I'm not talking the selective focus you use when your trying desperatly to read the last 20 pages of your book when your child is chatting you up about barbie's latest fashions... I'm talking... Photography!
Selective focus is a technique used in photography. You focus n a specific part of a subject that you want to highlight or emphasize, and you let the rest of the image fall into a the blur of the background. This is the effect created by a shallow depth of field.
Did I lose you already?
It's really simple... all those images where you see one part in focus, and everything else blurred... the photographer has used their aperture control to create that effect.
There are three things that go into creating images like this. The First... is your F-stop ( Known more commonly these days as your Aperture setting)
Aperture Control: The A mode on your camera, or part of what you control in full manual. Aperture is like the iris of your eye it opens wider or closes tighter to control HOW MUCH light gets into your image. The more light, the LARGER the aperture (the smaller the number)
Aperture controls the "depth of field" think of this as a field of flowers. Do you want the one an arms length away to be just as clear and sharp as the ones twenty feet away? This would be done with a small aperture (Big number like f/22). If you want to highlight one of those flowers that is an arms length away and let the rest fall to blur then you use a large aperture (Small number like F/2)
2. Focal Length: There is a lot of tech and science that can be learned about this, as well as a lot of differences in the types of lens. However, what you NEED to know for this lesson is the Focal length refers to the wide angle, or telephoto number of you lens. When you say I have a 28mm lens, or a 18-55mm those numbers are the "focal length" of your lens.
A lens of a shorter focal length will create a greater depth of field than one of a longer focal length... so opt for a 28mm over an 85mm (if you have a more "all purpose" lens like an 18-55 - keep the lens in the 18mm position, this will allow you the largest aperture setting as well)
2. Distance: The closer you are to your subject, the less depth of field that will be generated.
This is a great sample of blurring out a busy pattern in order to draw your eye to the beads on the lanyard. You can see the smallest of them in the loop clearly, as well as the cute bee and bugs. While leaving the foreground and background to blur out.The further away your background is the more blur you will achieve.
There are a lot of moments in photography where you will want to utilize this technique. The most common are...
Macro or Product Photography.
Like the two examples I have given above, I have used this as a powerful way to draw the viewers attention to a very specific part of the image - my main focus of the subject matter.
You can use this to create a general feel of mood in your image, putting the emphasis on your subjects face (or in animal photography the eye, specifically)
Fixed lenses (the ones with only one focal length number such as 28mm) are widely known to have large apertures (like 2.0 or 1.8) in order to specifically create these types of effects! Does that mean that if you don't have a manual camera or a lens like this that you can not achieve the same greatness...
Of course not!
You will find even on point and shoot cameras the ability to select a Flower mode (macro), a portrait mode (the shape of a head) at the very least... many camera's these days have even more to choose from that specifically adjust your camera's aperture setting to create this effect. Those "preset" modes are simply programmed combinations of aperture and shutter settings to achieve a specific effect. You will get a lot more use out of your point and shoot camera if you learn a few basic ones like these!