Depth-of-Field Explained

Depth of Field Explained

By William Lulow

“Depth-of-Field” is a term used to describe how much of a given image is “acceptably sharp.” Another way of saying this is that the term refers to the distance in front of and in back of the subject that is rendered “in focus” by a particular lens.

One of the factors that influences depth-of-field is lens type. There are several characteristics of all lenses that affect its ability to render most of any scene “in focus.”  One is the actual focal length of the lens itself. When the elements of a lens, which I have spoken of in an earlier blog post, are closer together (as in a wide-angle lens), the lens can capture more of a scene AND render most of that scene in focus. A lens with many of its elements further apart (as in a telephoto lens), is much less able to show most of a scene in focus. It is more able to focus on one element and render it sharp while the rest of the scene is more or less out of focus.

So, wide angle lenses tend to increase the depth-of-field in an image by making the subject appear smaller in the overall scene. Telephoto lenses on the other hand, tend to decrease the depth-of-field because they focus on a much smaller portion of the overall scene while reducing the physical distance and thereby making subjects seem bigger.

The other factor is the lens aperture. A smaller lens opening tends to concentrate the light rays being reflected from the subject on the sensor, thus making more of the scene “acceptably sharp.”  A larger lens opening would tend to  spread out these light rays more which renders the image less in focus. So, if you are looking for greater depth-of-field (more of the scene in focus) use a smaller aperture. If you are looking to make backgrounds out of focus with less depth-of-field, use a wider aperture. Also, it should be noted that the camera-to-subject distance also has something to do with keeping backgrounds soft. The closer the camera is to the subject, the softer the background will be. If the camera is far away from the subject, even though you are using a longer lens with a large aperture, still more of the scene will be in focus.

Now, if you are going to use a very small f/stop (i.e., a small lens aperture), you would necessarily have to compensate for the small amount of light hitting the sensor by decreasing the shutter speed (i.e., making the shutter speed slower) or using a larger ISO number. As you do this, you introduce a motion factor to the image. Sometimes you cannot hand-hold the camera at very slow shutter speeds to make “acceptably sharp” images with great depth-of-field. Therefore, you need to use a tripod to steady the camera and to ensure a sharp enough image. This also highlights what is referred to as the “triangle of exposure” because the three elements regulating exposure, aperture, shutter speed and ISO setting are all interrelated.

Here are two images I made recently with my Canon 135mm f/2 lens. On my cropped sensor camera, this lens acts more like a 216mm lens on a normal sensor:

The first image was made using an ISO setting of 100, a shutter speed of 1/320th of a second at f/2. (That is wide open for this lens). The second exposure was made using the exact same ISO setting of 100, a shutter speed of 1/13th of a second and an aperture of f/10.  Look closely at the azaleas. They are all in focus. But then look at the backgrounds. One is fairly sharp and the other is way out of focus. This is an illustration of how these two different settings affect the image’s depth-of-field. The one on the left with the very shallow depth-of-field really makes the flowers stand out against the background. Whereas, the one on the right presents a kind of confused message because all of the image is more or less, in focus. True the stems in the background are not sharp, but they are sufficiently in focus to be distracting to the image’s statement.  Both images were made with the camera on a tripod. Even with a fast enough shutter speed, this lens is bordering on being too heavy to hand hold successfully. It can be done, but not without a fast-enough shutter speed.

The bottom line is that if you want all or most of your image to be in focus, use a wide-angle lens and/or a small f/stop (aperture). Conversely, if you want to throw the background out of focus (sometimes referred to as “bokeh,”) use a longer lens and a bigger aperture (smaller f/stop number).

Here is an example of extreme depth-of-field. This image was made with my Canon 60mm macro lens stopped down to f/32 at about 1/2 of a second and ISO 400. Note that everything, front to back is sharp and in focus:

This image was lit with my studio strobe units. A softbox placed just behind the camera. When you want really great depth-of-field and you are shooting with a lens that can be stopped down to a minimum aperture, you most often have to add some light to the scene.

Tip: when you are shooting landscapes and want all of the scene to be sharp, use a wide-angle lens, focus in on the foreground, stop the lens down and just let the background go. Because of the nature of wide-angle lenses, you won’t be able to see that much detail in the distant background and the entire image will look sharp.