Blur photo

Why blur a photo

A blurred photo makes the perfect background: behind a headline, on a cover, in a slide, or in a YouTube thumbnail, blur makes foreground text pop and removes distracting detail. At low strength (3–8 px) you get a "depth of field" look similar to portrait mode on a phone; above 20 px the image melts into a wash of color, ideal as a backdrop.

Blur and privacy: when it's not enough

Be careful if you're blurring to hide information: a light blur over text, license plates, or numbers can sometimes be reconstructed. To truly redact sensitive data, use a very high blur strength or a pixelate effect with large blocks, which destroys the detail in a way that's far harder to reverse.

Blurring is not erasing

This is the most important point on this page. A blur is an operation reversible in principle: it mixes neighbouring pixels according to a known rule, and from a blurred image one can attempt to recover the original. On text, a number plate or a short string of digits the attempt often succeeds, because the possibilities are few and can all be tried.

It has really happened, more than once: credit card numbers, personal data and names reconstructed from images published «blurred». The most famous case involved a swirl blur, which is a perfectly invertible transformation, but an ordinary blur over a large digit is vulnerable too.

What to do when something must really be hidden

Cover it with a solid rectangle. It is not elegant, but it is the only operation that destroys the information instead of mixing it: under a black rectangle there is nothing left to reconstruct. Alternatively, pixelation with very large blocks works, where each block is a single colour and the details no longer exist.

Then there is the step almost everyone forgets: you have to save a new image and use that one. Covering a detail in a program and saving the project leaves the detail underneath; and it is the same reason a PDF with a black rectangle over the text still contains the text, readable by selecting it.

Where blurring is the right choice

As a background: behind a title, on a cover, in a slide, a blur makes what is in front stand out without making the image anonymous. It is why application interfaces blur the content behind a panel.

And as attenuation: blurring a cluttered background in a portrait, softening a distracting secondary element, giving depth to a photo taken with a phone that keeps everything in focus. In these cases reversibility is not a problem, because there is nothing to hide.

The radius, and how not to overdo it

A small radius softens, a large one turns the image into patches of colour. For a background behind text you need a radius large enough that nothing underneath can be read, otherwise the two images compete and the text becomes tiring. For a portrait effect, instead, you need the bare minimum, because an excessive blur reads as artificial.

One thing worth knowing is that blurring reduces the file size as well, because it removes detail: it is one of the few cases where an effect also improves loading times.

Nearby tools

To remove the hidden data of a photo there is Remove metadata, which is the other half of an image's privacy. To write over it Write on a photo, for dimensions Resize photo and for black and white Black and white photo.