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Blurring backgrounds in Photoshopin Tutorials |
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By nicodemus/Martyn (1,190) 
on May 30, 2002 9:46:37 AM CDT
The raw image The raw imageOften when faced with an image that has a distracting background you may decide that it could be improved by blurring that background, as if it were taken with a shallower depth of field (DoF) in the first place. It should be noted that it is always better to create shallow DoF in camera, since the lens exhibits a characteristic known as bokeh (a japanese word), due to the physical make up of the lens. This is recognisable particularly when points of light become circles, often with a slightly darker centre than edge - and in the case of mirror lenses, have a small circle missing in the centre. But that issue aside, there are ways of producing a decent finish and ways of messing it up. I hope to demonstrate in this article a technique that I made up, but I'm sure many other photoshop experts have come up with in their own way. So here is my image to start with, you can see that the wall and bin behind Alison are a bit blurred, but could suffer a bit more to reduce the distraction from this girl with a gun (it's only a prop, don't worry)
How to mess it up - the haloWhat the beginner will often do in this case is select the background and blur it, often by selecting the subject and inverting the selection. If the beginner has been going a while they even select the subject, copy and paste it on to a new layer and blur the whole background layer. Either way, they are not going to avoid the halo effect, where colour from the subject is blurred into the background.
You will see this in my example, where the dark hair and boa blur out into the wall. For reference, this is a 4 pixel Gaussian Blur. This happens because Photoshop has to use pixels from outside the selected area to calculate blurred values inside the area. Not good - well, maybe you might derive some artistic expression from it, but many people will see it as a bad blur job in Photoshop - the choice is yours as ever.
Avoiding haloesTo avoid this halo effect, you must remove from which ever layer you blur the pixels which will bleed into the blurred area, it's that simple. The way I start this is to select the subject - often a long process with the selection tools - and copy it to a new layer. You can then hide that layer, so that the background layer is not obscurred by it. The next step is to gently use the smudge tool to smear the background into the area hidden by the subject, in order to reconstruct a background, as if the subject had not been there when you took the picture. You don't have to remove it completely, just enough to remove the pixels which would foul up your blur effect. If you intend a 10 pixel blur, then a region of 10 pixels inside the subject's outline is enough. In fact it helps to make an outline of the subject on a new layer, using the Edit>Stroke command while you still have the subject selection active. In this example I made an outline in yellow and set it's layer transparency to 50%.
Yes it looks odd, but look at what I've done here. Wherever there is any distinguishable feature in the background, like the mortar between the bricks, and the timber on the walls, I've tried to continue that into the hidden area.
Finally the blurAll that remains is to add the blur to the background layer.
As you can see there is no halo created around Alison's head in this version. All we need now is blur filter which creates lifelike lens bokeh and we're away. From CS 1.0 onwards, Photoshop has included the Len Blur filter, which does precisely this, simulating the defocussing of real world optics. This provides a very believable blur, and you can even use a depth channel to vary the effect and provide even more illusion of depth. I hope people find this useful, it can be used with all sorts of blur, just as long as you work out which pixels will unwittingly be drawn into the blur calculations by Photoshop. Have fun!
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