Don’t splash out on expensive image-editing software! How to edit photos, using Preview

You may only use it to view images and read the occasional PDF, but Apple’s Preview application is capable of so much more!

If you familiarise yourself with some of Preview’s lesser-known features, then you can use it to perform a range of visual edits, such as resizing, cropping, and even removing the background of your images!

Why splash out on an expensive image editing software, when you can use a free application that came pre-installed on your Mac? In this article, I’ll show you how to edit all of your images, using Preview.

Resize your images

There’s lots of reasons why you might need to resize an image – maybe you want to free up some space on your hard drive by shrinking all of your high-resolution images, or perhaps you’re uploading the image to a website that has strict criteria about the kind of images that it accepts.

You can use Preview to quickly and easily resize an image by pixels, inches, centimeters, millimeters, points or percentage:

  • Launch the Preview application, if you haven’t already.
  • Select “File > Open…” and choose the image that you want to edit.
  • Select “Tools > Adjust Size.”
  • Open the “Fit into” dropdown and choose “Custom.”
  • Open the “width/height” dropdown and select the unit of measure that you want to use. You can choose from pixels, percent, inches, cm, mm, or points.

  • Use the accompanying “Width” and “Height” fields to specify exactly how big this image should be.
  • When you’re happy with your selection, click “OK.”

How to crop (and un-crop!) an image

Cropping can often improve a photo, allowing you to eliminate empty, boring or distracting space surrounding the main subject of that photo.

Preview can crop an image, with the click of a button:

  • Using your cursor, click and drag to highlight the section of the image that you want to crop around.
  • A new “Crop” button should appear in Preview’s toolbar; give it a click and Preview will go ahead and crop your image.

If you click the button only to realise that you’ve cropped the image incorrectly, then you can reverse the crop, either by:

  • Selecting “Edit > Undo” from the Preview menu bar.
  • Using the “Command+Z” keyboard shortcut.

Remove the background from an image

From time to time, you may need to remove an image’s background. Perhaps you want to replace your bland, everyday surroundings with something more exciting, or maybe you want to delete the background entirely, giving your image a transparent effect.

You can remove the background, using Preview’s “Instant Alpha” tool:

  • In Preview’s toolbar, select the “Instant Alpha” button (this should be the second icon, but if you’re unsure then hover over each icon in turn, until you see an “Instant Alpha” tooltip).
  • Click and hold inside the area of the image that you want to remove, i.e the image’s background.
  • Without releasing the mouse, move your pointer slightly. A red overlay should appear.
  • Keep moving your mouse until all the areas that you want to remove are highlighted red. If you start to select areas that you want to keep, then move your mouse in the opposite direction to reduce the selection. If you’re struggling to select the entire background, then remember that you can always remove the background in stages.
  • When you’re happy with your selection, press the “Delete” key on your keyboard.

Invert your selection

Are you struggling to select the area of the image that you want to edit? Perhaps the image contains a shadow that you want to lighten, an awkwardly-shaped item that you want to delete, or an area of overexposure that needs correcting?

When an area is strangely-shaped, it may be easier to select the rest of the image, and then use Preview’s “Invert Selection” tool to switch the selection to the area that isn’t selected.

To use Invert Selection:

  • Select the area of the image that you want to keep. Depending on the image, this may involve using Preview’s Smart Lasso tool, which allows you to draw any shape. If you need to use the Smart Lasso, then click Preview’s “Selection Tools” icon and choose “Smart Lasso” from the dropdown menu.

  • Once you’ve selected the portion of the image that you want to keep, select “Edit > Invert Selection” from the menu bar. This will flip the selection, ready for you to edit that area of the image.

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About the author

Jessica Thornsby

Jessica Thornsby is a technical writer based in Sheffield. She writes about Android, Java, Kotlin and all things Apple. She is the co-author of O'Reilly's "iWork: The Missing Manual," and the author of "Android UI Design," from Packt Publishing.

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