Point of Focus and Depth of Field
When a photographer uses the term "point of focus", he is referring to that object in a photograph at which he wants to draw the most attention. "Focus" itself refers to the amount of the image that is sharp. Both the point of focus and the amount of focus affect the resulting image. In short, altering each one changes the look of the final photograph.
In these two images, the point of focus was the same, the coneflower. However, the amount of the photo in focus, or the amount of sharpness, also called "depth of field", is different.
Let's deal with these subjects one at a time.
POINT OF FOCUS
Every photograph has a specific location at which the photographer wants the viewer's eye to rest. Seen through the lens of the camera, this can be a single point or multiple points. In upper-end digital cameras, photographers have the option to choose between the number of these points and their location.
In these first two examples, I have only one focal point, as indicated by the red square in each. However, I deliberately altered its location. As you can readily see, this greatly modified the look of the photo. One is visually more pleasing than the other.
Here is an important tip. If your camera doesn't allow you to move your point of focus from within the lens, you can still do so. Depress your shutter button halfway to lock the focus on your chosen focal point, and then physically shift your camera to arrange for the composition you desire. Be careful though not to alter the distance from your lens to your focal point, as this will disrupt the overall sharpness of the image.
In the flower photos above, my choice of a focal point was simple. I wanted to feature the forward flower. However, what point would you choose for a wide-angle scene? In these situations, when you desire to show the general appearance of a location, it often works best to set your camera to use a wider focus range, it sampling from multiple points. In some cameras, this is called "dynamic range" focus.
In the photo below, there are ten separate squares. The camera sampled from each of these areas to give the image the largest amount of sharpness. I also used a smaller aperture to enable more of the objects in the image to be in focus, which brings us to our next topic.
DEPTH OF FIELD
Having a sharp photograph is dependent on something called "depth of field". To put it simply, depth of field is the amount of the photographic scene that is in focus, or sharp. It can be greater, with much of the scene in focus, or shallow, where very little is in focus. Depth of field changes with the angle of the camera's lens, the distance from the lens to the objects in the image, and the camera's settings. It is especially dependent on the aperture. (Remember aperture is the size of the opening of a camera's lens, and it controls the amount of light entering into the lens at one time.)
Looking back at my very first example, the only difference between those two flower photographs is the aperture. In the image on the left, the aperture was F25. On the right (also above), the aperture was F5.6. F5.6 is a larger aperture (opening), thus it lets in more light quicker (causing a shorter shutter speed), and it gives a shallower depth of field. Shallow depth of field places more of the areas outside the point of focus into blur.
In reverse, F25 is a smaller aperture. Less light enters through the lens, requiring a longer amount of time, and there is more depth of field. Having more depth of field places a greater number of the objects in the scene into an area of focus. We can see then that the point of focus remained the same in both images, but the look between them is different because of depth of field.
So which level of depth of field to choose? Well, that depends on your subject. In landscape scenes, you generally want more depth of field. This will make more of the objects in the scene to be sharper. Think about it this way. In a landscape, for instance, a city street scene, the many elements in the scene are at different distances from the camera. One building might be six feet away, a street lamp 12 feet away, and a pedestrian passing by at about 15 feet. If your depth of field is too shallow, then the areas that should be sharp will be blurry instead. The "blur" in that photograph will cause the photograph to look like it is out of focus, which is not what you intended at all. By choosing a smaller aperture for this photograph, the subjects that are at varying distances will appear to be in focus because there is a greater depth of field.
On the other hand, there is also an advantage to using shallow depth of field. Shallow depth of field isolates the subject from its background and foreground. Things that might be distracting to a viewer disappear, and the eye is drawn instead to the object in greatest focus. Portrait photography often benefits from a shallow depth of field. However, the most common use of shallow depth of field comes in the area of macro photography. The rule is - the closer to an object your camera lens is, the shallower your depth of field will be. This can be a problem especially when working with tiny subjects, insects, for example, where you want the eye to be in focus.
In the photo of this butterfly, you can see that the backward edge of its wings is slightly out of focus. This is a direct result of having a shallow depth of field. Shallow depth of field also caused the background flowers to become blurry. This makes the butterfly and the flower the main subjects of the photograph as intended.
Understanding "point of focus" and "depth of field" is essential to creating photographs. Each one of these affects the other, and together they change the final look of a photograph. Additionally, understanding them gives you, the photographer, more decisions to make and more choices about how you want your photograph to look. Knowing your choices and being able to choose them effectively makes your photographs to be more YOUR photographs, and not just the result of your simply pressing a button. Instead, YOU become the photographer, not the camera. And that is really what photography is all about.