Lens manufacturers pour large chunks of money into research and development of faster and sharper lenses. Zooms are now the standard where at one time, if you wanted a sharp image, they were considered amateurish at best. The sharp zooms of today combined with precise autofocus technology allow tack-sharp images to be made all the time. Ironically, out-of-focus areas in a photograph are equally as important as sharp ones. Perhaps you've heard the term, bokeh. It's a Japanese word that means blur and relates to the aesthetic quality of the out of focus areas of the photo. Herein lies the connection with selective focus.
Selective focus images have shallow depth of field. The technique is often used in situations where the background would be distracting if it was sharp. Some of the ways to render an out-of-focus background are to use wide-open apertures, long lenses, get the main subject away from the background, move closer to the subject, and change your image-making angle. The purpose is to render a sharp subject or section of a subject immersed in a wash of color. It allows the subject to pop off the page, so the viewer zeros in on the area that's sharp.
With macro subjects, selective focus is frequently used. This is true for two reasons. Aesthetically, it's a powerful way to draw the attention of the viewer to a specific spot of the subject. Technically, it's more difficult to attain a lot of depth of field in that the subjects are small. This dictates shooting at small apertures that often involves the use of flash or other intricate lighting set up. On a large scale, place a person in the middle of a field of wildflowers. Photograph him or her with a long lens, create distance between you and the subject, select a wide-open aperture, and put the focus point on the person. The foreground and background flowers fall out of focus and just the subject is sharp. To reiterate, selective focus is used to deemphasize foreground and background elements and render a targeted element sharp.
The selective focus technique works particularly well on subjects with repeating patterns. For instance, picture a picket fence with similarly crafted tops. Zoom in to include a section of about 20 posts and focus on just the eighth one in. Select a wide-open aperture and press the shutter. The only part of the image that should be sharp is post number eight. The foreground and background will progressively fall more out of focus. The same effect can be made with stadium seats, rows of trees, lines of cars or wherever there's depth of similar subjects. Use this principle for all your future photography sessions to apply the selective focus technique.