How to blur the background of a photo

Blur the background in seven steps

1. 1

   Open the photo

   Go to lilcactus.app and choose File› then Open…, or drag the photo onto the window. To follow along exactly, open the sample photo from this page.

2. 2

   Select the person and check the outline

   Choose Select› then Subject. Lil Cactus finds the person and outlines them with a moving dashed line; the first time it can take a few seconds. Look along the outline for background caught inside it.

   In the sample, a small triangle of pink flowers between the bag strap and her arm, at the bottom left, is inside. Take it out: right-click the lasso button in the toolbar and choose Polygonal Lasso, then choose View› then Zoom In a few times and scroll to the triangle. Hold Alt or ⌥ and click one corner of it, let go of Alt or ⌥, click the other corners, and click the first corner again. Holding Alt or ⌥ at the first click takes the shape away from the selection.

   

   

3. 3

   Copy the photo

   Choose Layer› then Duplicate Layer. A layer called Background copy appears above the photo in the Layers panel and is selected; the outline stays.

4. 4

   Delete the person from the copy

   Choose Edit› then Clear, or press Backspace or ⌫. The copy now has a person-shaped hole. Nothing seems to change, because you see her through the hole, on the photo underneath.

   

5. 5

   Deselect

   Choose Select› then Deselect, so the blur covers the whole copy. Use the menu: its shortcut Ctrl+D or ⌘D adds a bookmark in many browsers.

6. 6

   Blur the copy with Lens Blur

   Choose Filter› then Blur› then Lens Blur… On the Depth map tab, set Source to From transparency: Lens Blur then keeps the hole out of the blur, so none of her colors spread into the street. On the Aperture tab, check that Radius is 30 px, a strong blur for this 1536 px photo. Click Apply.

   

7. 7

   Check the result, then save

   The café, the bicycles and the people behind her are soft, and she stays sharp. Lens Blur mixes light the way a lens does, so the blurred street also comes out a little brighter (about 10 levels out of 255 on average here, more in the shade).

   Choose File› then Export› then JPEG… (or another format) and click Export for a flat copy; the file you opened doesn’t change. File› then Save writes the flattened result back into the file you opened; for a JPEG the first save opens the Save Image settings. To keep the two layers, choose File› then Save As› then Lil Cactus Project… If sharp bits of background show between loose hair, see Hair and other fine edges below before you save.

   

Why not blur the photo directly?

With the person selected and then swapped with Select› then Inverse, Lens Blur changes only the background, but it still reads the pixels next to it, including hers. Her colors spread into the street around her: in the sample, a yellow band about 10 px wide forms beside the sweater. Deleting her from the copy and choosing From transparency keeps her colors out of the blur.

Hair and other fine edges

Gaps between loose strands of hair are part of the person’s outline, so the background showing through them stays sharp. In the sample this is clearest on the left of her hair, above the shoulder, with a sharp pink flower at the shoulder, and it shows at 100%.

To soften those spots, click the Background row in the Layers panel (the sharp spots are on the photo underneath). Right-click the retouch button in the toolbar (the one with the stamp) and choose Blur. Set Size to about 24 px and Strength to 40%, then brush two or three times over the gaps between the strands and the flower, staying off the sweater. Strands under the brush soften as well, to roughly half their crispness: at 100% this reads as hair falling out of focus, but zoomed in you can see where you brushed, so keep the strokes on the gaps.

Before step 3, Select› then Refine Edge… with Output set to Selection can keep a few more strands; check the result for small sharp specks of background. Removing a background explains Refine Edge in detail.

How much blur to use

• Radius is in pixels, so a bigger photo needs a bigger number for the same look: 30 px on a 1536 px photo is about 2% of its width; on a 4000 px photo, try 60 to 80. For a subtler look, use about half.
• Shape sets the outline that bright spots spread into, a hexagon by default.

Fix common problems

Her outline looks hazy, as if the street spreads over it

Source was left on None, uniform blur. Choose Edit› then Undo (Ctrl+Z or ⌘Z) and apply Lens Blur again with From transparency.

Part of the person is blurred

The selection missed it, so it stayed on the copy. Undo until the person is back on the copy, then before Edit› then Clear add the missing part: hold Shift or ⇧ and click round it with the Polygonal Lasso.

Edit › Clear is grayed out

The copy is locked or a Smart Object. Unlock a locked layer with its lock in the Layers panel. For a Smart Object, choose Layer› then Rasterize› then Rasterize Layer and click Convert to Pixels. Then continue. More in Smart Objects.

Coming from another editor

Photoshop

Select › Subject, then Filter › Blur › Lens Blur…, whose depth map can use a layer’s transparency.

Background blur: questions and answers