What is Depth of Field and why should you learn about it?
Depth of field refers to the distance in front of and behind the subject (focal point) of the photo that remain in 'acceptable' focus.
Have you ever seen a photo of, lets say, a flower where the flower was in clear
focus while the background (and perhaps the foreground) was out of focus? It really made that flower the focus of attention as your eye is not distracted by details of the background. This is an example of using a shallow depth of field to enhance a photograph. A shallow depth of field means that only objects within a small distance range are in focus.
Now think of the landscape photos you may have taken. How much of the photo was in focus? Odds are most if not all of the photo was in focus. This is usually what we want in a landscape photo and is an example of a deep depth of field.
What causes the changes in depth of field?
Basically, changes in any one or a combination of the following affect the depth of field :
- the aperture size
- the distance to the subject
- film/sensor size (some say focal length of the lens is the third factor)
Aperture (the opening through which the light passes) size is controlled on SLR (single lens reflex) cameras by adjusting the f-stop. The lower the f-stop, the wider the aperture and the shallower the depth of field. The higher the f-stop, the smaller the aperture, the deeper the depth of field. In digital cameras (non-SLR, also called consumer digital cameras) there is no f-stop control as such. Most consumer digital cameras have a setting for portrait or close-up/macro and one for landscape. These settings change the aperture size and thus the depth of field.
All other things being equal, the farther away you are from your subject the greater the depth of field. When you are taking a close-up photo (assuming your camera has manual focus) you will have to pay close attention to proper focus as the depth of the area that will be in focus is much less.
The third item mentioned above is film or sensor size. Digital cameras use sensors to capture the image. These sensors are very small, for example ¼ inch. Film,on the other hand is much larger. The most common size is 35 mm or about 1 ¼ inches. The smaller the media used to capture the image, the greater the depth of field. Generally speaking, digital cameras have less of a range of depth of field than do cameras that use film.
What is the easiest way to change the depth of field?
Using an SLR camera, the easiest adjustment to make the change the depth of field
is to change the aperture size. If you have a consumer digital camera you may find it difficult to create a photo with the subject in clear focus and the background really out of focus due to the small sensor size and the relative lack of aperture control. Try taking photos of the same subject using the portrait mode and the landscape mode and see what sort of difference you can achieve.
With an SLR camera you can experiment by changing the f-stop to see the difference in the area that is in focus. If you have a through the lens view finder, your lens probably has a ‘preview’ button that stops the lens down
allowing you to see the effect as you look through the view finder. Note that when you change the f-stop the amount of light reaching the film or sensor changes. You will have to adjust the shutter speed to compensate for this – the faster the shutter speed (higher the number) the less light with reach the film.
Take a look at the lens on your SLR camera. There are usually three sets or
makings. The one closest to the camera body is the aperture adjustment ring with the f-stops marked on it. On the most common lenses the range is from about 2 to 16. The outermost ring controls the focus and is marked in feet or meters or both.
Between these two adjustments is a third ring that does not move. It has the f-stops marked on it but these marks start from a central point and go in both directions. This ring gives you a guide as to the depth of field.
For example, with my 35 mm lens, focused on a subject 5 feet away using an f-stop of 8, when I look at the middle ring, the 8 on the left of centre is at about 4 feet and the 8 to the right of center is at 7 feet. My depth of field in this case is almost 3 feet. Notice that the distance that is in focus is not evenly
distributed in front of and behind the focal point. Generally there is more depth of field behind the focal point than in front of it. In this instance, the focal point if 5 feet away and there is a bit more than one foot in front and 2 feet behind that are in ‘acceptable’ focus.
The best way to get a feel for depth of field is to take photos with various combinations of f-stops and shutter speeds. Remember to note what combinations you used so when you view the results you can match cause and effect.
