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images - what is dpi, ppi & resolution

ThePrintGroup Resolution Scale Image2

When supplying artwork and images for print, you will often hear the words “high resolution” and “300dpi”. Let’s take a closer look at what this actually means. We’ll explain the differences between image quality for web vs image quality for print, and the correct terminology regarding size and resolution.

DPI, or dots per inch is the terminology used when describing the resolution of a printed image. It indicates the number of dots in a printed inch. The image is of higher quality (in sharpness and detail), if the image has a high dpi value. This term is often misused, as when someone asks for a 300dpi quality image or graphic, they usually mean 300ppi.

PPI, or pixels per inch is the terminology used to describe the pixel density of an image on the screen, as opposed to the dpi of a printed image or graphic.

Image resolution refers to the number of pixels in an image, or the detail it holds. A pixel is just one unit of the whole digital image and the resolution depends on the size of the pixels it contains. The higher the resolution, the more detail. Resolution can also refer to the width and height of an image as well as the total number of pixels.

For example:
- Image 1 is 297 x 210mm (A4) and 300ppi.
- Image 2 is 50 x 35mm and 300ppi.

While both images offer the same ppi resolution, the width and height of each image is different. Each image will only remain at 300ppi at its original size. The ppi resolution of each image will change depending on if you are making the file smaller or larger. If you try to increase the width and height of image 2 to fit a 297 x 210mm (A4) area, you would effectively be reducing the image quality to 50ppi and dramatically reducing sharpness and detail of your printed material.

TPG Resolution Comparison

So when you are asked for a high resolution, 300ppi image, it is best to purchase or supply, the largest size (in height and width) available. This will make it much easier to resize without effecting the ppi resolution and quality of the print.

Now that we’ve briefly touched base about these basic concepts of ppi resolution, it’s easy to see how an image may look great on your screen, however be completely unsuitable for print. For images to look great on a screen, they generally only need to be 72ppi, but again the rules of image width and height still apply here.

Another great example of this is when web graphics are copied and pasted into a document or brochure by a novice preparing the file for print. The image might look great on screen, but will often be pixelated and poor quality when printed. This is because the ppi resolution of the image is most likely to be 72ppi and recommended resolution for a great result in print is 300ppi.