Depth of field in Photography

Depth of field, in photography, is the sharp area in front of and behind the subject on which the lens is focused.

Any photograph has some sharp area (we call this area in focus) and some unsharp or blurred area (we call this area out-of-focus, or bokeh). We can imagine that the area/field in front of our camera is made up of infinite number of planes. All those planes will not have the same sharpness.

Table of contents:

• ● What is Depth of Field
• ● Depth of Field w.r.t. Aperture
• ● Depth of Field w.r.t. Lens to subject distance
• ● Depth of Field w.r.t. Focal length

Note : A shallow or small or thin or narrow or minimum Depth of Field captures a very small area in focus. A very small portion of the scenario will be sharp, major portion will remain out-of-focus.

On the other hand, a deep or large or thick or wide or maximum depth of field captures a large portion of the scenario in focus, i.e., major portion of the scenario will have acceptable sharpness.

Throughout the explanation of depth-of-field in this document, the words like shallow, small, thin, narrow, and minimum do have the same meaning (less depth-of-field). Whereas the words like deep, large, thick, wide, and maximum mean more depth-of-field. This holds true w.r.t. the term depth-of-field.

Three important factors, namely

1. aperture,
2. focal length
3. lens-to-subject distance and

are responsible for the depth of field in a photograph. In fact, those last two terms together constitute the magnification factor of a lens, which has very important contribution towards creating depth of field for a photograph.

Effect of Aperture on depth of field

The effect of aperture on depth of field is discussed in details this page. Let us check some facts about the second and the third factors which effect depth of field in photography.

Effect of Lens-to-subject-distance on depth of field

The distance between camera/lens and the subject (in focus) effects the overall depth of field in the photograph. If camera-to-subject-distance is very small (subject is closer to camera), we get a shallow or small depth of field; and if this distance is large (subject is further away from camera), we get a large depth of field. In both the cases aperture will be playing its own role.

Imagine the focus-point to be on the first flower. When camera is moved closer to the focused subject, we get a very shallow depth of field. And when moved away from the focused subject, a large depth of field is achieved.

This means when you place your camera very close to the subject, only a small portion of the scenario will be in focus, the rest of the scene will be out-of-focus, even if a narrow aperture is applied, or very wide-angle lens is used.

But, when you move your camera away from the subject(to be focused), the depth of field will be getting larger; moving the subject away from the camera/lens also will do the same.

Please check the following image and its cropped version.

The camera was very very close to the first cabbage (in focus). The wide angle focal length of 10mm(DX) and short camera-to-subject-distance together have produced a nice perspective.

But the latter has caused the far scenario to be a little out-of-focus (as seen in the above cropped version) inspite of the fact that a small aperture was used.

Now compare the following two images which were taken seconds apart; both the images have the same settings. They are different just in composition/framing.

In case of the first image, the camera was hold normally, the flowers were a bit far from the camera/lens. This resulted in a overall acceptable sharp image, i.e. depth of field was deep/large.

For the second image, the camera was hold very close to one of the flowers. As a result, I got an image which has a smaller depth of field. This is clearly visible if we check the further flowers and leaves in the scenario. Inspite of using a small aperture like f/11, the further elements have become blurred because of the less lens-to-subject-distance.

Lens-to-subject distance plays a very important role in capturing different depths of field and if used wisely, can produce nice artistic images.

Effect of focal length on depth of field

Wider or shorter the focal length (12mm, 18mm ...), deeper or larger the depth of field. But with a long focal length (85mm, 200mm ...), there will be a shallow/small depth of field. This is true when we position the camera at same place.

The focus is on the middle ball at all focal lengths. With a wide or short focal length, we get a deep/large depth of field. With a long focal length, there will be a shallow/small depth of field.

Please read this page parallelly for more details about depth-of-field.

When we increase the focal length of a lens, in other words, apply a longer focal length,

1. Everything in the scenario is magnified,
2. Field of view narrows down, and
3. Depth-of-field reduces.

When a wider/shorter focal length is applied, the oppsite happens.

Observe the change in depth-of-field when focal length changes, aperture kept constant, in the following animation.

Different shutter-speeds were used to keep the exposure constant. Since those images were shot outdoors, some difference in overall exposure may be found.

For the above images, I stood at the entrance of a monastery, upto where it was permitted. So the camera was, more or less, at the same place. Different focal lengths were used to capture the activities of the monks. Focus was set on the 3rd monk from the front.

If you look closely at the furthest decorated wall (marked with yellow in the first image), you will find that, longer the focal length, blurrier the farthest wall. At the longest focal length 187mm eqv. in the 4rth. image, the blur effect is found to be highest.

The same behaviour can be observed in the above three images. The farthest wall (marked with yellow circle in the first image) is blurriest at the focal length 330mm eqv. in the last image.

If any of the three variables is changed in the setting, the depth of field of the photograph also will change.

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