Pixel, PPI, DPI, resolution — if you work with images, you've probably seen these terms before. Knowing what each one means will save you time with images, and help you to make the best choices for your book.

Resolution

PPI vs DPI

Checking image resolution

Fixing image resolution

Resolution

  • Resolution describes the degree of detail and clarity in a image.

  • A high resolution image shows things clearly, and with a lot of detail.

  • Image resolution is measured in PPI or DPI.

PPI vs DPI

PPI and DPI are often used interchangeably, but they are not the same.

PPI

PPI describes the number of pixels in one inch of a digital image.

  • A pixel ("picture element") is the smallest single part of a digital image.

  • Zoom in on an image and you may see a mosaic of tiny squares. These are pixels.

  • Higher PPI images are clearer, because the pixels are denser.

DPI

DPI describes the number of ink dots in one inch of a printed image.

  • Printers print by transferring dots of ink or toner to paper.

  • To print your image, a printer translates pixels into physical dots.

  • Images print best if they are 300 PPI at the width and height you need.

Pixel count

Pixel count is the number of pixels across the width and height of an image.

  • A 4 × 6 in digital image at 300 PPI has a pixel count of 1200 × 1800.

Checking image resolution

You can find the resolution on any image, and decide if it is high or low.

On a Mac:

  1. Open your image in Preview.

  2. Select Tools from the top menu, then Adjust Size.

On a PC:

  1. Right-click on your image, then select Properties.

  2. Select the tab Details.

Fixing image resolution

Say you've checked your image and the resolution is low, but you still want to use it in your book. What can you do?

There are two ways to fix images. Both involve sacrifice — either you must make your image smaller, or plump it up artificially. The lesson is if you can take a new photo, re-scan your picture, or export your image again — you should.

Reduce image size

If your image is very large, you can increase the resolution by making the image smaller. A smaller size means the pixels must fit into less space, which raises the PPI.

On a Mac:

  • Open your image in Preview.

  • Select Tools from the top menu, then Adjust Size.

  • Make sure Resample image is unchecked.

  • Enter a new value for width or height. The resolution will increase.

  • Or, enter a new resolution. The image width and height will reduce.

  • When done, click OK.

Keep the same image size

You can upsample an image to increase the resolution without changing the image size. Upsampling adds pixels — the software creates new pixels in your image to maintain the same quality at a larger size.

On a Mac:

  • Open your image in Preview.

  • Select Tools from the top menu, then Adjust Size.

  • Check the box beside Resample image.

  • Enter a new resolution.

  • When done, click OK.

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