Depth of Field
Depth of field is a fairly controversial topic. I am writing this post after much research and experimentation. Here is what I understand about it:
Depth of field is the range of distance within the subject that is acceptably sharp. In simpler terms, it is the portion of a scene that appears sharp in the image. There are three main factors that control the depth of field: 1) Aperture, 2) Distance from the subject and 3) Focal Length . The last two factors are interrelated as I will explain later. Let’s discuss each of them in detail.
1) Aperture: The rule of thumb is as the aperture increases, the depth of field decreases (or become shallower).
The effect of aperture on depth of field is shown in photographs below (Photo 1). The two shots were taken at same focal length (= 70mm) and subject distance. The first photograph was taken at bigger aperture (= f/5.0) than the second photograph (aperture = f/16.0). We observe a shallower depth of field in the first photograph than the second (The wall is more blurred in first shot). A cartoon showing change in the depth of field with aperture can be found here.
2) Distance from the subject: The rule of thumb is as the subject distance increases, the depth of field increases (or become deeper).
There is a simple experiment to demonstrate this effect. Hold your hand at arm’s length in front of your face. Even when you are focusing on your hand, most probably you can see the background in reasonably clear focus. Now slowly move your hand towards your face. Keep your eyes focused on the hand. Notice how the background blur increases as your hand moves towards your face. The effect can be enhanced by placing an object (like a laptop) at arm’s length right behind your hand. Moving your hand closer will slowly blur the object and hence indicating a change in depth of field.
3) Focal Length: The rule of thumb is as the focal length increases, the depth of field decreases (or become shallower). However, this is a controversial topic. Few people contest that the depth of field is independent of focal length. Let me explain this:
As I said in beginning of the post, the focal length and distance from subject are interrelated. Let me explain that first. In my earlier post I discussed about the field of view (extent of view imaged by the camera) and it’s dependence on the focal length. Basically, longer the lens, smaller the field of view. However, I didn’t mention the fact that field of view also depends upon the distance of camera from the scene or the image. Closer the camera to the scene, lesser the field of view. So effectively, you can shorten the field of view by changing the focal length (zooming in) or by moving closer to the image. Similar concept applies to the depth of field, however it is not as intuitive. The point of this discussion is that the depth of field varies with the distance from subject and this distance is related to focal length.
The controversy surrounding the dependence of focal length arises when photographers change the focal length and distance from the subject simultaneously. They are two opposing effects. Hence if you change the distance from the subject as you change the focal length, the depth of field will remain the same. For example, the depth of field of shot taken at focal length of 100mm and subject distance of 10ft will be same as the depth of field of shot taken at focal length of 75mm and subject distance of 7.5ft. You can check this result here (Depth of field for both shots will be 0.5ft for 35mm film and f/2.8 aperture).
So in order to test whether the focal length affects the depth of field, we need to keep the other two factors constant. In the photographs shown below, I kept the aperture (=f/22.0) and distance from the subject constant. I took these two photographs at focal length 135mm and 75mm. I cropped the second photograph (with focal length 75mm) to keep the same field of view. The first photograph have a shallower field of depth than the second photograph (Observe the yellow leaves in the background. They are more in focus in second photograph). Hence, the focal length does affect the depth of field. And it is inversely proportional to the depth of field.
Now we know how to control the depth of field. Next question is how to apply this information, that is, when to use a shallow depth of field and when to use a deep depth of field. This is a subjective decision and hence, vary from photographer. However, there are a few rule of thumbs.
A shallow depth of field is useful for taking portraits and close-up shots of object. The distracting background is thrown out of focus to draw viewer’s attention to the main subject. Two examples of shallow depth of field are shown below (Photo 4 and Photo 5)
On the other hand, a deep depth of field is useful for taking landscapes and architectural photographs. It shows the details of entire scene instead of focusing at one particular object. An example is shown below:
This covers basics of the depth of field. The complete discussion of the topic will include depth of focus, circle of confusion and hyperfocal distance. However they are advance topics. I will deal with them some other time.
Tip: New DSLRs usually have a depth-of-field preview button. You can use it to preview the area of acceptable focus. It darkens the area with low focus.
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