Subject isolation is a technique in which you make your subject stand out distinctly from the background. This technique is most commonly used for portraits or close-up shots, but it can work in any scene if you know what to do and have the right tools. The two fundamental methods for creating subject isolation are narrow depth of field (DOF) and post-processing effects.

Method 1: Narrow Depth of Field

Using a fast telephoto lens at close range causes the background to go completely out of focus. By orienting myself perpendicular to the cormorant, I was able to get most of it in focus (click to enlarge).

Depth of field is the perception that objects in front of or behind the exact plane of focus are either sharp or soft. Depth of field is an optical property, and it depends on three things: lens aperture, focal length, and distance to your subject. While all lenses can be stopped down to maximize depth of field, you’ll need a wide aperture (fast) lens to minimize DOF. Most budget lenses offer maximum apertures of f/4 or smaller; to really get good subject isolation, you’ll want a lens that has f/2.8 or faster. Some of the classic “portrait” lenses offer fast apertures of f/2 or f/1.4.

While aperture is important, focal length is another determinant of DOF. A telephoto lens will have a narrower DOF at a given subject distance than a wide-angle or normal lens. This helps to explain the common “portrait” focal lengths of 85mm, 105mm, and 135mm. Two of my favorite portrait lenses are the 85mm f/1.4 AFS G Nikkor and the 200mm f/2.0 AFS G VR Nikkor. If you’re on a budget, the 85mm f/1.8 Nikkor is also excellent. DX users will find an extreme bargain in the 50mm f/1.8 AFS Nikkor.

Lastly, subject distance is a component of DOF. Regardless of the aperture and focal length you choose, the closer you are to the subject, the shallower the DOF will be. In fact, with extreme close-ups, the DOF is so shallow that many macro photographers will stop down to f/22 or greater just to try to get more of their tiny subjects in apparent focus.

The best way to limit depth of field, therefore is to use a fast telephoto lens at a relatively close subject distance. Shooting for limited depth of field requires some practice, because you will be dealing with a very shallow range of perceived focus. Because the focal plane will be very thin, you’ll want to orient yourself as perpendicular to the subject as possible. If the subject extends beyond the focal plane, it will quickly go soft. For portraits, make sure the subject’s nearest eye is in focus. For inanimate subjects, make sure that you define the area of interest by focusing on it precisely. Live view focusing can often help in these situations, especially if you’re using a tripod and the subject is static.

Another thing to check when you are using limited DOF is the corners of the frame. Look for distracting background elements, especially large bright areas. Usually you can reposition yourself very slightly and eliminate the unwanted background, especially if you’re using a telephoto lens.

Method 2: Post-Processing Techniques

If you don’t have a fast lens or a scene that works well for isolating your subject, you can use post-processing techniques to draw the viewer’s attention to your subject, away from the background. One of the simplest techniques is the “vignette” effect, which can be applied manually by darkening (or lightening) the corners of the frame, or using plug-ins like Nik Software’s Color Efex Pro 4. The advantage of using plug-ins is that it is very easy to create a wide range of edge effects. For example, Color Efex Pro 4 offers four different filters for creating edge darkening/lightening effects:

• Darken/Lighten Center
• Vignette
• Vignette: Blur
• Vignette: Lens

Each of these filters produces a slightly different effect using different methods. If you don’t have a plug-in to create a vignette effect, you can simply use the brush tool in Photoshop, paint with either black or white in a new layer, and adjust the layer opacity to taste. Often times you’ll find that you don’t need to make the corners extremely dark to see a noticeable effect. Try toggling the layer visibility on and off to preview the effect at different opacities.

I used the "Vignette: Lens" filter in Color Efex Pro 4 to add the corner darkening to this image (click to enlarge).

Darkening the corners in this image diminishes the busy background of the aircraft hanger. I used the "Vignette" filter in Color Efex Pro 4 to produce the effect (click to enlarge).

There are also software tools that try to emulate narrow DOF by blurring the image. One such tool is Alien Skin’s Bokeh software. With this software, you can recreate the look of different apertures and focal lengths and define the area of the image that is to remain sharp. In my experience, Bokeh does a very good job on most images… better than what I could achieve manually using the on-board blur tools in Photoshop. When adding blur to an image, be very careful not to get too extreme. Usually extreme settings will end up looking incredibly artificial.

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