you want to discuss the DPI of a digital image, YOU MUST ALSO assign a physical size to it. You can’t simply say, “This image is 200 dpi.” You have to say something like “This image is 200 dpi at 4 inches by 6 inches.” It is important to remember that a digital image has no absolute size or resolution. Think about this, when do you typically discuss DPI? The answer you should be thinking about is “When I want to print the image.” This is where DPI comes into play because a printer may output at 150 dpi, 200 dpi or 300 dpi and each would require a different file size to print the optimum print. You may also be thinking that you discuss DPI when you scan a photo, but we will get to that later. For uploading to social media, 150 dpi is adaquate.
The issue that causes all this confusion is that many users interpret a photo editing program’s reference to DPI as a measure of “resolution” but this is actually the displaying the OUTPUT or printing resolution, not the resolution of the digital image. Has this happened to you: You had your photos scanned by ScanMyPhotos but when you open the file in Photoshop, it says that it is 72 dpi and you clearly paid for 600 dpi? Before you start writing a strongly worded letter to ScanMyPhotos, look at the width and height? Does it show that the photo is 50″ by 33″? We all know you did not send a 3 foot by 4 foot photo for scanning so what happened? Nearly all monitors can only display 72 dpi so most programs default to showing 72 dpi.
So, when working in Photoshop for example, the first thing you want to do when looking at File->Image Size is to TURN RESAMPLE IMAGE OFF. If you then turn your attention to the rest of the dialog box, you can see it connects Resolution, Height, and Width together while Pixel Dimensions is separate and uneditable from the Document Size. As long as you have Resample Image turned off, if you change any one of the values for Width, Height or Resolution you simultaneously change the other two, but the Pixel Dimensions will always remain the same. As the resolution goes up, the width and height go down, and vice versa, because a digital image has no absolute size or resolution. All it has is a certain number of pixels in each dimension that will be displayed on a monitor or screen. Click on the image below for a detailed look at the relationship between size and DPI and how it doesn’t affect the actual pixels in the image.