Control the depth of field

A good composition is about including the important elements of your photo and excluding what can distract the observer.
To obtain this result you have to control the depth of field, which means render clearly the useful elements and put out of focus the others: for example doing portraits is important to reduce the depth of field in order to focus attention on the subject and blur the background.
To improve your use of this technique, you have to keep in mind the different parameter that increase the depth of field:
- size of CCD
- distance of the subject
and the parameter that decrease it:
- opening of your lens
- focal length
If your camera isn’t a SLR, the small size of the CCD makes difficult to get a short depth of field, so you have to get the most from the other factors, using the maximum opening and your longest focal.
Point and Shoot cameras have only automatic selection of the opening, so the only option is to use some preset like Portrait or similar.
Camera with a semiautomatic setting can use the Aperture Priority mode and set the widest opening, like F/3 or something.
The right focal length to decrease the depth of field is something like 150mm or more, so if you have a 10x zoom you have to use at least half of its range, and if you have a 3x you must go to the max.
If everything fails try to take advantage by the digital manipulation technique, shortening the depth of field in editing phase using tools like Photoshop.
The technique is simple, but you need to master some basic Photoshop tool in order to:
- create a duplicate layer
- use the Filter->Blur->Gaussian Blur function
- create a mask on this layer to preserve the part of the image the need focus
With practice this technique can be very effective, at least with simple composition.