One of the creative tools available to photographers is the ability to restrict the depth of field in a photograph. Depth of field is the range of distance for which the subject is rendered in acceptably sharp focus in the photographic image. The photographer has a number of ways to regulate the depth of field when he takes the photograph. He can choose a small aperture (large ƒ stop number) which will increase the depth of field and conversely choose a large aperture (small ƒ stop number) to decrease it. Another regulating factor is the focal length of the lens being used. Longer focal lengths or telephoto lenses produce a tighter depth of field than wide angle or short focal length lenses. Since most digital cameras come with fixed lenses you change the focal length using the zoom, zooming in increases the focal length while zooming out decreases it. The third factor in determining the depth of field is the distance the lens is focused to. The further the subject is from the lens the greater the depth of field.
The above is common knowledge to SLR photographers and while useful to digital photographers less helpful to the vast majority who use the current generation of non SLR digital cameras. This is because digital camera sensors are much smaller than the 35mm film plane used by SLR cameras. For example, a typical digital camera CCD chip area is usually in the neighbourhood 7.2mm x 5.3 mm compared to the 24mm x 18mm 35mm film frame. As a result digital cameras use lenses of a much shorter focal length than SLR cameras to achieve the same angle of view. Typically a 10X zoom on a digital camera would require a 70mm lens while an SLR would require a 370 mm lens to achieve the same angle of view. Add to that the fact that most digital camera lenses have a very limited ƒ stop range and we have a real problem controlling depth of field. As figures 1.0 and 1.1 demonstrate it's much easier to present a clean non distracting background using an SLR than it is with a conventional digital point and shoot camera. Both photos were taken at ƒ5.6 but figure 1.0 was taken with a digital point and shoot camera while figure 1.1 was captured using a digital SLR.
This is where the control the digital darkroom affords you comes to the rescue. In PSP we can use gaussian blur combined with layer masks and/or selections to realistically mimic a restricted depth of field. Accomplishing this in the photo above would be relatively easy. Since the background is located at a constant distance from the subject it's simply a matter of selecting out the foreground elements and applying a blur to the rest of the image. As you can see the SLR photo has a less distracting background making main subject stand out better.
Another reason to decrease the depth of field in a photo is to give it a more three dimensional look. When using a long lens on a distant subject the depth is compressed, that is things that may be hundreds of meters behind the subject appear to be quite close, distance is compacted which can cause the photo to look very flat. In this case adding lens blur to the background will give the scene a more three dimensional feel. We can't just select out the background and add the blur because a natural depth of field needs to fade gradually from sharp focus on the near ground to full blur on the far ground. This is where a mask layer comes in handy.
Below is an example of a photograph with a busy background where that gradually merges with the foreground. Let's run through the steps needed to add some depth to this image. If you want you can download a 1600X1066 copy of the moose photo (300kb) at http://ronstoons.com/moose.zip and follow along.
In some cases this method can be used as an alternative to cloning out power lines and other distracting elements in a busy background.
Using different types of gradient masks will allow you to focus the sharp area where you need it.
