Almost every time I receive a picture from a client, they ask, “Is this image big enough?” If it isn’t, the jargon begins to fly: resolution (or “res”), pixels, pixelated, screen resolution, print resolution, DPI, PPI … whew! Clients are often confused because the image looks fine on their computer and they can’t understand why it won’t look great when it is printed for the final piece.
Let me try to explain some of the technical jargon, as simply as possible, as it is related to the field of graphic design.
Image resolution describes the amount of detail an image holds. Higher resolution images are crisper and more detailed. In a lower resolution image, the fine differences in color disappear, edges become blurred, etc. There are many kinds of resolution that can apply to film, television, etc., but the two types we are concerned with here are print resolution and screen resolution.
In general, resolution is measured in ___ per ___, with those blanks depending on the medium. Screen resolution is measured in pixels per inch (PPI). There’s another piece of jargon – a pixel is a tiny square of color. A monitor uses tiny pixels to assemble text and images on screen.
The optimal resolution for images on screen is 72 DPI. Increasing the DPI won’t make the image look any better, it’ll just make the file larger, which will probably slow down the website when it loads or the file when it opens.
Print resolution is measured in dots per inch (or “DPI”), which means the number of dots of ink per square inch that a printer deposits on a piece of paper. So, 300 DPI means that a printer will output 300 tiny dots of ink to fill every square inch of the print. 300 DPI is the standard print resolution for high resolution output.
How the two work together
Documents begin on screen and either stay on screen, or they are printed. If the document will stay on the screen (like a website), you just need to worry about screen resolution, so your images should be 72 PPI.
An important note: Sometimes the terms DPI (print) and PPI (screen) are used interchangeably. So, don’t be confused if someone refers to a 300 DPI image that is on screen, because pixels per inch (PPI) translate equally to dots per inch (DPI).
If you’re going to print the document, you need to make sure the images are 300 DPI at 100% of the final output size. This sounds more complicated than it really is.
We already know that 300 DPI images have 300 dots per square inch. So, if we have an image that is 300 pixels by 300 pixels — we can print it at 1″ × 1″ at 300 DPI. If this is the final output size, we are good to go. If the image was intended to be printed as a 2″ × 2″ image, we would be in trouble, because it would output at only 150 DPI. If this was intended to be a 5″ × 5″ image, we are in trouble, because the image would output at only 60 DPI.
The bigger we try to print the 300px × 300px image (note: px is the abbreviation for pixel), the more pixellated it becomes. Pixellated is a term used to indicate the degradation of the image – the eye can start to see the individual pixels, and the edges become very jagged.
Here is how a 300px × 300 px image, printed at 1″ × 1″ at 300 DPI:
Here is how the same 300px × 300 px image, printed at 2″ × 2″ at 150 DPI would look (I had to crop in on it for the purposes of the post, but you can start to see the pixellation:
Here is how the same 300px × 300 px image, printed at 5″ × 5″ at 60 DPI would look (I had to crop in on it for the purposes of the post, but you can really start to see the pixellation:
As I hope you can see, the original image size doesn’t matter as much as what the DPI will be when the image is printed out at 100%. The image is perfectly fine at 1 inch by 1 inch, but it looks terrible at 5 inches by 5 inches because the DPI is only 60.
How can I figure out the DPI of an image?
Math. And unfortunately, it has little do with the overall file size of an image (like whether the image is 1MB or 10MB). In general, a bigger file size is better to give your designer, but there’s a more exact method to it. We need to find the overall dimensions of the image, and then do some math.
On a Mac
- Right-click (or control-click) on an image.
- Select “Get Info.”
- Under the “More info” tab, look for Dimensions.
You should see a number like “1024 x 768” (some number x some number). These numbers show the number of pixels in the image (width x height)
On a PC
- Right-click on image icon.
- Select “Properties.”
- Click the “Summary” tab in the properties window.
You’ll see values for the Width, Height, Horizontal Resolution and Vertical Resolution.
(You can see screen shots here). Ignore the Horizontal and Vertical Resolution values. Just pay attention to the overall Width and Height
So, if you want to print an image that is 1024 × 768 (listed as Width=1024px, Height=768px on a PC), you need to divide each value by 300 to see how many inches you can print at 300 dpi.
1024 ÷ 300 = 3.4133″ (width)
768 ÷ 300 = 2.56″ (height)
So, you could print this 1024px × 768px image at 300 DPI at a size of 3.4133″ × 2.56″ – any bigger than this, and you risk the image becoming pixellated. Sure, you can enlarge the image a teeny, tiny bit beyond this size, if you need to, but it’s best if you don’t.
Designers, did I forget anything? Non-designers, does this make sense to you? Is there anything that is still confusing you?
Editor’s note: The original version of this post was published May 25, 2010.