Photographers use blur (or bokeh) for a variety of reasons: to simulate depth of field, to add interesting visual artifacts, to simplify them and to change the quality of their expression. In the past, blur was controlled almost entirely through exposure; now it can also be controlled during postprocessing, giving photographers an unprecedented array of options and ways to customize the look and feel of their images. Knowing what you can do, how far you can go and when you can do it may change the way you shoot, one time, sometimes or all the time.
There are many blur filters in Photoshop: Field Blur, Iris Blur, Tilt-Shift, Gaussian Blur, Lens Blur, Motion Blur, Radial Blur, Shape Blur, Smart Blur and Surface Blur in the Filter > Blur drop-down menu. The choices are extensive, and it pays to familiarize yourself with your options by experimenting with them; you’ll find you have an extraordinary set of options that you can modify and combine creatively. If you only use the filters Gaussian Blur and Lens Blur, you’ll still have game-changing control at your fingertips, once you learn how to extend and modify them.
There are several important nondestructive strategies you can use to gain more control over all filter effects that will enable you to go further in your explorations and generate more sophisticated and compelling results. Try one or all of the moves in this classic progression. Apply a filter to a duplicate layer and then modify its Opacity, Blend Mode and Blend If sliders, and add a layer mask.
While many of these strategies (such as Opacity and Blend Mode) can be adapted for use with Smart Filters, Smart Filters present a few limitations. Lens Blur (the most photographic in appearance) is currently unavailable as a Smart Filter. You can’t use the Layer Styles Blend If sliders with Smart Objects or Smart Filters. You can only have one mask for a set of Smart Filters.
Blend Modes control how each layer blends with layers below them and will give you more control over the final appearance of each filter effect. While the list of Blend Modes is 27 items deep, eight Blend Modes are particularly useful for focus effects: Normal, Lighten, Screen, Darken, Multiply, Overlay, Soft Light and Hard Light.
Normal adds no extra benefits, but keep in mind that, by planning to reduce opacity, you may choose to apply a filter aggressively. Lighten and Screen are the most useful additions as they tend to produce effects that simulate traditional camera blur and soft-focus effects best. Lighten and Screen both reveal values of the top layer only if they’re lighter than those below them, and both will lighten darker areas of an image, creating a soft haze that becomes pronounced in shadows; Screen is more intense and may clip highlights if opacity isn’t reduced. Darken and Multiply generate effects that spread darker values into lighter values; while similar effects can be created in the traditional darkroom with split-focused and defocused exposure, the results are often more graphic than photographic, more gothic than romantic. Just as Screen is more intense than Lighten, Multiply is more intense than Darken and may clip shadow detail if opacity isn’t adjusted, again, either globally or locally. Overlay, Soft Light and Hard Light all affect contrast, and lightening and darkening the image with values above and below 50% gray. Soft Light is perhaps the fourth most useful Blend Mode after Normal, Lighten and Screen for blurring effects. Familiarizing yourself with these four and carefully evaluating the differences between them is a worthwhile investment of your time. Remember that you can adjust the brightness and contrast of the image after the blur effect is generated, so don’t be afraid to work aggressively and get the effect you really want.