The Importance of DPI

When it comes to digital imaging, and image is never just an image. Image files all have a variety of properties, such as colour models, dimensions and, most importantly for this article, DPI. Quite simply, DPI stands for "Dots Per Inch" and is a property used to prepare digital images for print.

Basically, when things are printed in full process colour (CMYK), the printing is done in four stages - one pass for cyan, one for magenta, one for yellow and one for black. In each pass, tiny little dots are printed on the page, according to how much of that particular colour is needed across the page. The closer the dots are to each other, the stronger the colour will be. Conversely, the further apart the dots are, the more the paper will be visible, an the lighter the colour will become.

The following image is a magnification of a printed page, and gives you some sense of how the different coloured dots work together.

Now, the importance of DPI comes in because, the higher the DPI, the more dots can be effectively printed across a single square inch of paper. The higher the DPI, the clearer and more detailed an image will be (up to a point - at a certain point, the difference in detail will be come imperceptible to the naked eye). Generally speaking, 300 dpi is ideal, and many printers will recommend submitting your artwork at 300 dpi.

Unfortunately, many images come to us at 72 dpi, which is fairly standard for on-screen display. Before these images can be used in any designs, they need to be converted to 300 dpi. Seems pretty simple, right? Especially since programs like Photoshop have image resampling/sizing options, like this:

Be careful! While this is the Photoshop tool that you will need to use to resample your images, there are a few things that you need to keep in mind.

First, be very careful of up-sampling. Up-sampling basically involves increasing the overall size of the image, and in the vast majority of cases also involves a loss in quality. With this kind of enlargement, the graphics program has to redistribute a finite amount of colour across a larger amount of "space." Invariably, this leads to a pixelated and/or blurry appearance.

Second, be careful of simply changing the DPI from 72 to 300. Making this change is necessary, but you will still want to watch the dimensions of the image itself, preferably in pixel units (as opposed to inches, millimetres, points etc.). You will probably notice that the size of the image increases as you increase the DPI. This is because DPI is directly associated with the number of pixels in an image. If you look at the dialogue box we looked at earlier, you will notice that the DPI is set at 72 and the size of the image, in inches, is 6.25" squared. If we recall that DPI stands for Dots Per Inch, and multiply 72 by 6.25, the resultant product is 450. Now, notice that the size of the image in pixels is 450 squared.

Essentially, then, a pixel is theoretically comparable to a dot, and as such, when we increase the DPI of an image, we also increase the number of pixels in the image. This in turn, results in up-sampling and all of its disadvantages.

The solution is to increase the DPI without any increase in image size. In some programs, like Corel Photo-Paint 8, you can select an option called "maintain original size." Selecting this option first will allow you to modify the DPI settings without actually affecting the image's pixel count. In Photoshop CS2, simply uncheck the Resample Image option. This will allow you to adjust the DPI or the image's relative size (in inches, millimetres, points etc), without actually affecting the pixel count.

Bear in mind that, as you increase the DPI of an image, its digital size may stay the same, but its presumed printed size will shrink proportionately to the DPI. So if we use our same example of an image that is 450 pixels, squared, its image size relative to its DPI would be as follows

This has two important implications. First, it means that the image that was very large at 72 DPI, will be less than a third of that size at 300 DPI. As such, it is a good idea to purchase or create large images that can be scaled down to meet your size requirements. Second, remember that DPI determines the size of a printed image, and while it is possible to further down-sample an image, avoid trying to scale an image to a size larger than that defined by its DPI. If you need to make the image a bit larger, consider lowering the DPI down to around 250 or so.

Basically, what it all comes down to is to be careful when resizing and/or resampling your images. Avoid attempting to upscale images and be sure to maintain your image size when resampling from 72 DPI to 300 DPI. If you can follow these directions, you should be able to avoid many of the sampling nightmares that so many beginners and do-it-yourselfers face all the time.