Maybe I’m being too persnickety, but dots (from the abbreviation dpi) are not pixels (of the abbreviation ppi). Just like caterpillars are not butterflies. So I’m going to take you on a little geek-out trip into the dot/pixel world.
Let’s start with your computer. You get a digital file from a graphic designer and on the page is a great big letter “i” in a gray colour. How does your computer (or tablet or smartphone) display that “i”? Let’s take a real close look at the edge of that letter. The computer divides every document into little squares, and each square is filled with a colour. These squares are called pixels. On the image to the left, you can see the squares that fall outside the letter remain white, while the squares that fall inside the letter are completely filled with the gray colour. A pixel is always filled completely, and is only filled with one colour, never several shades of colour.
Let’s compare that to what this would look like printed. Your printing device will also divide your page into invisible squares. Only this time the square is the target area for a dot of ink. You can see on the second image, where the gray “i” is there are black dots filling approximately half the area of their assigned square. Since the letter is 50% black, then the black dot covering half the area makes sense. Remember, this dotted view is really zoomed in. When viewed normally, the black dots and the white space around them blend in our eyes, and we see gray.
When you type a darker gray letter, then print it, the printer issues a bigger glob of ink and the dot fills most of it’s assigned space. If printing a colour, the printer will drop up to four drops of ink (a cyan, a magenta, a yellow, and a black) to achieve the various colours. An orange peel would be made up of squares each with a drop of magenta and yellow in various sizes. So, printing is done with dots. It is the size and colour of the dots that create the shades and tones of an image.
What all this means is that dots have nothing to do with digital files, nor the resolution or quality of that digital file. Unless you are receiving something printed, a digital image cannot be described as 300 dpi, because there are no dots whatsoever. The file you get from a designer or the digital image you use does not have dots, thus cannot have a dots per inch value. Dots are only when ink or toner is laid down on a piece of paper.
I could go on about the incorrect use of 300 dpi digital files, like 300 dots per inch would produce a poor quality print (draft on most home and office printers), but let’s leave it there – dots per inch or dpi does not apply to digital files.
I can hear you saying, “Fine, I want an image that is 300 pixels per inch, if that makes you happy.” And I would reply, “It’s not that simple.” Yes, I can hear the sound of face plants in palms.
What is actually important to know about an image is its resolution. And no, resolution is not the same as the pixels per inch (and definitely not the printer’s dpi). Let me explain. Let’s say I have an image of a bowl of strawberries and there are 1000 pixels across and the same down. So, 1000px by 1000px is the resolution. You can make that image 10 inches wide by 10 inches long. Those 1000 pixels are now evenly spread across 10 inches. Therefore, in each and every inch there are 100 px or the image is now 100 pixels per inch (ppi). On the other hand, I could make that same image only 2 inches by 2 inches. That means the 1000 pixels are now squashed into only 2 inches – or 500 pixels per inch (ppi).
So, the same 1000 pixels can be squashed into a small space resulting in a high pixels per inch (and would render a quality printout) or can be spread out (and render a poor quality print).
Looking at the image above of sixteen red pixels (that’d be a slice of the image of strawberries), in the first line, those pixels are stretched out over 4 inches resulting in only 4 ppi and the pixels are quite large. On the second line those same 16 pixels are squeezed into 2 inches and it is now 8 ppi. It is the same 16 pixels, but now a lot smaller and this higher density of pixels renders more detail and a crisper image. On the last line, all 16 pixels are squashed into one inch across, or 16 ppi. As the number of pixels in an inch increases, the quality of the print increases.
What is really important to note, is that any image can be made to be 300 ppi. It might be really tiny and totally unusable, but nonetheless, it’s still 300 ppi. So what is more important information for you to have is the total number of pixels across and down. Is there enough pixels to be 300 ppi at the size you need? To make it really easy for people buying my products, I list the resolution of my images with the pixel dimensions as well as the inch dimensions when it is sized to 300 ppi. It looks something like, “3600px by 3600 px, 12 inches by 12 inches at 300ppi.”
So remember, dots only apply to printers and printed pieces. Since digital files are made of pixels, I cannot possibly claim to be selling something that is 300 dots per inch, when I have no idea how it will be printed. What a good designer will do is provide you with the resolution (pixel dimensions) and you can divide those numbers by 300 to decide if there are enough pixels to print the image at the size you wish. In practise, for home and office printers, anything above 150-200 ppi is probably sufficient.
Next time I’d like to cover the difference between images and graphics, or more simply put, the difference between jpgs and gifs/pngs.