Depth-of-Field Revisited

Depth-of-Field Revisited

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 or aperture.

One of the factors that influences depth-of-field is lens type. There are several characteristics of all lenses that affect their 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 earlier blog posts, 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 apparent physical distance and thereby making subjects seem bigger. This becomes more important if you are using a zoom (variable focus) lens because it can change depth-of-field very quickly as you go through the different focal lengths. Also, if you have your camera set to  AUTOMATIC, this will also influence how any particular lens behaves vis-a-vis depth-of-field. 

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 or a shorter lens or both for maximum depth. If you are looking to make backgrounds out of focus with less depth-of-field, use a wider aperture and/or a longer lens. 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 because there is consequently more distance behind the subject than in front of it. 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 because the lens will actually compress the distance.

Now, if you are going to use a very large f/stop number like f/16 which is really a very small aperture in order to increase your depth-of-field, 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 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 rhododendrons. 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. (This light I often refer to as the “fourth side of the exposure triangle.”)

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, much detail in the distant background will be too small to see clearly and the entire image will look sharp. Here is a good example of that:

I basically set the camera on a tripod focused on the near grasses, since they were closer to the lens and would appear larger in the image, found the correct exposure (f/8 @1/200th of a second, ISO 100, 20mm lens). So even at f/8 look at the depth-of-field obtainable with this wide-angle lens! The sailboats as well as the hillside in the background all seem to be in focus enough to present the whole scene.

Here is another example of that technique. The focus is all on the foreground grasses, yet the effect of the image is that it is all sharp. Look also at how much of the sky is included:

DEPTH OF FIELD CHART:

This is a handy chart to help determine what apertures you would need for maximum DEPTH-OF-FIELD.

Now when photographing people for portraits, a medium telephoto lens will render physical attributes like faces more normally than would a wide-angle lens which usually introduces some distortion. The telephoto lens will also render the background soft (“bokeh”) which makes the subject stand out more. Here is an example:

This image, shot for a magazine cover, was made with my 135mm f/2 lens at quite a distance from the subject. Exposure was f/5.6, ISO100 @ 1/125th of a second. This is what the setup looked like:

Here you can see that the background was rendered very soft (out of focus) which made the face really stand out. This is the effect you want for most portraits. Above is the setup for this cover shot.