Have you ever grabbed an image off the internet, then printed it, only to find it looks awful on paper or is the size of a postage stamp? The image looked great on your computer screen, so why does it print so badly? The reason is image resolution. Image resolution describes the detail an image holds. The higher the resolution, the more detail is in the image.
Images on the internet are low resolution: they need only be 72 ppi (pixels per inch). The reason is that they are displayed by screens that are 72 ppi. Printers, on the other hand, print at a much higher resolution: the lowest quality printers start at 240 dpi (dots per inch, literally the number of toner dots the printer puts on the paper per inch). That means when you send a 72 ppi image, it does not have enough data to be high quality, and it prints badly.
Here is an image that is low resolution. You can see how the image is all blocky: there just isn't enough data to fill in all the details.
The term “image resolution” means how many of your image’s pixels will fit inside each inch of paper when printed. Obviously, since your photo has a fixed number of pixels, the more of them you squeeze inside each inch of paper, the smaller the image will appear on the paper. Likewise, the fewer pixels you print per inch, the larger the image will appear on paper. The number of pixels that will be printed per inch is known as the resolution of the image, or “image resolution”. Image resolution has everything to do with printing your image. It has nothing to do with how your image appears on your computer screen, which is why images you download off the internet usually appear much larger and higher quality on your screen than they do when you print them.
How do you find the image size?
Open the image in Photoshop. Under the menu option Image (from the very top bar of options) choose Image Size. Up pops the following window:
The image size has two main sections: Pixel Dimensions at the top and Document Size at the bottom. Pixel Dimensions tells us how many pixels are in our image, Document Size tells us how large the image will appear on paper if we print it. In this case, we have an image that is 1200 pixels wide and 800 pixels tall. That may seem like a lot (1200 x 800 = 960,000 pixels!), and it is if the image is just going to be displayed on a computer screen. But just because it looks nice and big on the screen doesn’t necessarily mean it will print nice and big, at least not with any degree of quality. Let’s take a closer look at what the Document Size section is telling us.
The document size section tells us two things: what the current resolution is, and how large or small the image will appear if we print it based on that resolution. Currently, our resolution value is set to 72 ppi, which means that out of the 1200 pixels that make up our photo from left to right (the width), 72 of them will print inside each inch of paper, and out of the 800 pixels that make up the image from top to bottom (the height), 72 of them will print inside each inch of paper. The value in the Resolution box is for both width and height, not the total number of pixels that will print. In other words, for every square inch of paper, 72 pixels from our image will be printed from left to right and 72 pixels will be printed from top to bottom. The total number of pixels printed in every square inch of paper would then be, in this case anyway, 72 x 72 (72 pixels for the width times 72 pixels for the height), which gives us 5184 pixels!
But once again, 72 ppi is low resolution. The image would print out at a large size and look awful, all blocky and blurry. So how high of a resolution value do you need for professional quality printing? The generally accepted value is 300 ppi. Printing an image at a resolution of 300 ppi squeezes the pixels in close enough together to keep everything looking sharp. In fact, 300 is usually a bit more than you need. You can often get by with a resolution of 240 ppi without noticing any loss of image quality. The professional standard, though, is 300 ppi.
Let’s take our same image then at 1200 pixels wide by 800 pixels high, change our resolution from 72 ppi to 300 pixels/inch, and see what we get. Here’s the Image Size dialog box again showing the new resolution of 300 ppi. Notice in the Pixel Dimensions section at the top that we still have 1200 pixels for the width and 800 pixels for the height. The only thing that’s changed is our resolution, from 72 to 300:
With our resolution now increased from 72 to 300 ppi, this means that out of the 1200 pixels that make up our image from left to right, 300 of them will now print inside every inch of paper, and out of the 800 pixels contained in our image from top to bottom, 300 of them will now print inside every inch of paper. Naturally, with so many more pixels squeezing into each inch of paper, we’d expect the photo to print much smaller, and sure enough, the Document Size section is now showing that our photo will print at a size of only 4 inches wide by 2.667 inches high.
Where did those new width and height values come from? Some simple math is all we need:
1200 pixels wide divided by 300 pixels per inch = 4 inches
800 pixels high divided by 300 pixels per inch = 2.667 inches
The photo will now print much smaller than it would at a resolution of 72 pixels/inch, but what we lose in physical size, we more than make up for in image quality. At 300 pixels/inch (or even 240 pixels/inch), we’d enjoy sharp, detailed, professional quality print results.
Please note: You can get away with resolutions as low as 150 ppi. It is not professional quality, but it is good enough for sketches.
So for the poster project, let’s say you want to have an image that covers the entire picture plane. That means you need it to be at least 11″ x 17″ in size, and at least 150 ppi. Here’s the math:
150 pixels per inch x 11 inches wide = 1650 pixels
150 pixels per inch x 17 inches wide = 2550 pixels
How large will the final image need to be?
1650 pixels x 2550 pixels = 4,207,500 pixels (4.2 MP)
So if you have a 5MB camera, you could use a photo you took with it for your poster. You would only be able to use it at full size: if you were to blow it up, it would no longer be at the right resolution and the image quality would degrade.
Some Rules of Thumb:
- Resolution and image size are inversely proportional to each other. Enlarge an image, the resolution decreases; reduce an image, the resolution increases. Example: a 2 x 2″ image at 300 dpi (acceptable) enlarged to 4 x 4″ has a new resolution of 150 dpi (unacceptable for professional needs).
- Low resolution images print fuzzy, jagged and blurry. I will mark off for low res images.
- The settings used during the original “capture” of an image (ie: scanning, digital camera, etc) determine its base resolution. Resolution can only be improved by decreasing the image size, or by recapturing the image at a higher quality setting.
- Web images are predominately low resolution (72-96 dpi) GIF or JPEG files. This resolution is good for quick transmission over the internet, but is not acceptable for use in printing. Do not save images or graphics from a website to use in your print project!
- Upsampling is when a low resolution image is saved to a higher resolution with no changes in dimensions. Upsampling adds more pixels/dots per inch (dpi), but creates blurry images, ugly blocks of color, and high contrast in images. The only way resolution can be improved is by decreasing the image size, or by recapturing the image at a higher quality setting.
Image Resolution And Print Quality
I used this heavily to write this lesson, even copied entire chunks. My apologies, but it was hard to improve upon the original.
About Image Resolution
This is also a great article, I grabbed some things from it verbatim.
