How to Blur Images on Mac Using Preview (and Better Alternatives)

Blurring an image is a fundamental photo editing technique that can help you soften distracting details, create a sense of motion or depth, or direct the viewer‘s attention to your subject. On macOS, every Mac includes the built-in Preview app, which has basic tools for annotating and adjusting images. So can you use Preview to blur a photo?

The short answer is: kind of. While Preview doesn‘t have a dedicated blur tool, it does include an option to reduce the sharpness of an image, producing a subtle blurring effect. However, this method has some big limitations compared to the blur filters in full-featured image editors.

In this article, we‘ll walk through the process of "blurring" an image in Preview using the Sharpness adjustment. We‘ll also take a deeper look at how this option differs from real blur filters, and explore some alternative tools that give you much more control over the blurring effect. Let‘s dive in!

Blurring an Image in Preview with the Sharpness Slider

If you just need to slightly soften an image, Preview‘s Sharpness slider can get the job done in a few quick steps:

  1. Open the image you want to blur in Preview. You can do this by double-clicking the image file in Finder, or by launching Preview from your Applications folder and using the File > Open menu.

  2. In the Preview menu bar, go to Tools > Adjust Color (or press Shift+Command+C on your keyboard). The Adjust Color window will appear, displaying a histogram of your image along with several options for modifying the colors and tones.

The Adjust Color window in Mac Preview, showing options to adjust exposure, contrast, saturation, and sharpness

  1. At the very bottom of the Adjust Color window, you‘ll see a slider labeled Sharpness. Click and drag this slider all the way to the left, reducing the Sharpness value to the minimum of 0%.

The Sharpness slider in Preview's Adjust Color window, set to the minimum value of 0%

  1. As you move the slider, you‘ll see the image update in real-time to show the softening effect. The farther you drag to the left, the more the sharpness will be reduced, creating an approximation of blurring.

  2. When you‘re satisfied with the result, close the Adjust Color window. The softened version of your image will remain in the Preview window.

  3. To save your changes, go to File > Save in the menu bar (or press Command+S). Your blurred image will overwrite the original file. If you want to keep the original unmodified, go to File > Duplicate instead, then save the duplicate with a different file name.

Side-by-side comparison showing an image before and after reducing Sharpness in Preview

That‘s all it takes to achieve a basic blurring effect in Preview. But how does the Sharpness adjustment actually work, and how does it compare to real blur filters? Let‘s take a closer look.

Sharpness vs. True Blur: What‘s the Difference?

When you drag the Sharpness slider to 0% in Preview, it may look like the image gets blurrier. But what‘s actually happening under the hood is a bit different than applying a real blur filter.

Sharpness adjustments like the one in Preview work by increasing or decreasing the contrast along edges in the image. Edges are areas where light and dark tones meet, like along the outlines of objects or textures. When you increase sharpness, the light pixels get lighter and the dark pixels get darker, making the edges more defined. Decreasing sharpness does the opposite, reducing the contrast to make edges seem softer.

Diagram illustrating how the Sharpness adjustment affects contrast along edges in an image

A true blur filter, on the other hand, works by averaging out the pixels in an area to create a smooth gradient. Common blur filters like Gaussian blur place more weight on pixels near the center of the blur radius, resulting in a realistic softening effect. Other filters like motion blur average the pixels in a particular direction to simulate movement.

Diagram showing how Gaussian blur averages pixels in a radius around a central point

The key difference is that real blur filters create new pixel values by sampling and averaging their neighbors, while sharpness adjustments simply increase or decrease the existing pixel values along edges. That‘s why Preview‘s Sharpness slider can‘t achieve the same smooth, photorealistic results as a true blur filter.

To test this, try zooming in to 100% or more on an image "blurred" with Preview‘s Sharpness tool. You‘ll notice chunky, pixelated artifacts and strange halos around high-contrast edges. The effect looks especially unnatural on text, logos, or anything with fine details.

Closeup of an image softened with Preview's Sharpness slider, showing pixelation and edge artifacts

In fairness, Preview is designed as a lightweight image viewer, not a full-featured photo editor. The fact that it includes any adjustments at all is a bonus. And in many cases, slightly reducing the sharpness is all you need for a quick touch-up. But to create professional looking background blurs, soft focus effects, or motion blurs, you‘ll need a more capable tool.

Can Preview Blur Part of an Image?

Another major limitation of the Sharpness method is that it affects the entire image equally. There‘s no way to limit the blurring effect to a particular area, like a distracting background or a subject‘s skin.

The ability to selectively apply blurring and other filters is called non-destructive editing. It‘s a key feature of advanced image editors like Adobe Photoshop and Affinity Photo. In these apps, blurring happens on a separate layer or mask, leaving the original pixel data intact. This gives you much more control and flexibility to fine-tune the results.

