The physical focus is the point in the frame your lens is focused on. This should be the point of the image we want to be clear and sharp. But how do you know where the camera is focused?
Point and Shoot Cameras
Most cameras now have a little light that will blink at the point your camera has selected to focus on. However, that doesn't mean the camera has selected the right point to focus on. With point and shoot cameras, the preferred focus point is directly in the middle of the image. If your preferred focus point is not in the center of the frame (see Rule of Thirds), you can trick the camera into focusing on your preferred point. Simply center your preferred point in the frame and press the shutter release button half-way down to lock focus and exposure. Then recompose your image without releasing the button. Once the image is as you want it, press the shutter release button the rest of the way down to activate the exposure. Please remember that if your camera does not have a TTL (through the lens) viewfinder, what you see in the viewfinder will not be exactly the same as the image you take.
Most SLR cameras allow you a lot of options when focusing on a subject. Some allow you to switch between manual focus and automatic focus. And some allow you to select how the camera chooses its focal point. Like point and shoots, a small light will blink on the point where the camera is focusing. If that is not the point you prefer you can:
- Use the same method of recomposing as a point and shoot camera
- Switch to manual focus
- Override the focus selection point
Depth of Field
If you know your camera was focused on the correct point but some of your subject is not in focus, or you want more of the surrounding area in focus, you will need to work with depth of field. Depth of field explains how much of your image (from front to back) will be in focus and is controlled by your lens aperture. It is set with the F-stop control on your camera. Several factors work together to determine the depth of field of an image. These factors are discussed in detail in my Depth of Field article.