Earlier in the week I wrote a post about the Smart Blur filter in Photoshop. Today I’m looking at the Lens Blur filter.

The Lens Blur re-creates many of the depth of field effects familiar to photographers. The dialog box is fairly complex with lots of sliders and buttons and knobs to play with! This filter works by adding blur to an image to give the effect that some objects in the image stay in focus and others areas are blurred. You decide which areas are blurred by making a selection.

To help you re-create in-camera depth of field effects the Lens Blur filter lets you load a depth map to help specify the points of the image that are near to the camera, and those that are further away. You can use alpha channels and layer masks to create depth maps; black areas in an alpha channel are treated as though they’re at the front of the photo, and white areas are treated as if they’re far in the distance. You can choose an iris shape and this will also determine how the blur appears. To get to this filter in Photoshop, choose Filter > Blur > Lens Blur.

Don’t be intimidated by Lens Blur filter dialog box. It is a bit scary-looking the first time you open it, but just click on the preview box and you will see your changes as you make them on the dialog box. If you don’t like the changes you’ve made then just cancel out of the dialog box and your image will not be affected.

So now, I’m going to show you a practical example of the lens blur in action. I’m going to use it in conjunction with the alpha channels to make a selection.

1. Open an image that you want to create some depth-of-field with. I’m using a picture I took of a statue in front of the Eiffel Tower.



In this image I want to keep the statue in focus while blurring out the Eiffel Tower.

2. In the channels palette click on the Create New Channel button at the bottom of the palette. This will create a black alpha channel (called Alpha Channel 1).

3. Click on the eye icon next to the RGB channel. Now you can see the image AND the red overlay representing the alpha channel.


4. Select the Paintbrush tool and with the foreground colour set to white, paint over the areas where you want the filter to be applied. If you make a mistake, change the foreground colour back to black and paint back over it.



In this case, I’m painting on the horse statue, but when I go into the Lens Blur filter, I’m going to invert the selection made in the alpha channel.

 

5. When you’re happy with the alpha channel, click on the RGB channel at the top to make the image itself the subject of the filter.

6. Choose Filter > Blur > Lens Blur. This opens up the big dialog box. BUT DO NOT BE AFRAID!!


7. Make sure the Preview box is checked so that you can see what you’re doing.

8. From the Depth Map, set the Source to be Alpha 1 (this is your alpha channel that you made earlier), and then adjust the Blur Focal Distance slider.


For my picture, I set the Blur Focal Distance to about 90, but importantly I also checked the Invert box. Otherwise, the horse statue would become blurred and the Eiffel Tower would stay sharp. You can drag this slider up and down depending on how blurred you want part of the image to become.

As you can see there are lots of other sliders to play with, including adding noise, specular highlights, lens iris configurations. The best thing I can suggest here is start dragging and see what happens.

9. When you’re happy with your preview image, click OK to apply the filter. And Voila!


Finally, here’s some tips for painting with alpha channels for use with the Lens Blur filter.

  • The part of the image that you want to remain sharp (the subject) should always be painted black in the alpha channel.
  • Anything at the same distance from the lens as the main subject should also be painted black.
  • Areas farthest from the lens should be white.
  • Try applying grey or a gradient between the areas of black and white.

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