This is an important question when it comes to print; dpi or dots per inch is an older and general term that has been used sometimes incorrectly to apply to many things relating to digital images and printing. More recently, the term ppi or pixels per inch has become popular due to the description being more specific to the actual term itself. It doesn’t help when many programs still use the older terminology. Confused? Read on…
Dots per inch refer to the printer and its printing process and not the resolution of the image itself. Printers have a small and limited amount of ink colours to print from and they need to be able to mix up as many of the colours from the image as possible. To do this, each pixel on screen when printed is made up of a series of small dots from the printer. The higher the dpi from the printer means the more printed dots per inch and so the better the colour range and tonality of the image.
For example, a 1200dpi printer will print 1200 dots per each inch of the image. Generally, this should give much crisper and smoother blended colours as opposed to a 30ppi printer or setting in which only 300 dots would be printed. Most printers have the option to choose the output resolution depending on the type of image you want to achieve. You will save time and ink if you print a rough image at 72dpi which will probably appear jagged and blocky in colour and will use more ink if you want to print out a sharp and colourful photo for best which will also take longer to print. It’s common sense really; the more dots per inch, the better the printed image.
Pixels Per Inch
So, what is ppi or pixels per inch? Well, the term describes itself and is literally the number of pixels that make up each 1″ square of digital image and is the correct way of measuring the resolution. For example, if a square image is created at 100px x 100px and measures 1″, its resolution would be 100ppi because there are 100 pixels per inch of image.
The ppi will determine the quality of the image and there are a lot of sources of information that give various numbers as to what is correct for printing. In theory, the higher the ppi, the better quality the image and less chance of seeing a jagged and pixellated image (where you can actually see the pixels as squares making up the image rather than smooth blended colour). We usually recommend supplying/using images that are 300ppi at the correct size of final print for most items. Larger banners and posters often can get away with less if viewed at a distance as the pixellation will not be as apparent but will be visible on closer inspection.
A very brief explanation of resampling is needed here. Some programs such as Photoshop have the option to resample an image. Resampling will change the number of pixels per inch in an image. If you are told an image is too low in resolution, taking the image into Photoshop and resampling it by adding more pixels per inch to increase the resolution does not work. This is because Photoshop has to guess the pixels to add which can cause all sorts of strange colour and quality issues. Best to find another image or a better version of the one you are using. Resampling downwards to decrease the file size in most instances is fine, particularly when using the image on the web. Should you choose to change the file size and not resample, altering the ppi will change the final print size, but not the actual amount of pixels per inch.
For example, if we take the 100px x 100px image we talked about earlier and resample it by changing the ppi to 10ppi, the dimensions will not change. The image will still measure 1″ but the computer gets rid of the extra pixels so it now it contains 10px x 10px instead of 100px x 100px. If we choose not to resample and the same square image is enlarged in size, the resolution will reduce and if scaled down, the resolution will increase. So, if this square was enlarged to 2″, the resolution would half to 50ppi meaning there are only 50 pixels per inch and if the square was reduced to 1/2″, the resolution would double to 200ppi. You may also see cm per inch in some circumstances though this metric measurement is generally used less than inches.
It may seem pretty confusing on paper but is actually very simple in practice. The best way to get your head around ppi and dpi is to experiment with a program such as Photoshop, using the image size window and a decent printer with various dpi options.