What do we mean by focus
When light reflects from the object you are photographing, it passes into the camera through the lens. This light must be directed onto the image sensor exactly in order for you to achieve a sharp image.
This sharpness is achieved by moving the adjustment ring on the lens which directs the image more sharply onto the cameras image sensor. Adjusting the lens causes the various elements within it to move, and this is what produces a sharper image.
Most modern DSLR cameras have an autofocus system which uses an array of AF points. Your choice of AF points or group of points will help the camera to decide which object within the viewfinder should be the focal point of the image. However, if you are not concentrating, this can also be why you sometimes end up with an image where the main subject is blurred, if your camera uses the wrong AF point.
This can happen when your camera uses a point that is over a different part of the image to the area that you wish to be sharp. So you need to pay attention when composing the shot in the camera’s viewfinder.
These focal adjustments can be made either automatically, or manually, by using the auto, or manual switch located on the side of the lens. So selecting a single focal point to highlight the subject of the image can reduce the risk of this.
Using your cameras AF points
Most DSLR cameras allow you to select which focal points you wish to use, so you will have more control over which areas of your image are sharp . For some shots you may wish to use the full array of AF points, other times you may wish to choose fewer points
On my Canon DSLR pressing button 1 will display the AF points in viewfinder. Dial 2 then allows you to select the appropriate AF point / zone.
When shooting action shots, or moving subjects, you may choose to have more AF points active to increase the chance of the subject being in focus. For a portrait, or still life shot you will want to use fewer points to allow more precise focus on a specific area.
Check your cameras user manual to find out how to select specific user point / group of AF points.
Again with my Canon DSLR, AF selection showing the various AF point selections available. With my particular camera, a maximum of 81 points can be selected.
Depth of field.
Your choice of where to focus in an image can also affect the Depth of Field, i.e. the area of the image which is in focus from front to back of the image. This will also be dependent on the aperture setting being used. As a general rule approximately 1/3 before, and 2/3 after the point of focus will be sharp. Anything outside of this will be blurred.
Get out with your camera to practice the techniques discussed here.