A Computer Darkroom Tutorial

One of the most useful features introduced with Photoshop CS is the Lens Blur filter. Unfortunately Adobe failed to provide much in the way of guidance as to how it is to be used.

This short tutorial, shows one example whereby I have used the filter to replicate a widely used photographic technique called differential focus

Not long after publishing this tutorial a number of readers emailed me to ask if it was possible to obtain similar effects to Lens Blur by using the Gaussian Blur filter in earlier versions of Photoshop. The answer is NO!. Lens Blur works in an entirely different way to Gaussian Blur. So before we begin the actual tutorial it is worth demonstrating the main differences between the Lens Blur and Gaussian Blur filters.

The following explanation of how Lens Blur works  is based on a demonstration called Hocus Pocus Focus given by Russell Brown of Adobe at PhotoExpo 2003

Original Image	Gaussian Blur	Lens Blur

Notice that with the grid patterns Gaussian Blur simply softens the edges whereas the Lens Blur filter results in a blur effect that has a more geometric shape or pattern to it. Looking at the section of the image with the 4 circles (representing highlights of different sizes) we see that the shape of the blur effect in the Lens Blur example is that of a Hexagon. More interesting is that fact that with the largest circle we find that the centre of the Lens Blur example has remained white whilst Gaussian Blur has become a light grey. The actual shape of the blur effect is determined by the Iris Shape chosen by the user although not all images will show this effect to the same extent as shown above.

Lens Blur in Action

In this example the original image has a strong subject but since the background clutter is so detracting it fails to make any real statement. With a bit of forethought I could easily have ensured that the background was out of focus, but hindsight doesn't fix the problem I now face. Can the new Photoshop CS Lens Blur filter save the day?

Original Image

Step 1

We begin by making a selection of the subject. In the image shown below I used with the Magic Wand and Polygon Lasso tools.

 

Make the selection using whichever tool best suits the image

 

Step 2

• Create a Mask (Alpha Channel) using the Save Channel as Mask button found at the bottom of the Channels dialog.

 

Step 3

• Using Quick Mask mode clean up the mask so that the subject area is white and the background is black.

Step 4

• Choose Lens Blur from the Filter > Blur menu

• Once the Lens Blur dialog opens hold down the the Alt key and click the Reset button (Lens Blur will be set t the default values)

• Choose the Alpha Channel form the Depth Map: Source drop down menu

Select the Alpha Channel as the Depth Map

Step 5

• Clicking the area that we want to remain in focus (the statue) causes the Blur Focal Distance slider to automatically move from 0 to 255 (remember 255 is white).

• Next we increase the value of  Iris: Radius slider and watch the background (black area) go out of focus. Adjust blur to taste!

• Zooming into image so that we can view the subject and background at the same time enables us to determine the Noise Amount that needs to be added to the image so that the blurred area doesn't look unnatural.

The Radius slider located within the Iris section of the Lens Blur panel allows us to adjust the amount of blur applied to the image. In this example a value of 0 results in no blur and  a value of 255 is maximum blur.

Adjust the Blur Focal Distance, Radius and Noise Amount for effect

 

Final Image

There is much more to Lens Blur than I've shown in this tutorial e.g. the use of a black to white Gradient in lieu of the selection can be used to represent the effect of depth of field.