When we, as photographers, plan a shot, one of the fundamental decisions that we must make is what we want to be in sharp focus. What we desire to be in focus can vary radically from one image to another. For an expansive vista of the Grand Canyon, we may desire that everything from the foreground to the farthest recesses of the canyon be sharp. For a macro shot of a flower, we may want a very narrow range of focus so that the viewers’ attention is concentrated at a specific point in the image.
The issue of what is and is not in focus is referred to as depth of field. From a practical point of view, depth of field can be defined as the nearest point to the farthest point that appears sharp in an image. So, the question now becomes, “How can we control the depth of field?” Actually, there are several factors that we can control that will affect the depth of field.
The camera aperture has a very large impact on depth of field. Basically, the smaller the aperture is, the greater the depth of field that will result. So, if you want a large depth of field, you should use a small aperture. For a small depth of field, you will need a large aperture.
Increasing the focal length reduces the depth of field. Conversely, decreasing the focal length increases the depth of field. Thus, long lenses typically have small depth of fields, and wide angle lenses have large depth of fields. This is one of the reasons that landscape photographers often use wide angle lenses. A wide angle lens combined with a small aperture produces a very large depth of field.
The greater the distance is from the subject, the greater the depth of field that will result.
Lens sharpness has an impact on depth of field. Sharp lenses are sharper throughout the field of view. This increases the depth of field.
Sensor or Film Size
Okay, this one is a little bit more complicated. In one way, larger sensors in digital cameras or larger film in film cameras produce greater depth of field because the image from a larger sensor or piece of film does not need to be enlarged as much in order to create a print. The less an image is enlarged, the sharper it will appear. This increases the depth of field. On the other hand, cameras with larger sensors or film tend to use longer lenses which decreases the depth of field. So, what is the overall impact? Usually, the larger sensor or film cameras will produce a smaller depth of field. However, that is not a guaranteed thing as much depends on how the camera is used.
The general, the larger the print is, the smaller the depth of field that results. This is because the print has to be enlarged more from the original size of the sensor or film in order to create a large print. This softens the print detail and reduces the depth of field.
When it comes to the focus distance, there is one very special focusing distance called the hyperfocal distance. The hyperfocal distance is the shortest distance from the camera such that everything from approximately half that distance to infinity will appear sharp. Focusing the camera at the hyperfocal distance results in the greatest depth of field possible for a given combination of aperture and focal length.
There are a number of ways to determine the hyperfocal distance. Probably, the most common method is to use a depth of field guide. A depth of field guide is an inexpensive, small device that allows a photographer to determine hyperfocal distances as well as other depth of field parameters.
In addition, photographers that have older lenses may find that their lenses have depth of field scales engraved on the lenses. To set the hyperfocal distance, one simply aligns the infinity symbol with the color coded depth of field lines. Unfortunately, most modern lenses no longer have depth of field scales.
With the above information, you should be ready to head into the field with full control over the depth of field in your images.
If you are interested in learning even more about depth of field, please check out Ron’s full length depth of field article at Depth of Field