Image Resolution – DPI/PPI/SPI

If you are supplying image files or creating your own graphics, you need to know this.

Why do you need to know this?

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Enough said.

The Shortcut

The quick way to check if an image will work for your printing needs is to look at the number of PIXELS. Don’t worry whether it says it’s 300 or 72.

Formula: final print size in inches x 300 = # of pixels you need to print an image

So if the final printed size needs to be 5 inches across, you will need a digital file that has at least 1500 pixels across.

Terminology

DPI (Dots Per Inch) is probably the most misused and confused term for resolution. And it is probably the least important as well. It is actually your PRINTER resolution – how many dots of ink or toner a printer can place within an inch. Some scanners also use the term DPI, however, there are no dots in an image until it is printed.

The amount of resolution required depends on the type of printer used. For example, for conventional commercial printing, the resolution required is approximately 300 dpi. For ink jet printing, usually a resolution of 120-200 is all that is needed. That’s why you can print something on your inkjet and it looks great, then submit it for commercial printing and it sucks.

SPI (Samples Per Inch) is the official measure for image resolution in scanning.

So what term should you use?

Because Photoshop uses the PPI (Pixels Per Inch) for image resolution (go to Image menu, Image Size), this is what most people end up using to measure and describe resolution.

Technically, PPI is the DISPLAY resolution – how a monitor displays an image (how an image looks on your monitor is ultimately determined by the resolution of the monitor — the number of pixels it can display in a given area). But it can also translate well to image resolution.

What is important? Total number of pixels. The resolution can be set to anything (72, 300 or 3000 dpi), and as long as the total number of pixels in the file are kept the same, it’s all good.

When someone says they need a 300 dpi file, EVERY file can be measured at 300 dpi, whether is happens to be saved that way or not. For example, let’s say you just shot a picture with your spiffy new camera. It’s 3500 x 3530 pixels, but by default the image was saved at 72 dpi. So if you divide the numbers of pixels by 72 dpi, you will get the total number of inches the picture can be reproduced at.

Magnify a photo several hundred percent on your monitor and each square (the smallest element) you see is a single pixel (from the term “picture element”). This is the resolution that counts. The more pixels the photo contains per inch, the better it will look on your screen and print. It’s similar to the quality difference between a photo in a newspaper and magazine.

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Why can’t I use an image from my website?

High quality commercial printing onto paper requires at least 300 PPI. Web images are set at 72 PPI because that’s all that monitors need to produce a good visual image.

What about images from videos?

These are called “captures.” They work the same as web graphics. A typical video setting for for TV is 720 x 480 pixels. HD TV is higher, but even that is only 1028 pixels.  An image that would be printable at 5 inches across would have to be 1500 pixels across.

Why can’t I just enlarge an image?

Photoshop cannot accurately invent pixels that were never present in the image to begin with. If you take a photograph or scan that starts at 72 PPI and enlarge it to 300 PPI for printing. You have just added 228 pixels for every inch of your photo. Photoshop now has to approximate what those new pixels should look like based on the pixels around it. What you end up with is a blurry mess if you ask it to create to many because it has to start making pixels from other pixels it just made. Rule of thumb – don’t try enlarging an image more than 100%.

While insufficient resolution will result in lower quality printed images, too much resolution results in wasted information and unnecessarily large file sizes. I do recommend starting with scans and photos at a higher resolution than you think you need so that you can crop the image and use it for different purposes (posters, CD covers, etc.). You can always lower the resolution later.

THE BOTTOM LINE

I usually scan images for covers at 400-600 PPI. I set my Photoshop file resolution to that as well in case I have to create a large poster from it some day. A 5×5-inch CMYK TIF file at 300 PPI should be 8.58 MB. At 600 PPI, it should be 34.3 MB. Photoshop files will be even larger because of the additional layers and data it stores.

To find out if your image is large enough to print without pixelating the image, divide the number of pixels by the resolution. For example: # pixels / resolution  = size in inches that it will print without degradation

How much is too much resolution?

Too much resolution a) makes your files larger than necessary, b) increases the printing time and c) can have the effect of softening your images. The best solution is to have the maximum amount of resolution required based on your final image size, your printing method and line screen.

Checking Image Resolution

photoshopimageresolutiondialogbox

This is Photoshop’s Image Size dialog box.

  • Note the pixel dimensions shown in the upper portion of the Image Size dialog box: 1500 x 1500, 8.58 MB.
  • Note the physical dimensions 5×5 inches
  • Note the Resolution: 300 pixels/inch (ppi)

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