While Preview does support basic annotations like shapes and text, it doesn‘t offer true layer-based editing. So if you want to blur only part of an image, you‘ll need to use a more advanced tool that supports selections and masks, like Pixelmator Pro or GIMP (more on these later).

Example of an image with selective blurring applied to the background

Privacy and Security Considerations

If you‘re thinking about using a web-based tool like Photopea to blur sensitive images, it‘s worth considering the privacy implications. Unlike apps that run directly on your Mac, browser-based editors require you to upload your photo to a remote server for processing.

While most web apps have privacy policies prohibiting them from misusing your images, there‘s always a small risk of data breaches or hacks that could expose your files. Additionally, metadata like location tags or camera info may be inadvertently shared when you upload a photo.

For maximum peace of mind, it‘s best to use an offline editor running locally on your Mac to handle images containing faces, personal info, or confidential data. Standalone apps may also perform faster and offer more tools than their web-based counterparts.

Of course, even with an offline editor, it‘s important to exercise caution when saving or exporting your blurred images. Always double-check that no identifying details are visible before sharing a file publicly.

Choosing the Right Blur Tool for Your Needs

We‘ve covered the ins and outs of soft blurring images with Preview‘s Sharpness slider. And while it‘s a handy option for quick adjustments, it‘s clearly not the best tool for every blurring scenario.

To summarize, here are the main benefits and drawbacks of using Preview to blur images:

Pros:

  • Fast and simple, no extra apps required
  • Included free with every Mac
  • Fine for subtle softening of the entire image
  • Non-destructive if you duplicate the image first

Cons:

  • Not a true blur effect, just reduces edge contrast
  • Can introduce unwanted artifacts and pixelation
  • No control over blur intensity or radius
  • Can‘t selectively blur parts of the image
  • No advanced options like lens blur or tilt-shift

So when should you use a different blur tool? Here are some common situations where it‘s worth exploring alternatives to Preview:

  • Smoothing skin or reducing blemishes in portrait photos
  • Creating a shallow depth-of-field effect to isolate your subject
  • Adding realistic motion blurs to sports and action shots
  • Simulating tilt-shift or other lens effects for creative purposes
  • Applying blur effects as a non-destructive filter layer
  • Working with very high resolution images that require pixel-level precision

If any of those use cases apply to you, check out these alternative blur tools for macOS:

App Price Blur Features
Photopea Free Gaussian, motion, radial, smart, surface, lens blur and more
GIMP Free Box, Gaussian, motion, pixelize, tileable, variable blur
Pixelmator Pro $39.99 Box, Gaussian, motion, zoom, tilt-shift, selection-aware
Affinity Photo $54.99 Box, elliptical, Gaussian, motion, radial, tilt-shift, zoom
Acorn $39.99 Gaussian, motion, zoom blur
Adobe Photoshop $9.99/mo Gaussian, motion, radial, surface, smart, lens blur galleries

Of course, there are many more Mac apps and web tools with blurring capabilities. When in doubt, look for an editor that offers a wide range of blur options, supports non-destructive layers and masks, and includes tools for making precise selections. Don‘t be afraid to experiment with different apps to find the one that clicks with your editing style.

Putting It All Together

To wrap up, let‘s look at a few real-world examples of how you might use blurring to enhance your images:

  1. Portrait retouching. In close-up headshots, blurring the background slightly can make your subject pop. You can also selectively blur skin to minimize imperfections while keeping eyes and other details razor sharp.

Example of using blurring for creative effect in a portrait photo

  1. Tilt-shift miniatures. By applying a gradient blur mask from the top and bottom of an image, you can simulate the look of a miniature scale model. This works especially well for aerial cityscapes and landscapes.

Example of using tilt-shift blur to make a cityscape look like a miniature model

  1. Motion effects. Adding directional blurring to subjects in motion, like a runner or cyclist, can convey an exaggerated sense of speed and dynamism. For the most realistic results, use a path-based blur filter that follows the contours of your subject.

Example of using motion blur to convey a sense of speed and movement

The possibilities are virtually endless. As you can see, when used thoughtfully and deliberately, blurring isn‘t just a corrective tool for fixing soft focus. It‘s a creative technique all its own, on par with adjusting color and exposure.

So don‘t be afraid to experiment with different types and amounts of blurring in your photos. Try selectively blurring the background to emphasize your subject. See how motion blurs can jazz up action shots. And if Preview‘s basic Sharpness adjustment isn‘t cutting it, explore the wealth of dedicated blurring tools in apps like GIMP and Pixelmator Pro.

With a little practice and attention to detail, blurring can take your images from mundane to magical. Happy editing!

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