Depth of Field

We have already looked at both aperture and focus, and seen how the two concepts are linked. We’ll now take a closer look at this topic and especially at something called “depth of field”. Depth of field is something that usually isn’t immediately obvious to beginner photographers, but it is an extremely powerful concept once you understand how it works.

Two things determine whether an object appears in focus or not:

(1) How far the object is from the camera lens
(2) How far the camera lens is from the sensor

With the camera we can control the distance between the lens and the sensor to adjust the focus. We could also change our position, by moving closer or further from the subject of our photo. It’s clear that distance is a key factor in focus, and from this we get the idea of a focal plane. The focal plane refers to an imaginary plane located some distance away from the camera. All objects that fall on this plane appear in focus, while everything else will be out of focus. By adjusting the camera focus ring (i.e moving the lens closer or further from the sensor) we can move the focal plane closer or further away from us.

However, objects do not suddenly switch between being in focus or not. If you experiment with the focus ring on your camera you will notice that objects gradually become more and more sharp as you bring them into focus, and then gradually more and more blurred as you move them out of focus again. How do we decide if an object appears sharp or not? This is actually a bit of a judgement call, and often people refer to objects as being acceptably sharp. This means the object looks sharp and appears to be in focus to normal human eyes.

What all this means is that the focal plane has some thickness, or depth, to it. As we approach the focal plane objects gradually become sharper, until they cross the threshold of “acceptably sharp”. As we move across the focal plane and start to move away from it again objects gradually become more blurred, until we can no longer call the object acceptably sharp. The distance over which objects appear sharp is called the depth of field.

Notice how both the background and foreground appear blurred. Only the monkey and objects close to the monkey are acceptably sharp.

The depth of field can be changed – it can be very narrow (which is referred to as a shallow depth of field) or it can be very wide (which is a deep depth of field). As you may have realised, controlling the depth of field is very useful in photography, and gives the photographer a lot of control over the appearance of an image.

The key to changing the depth of field is aperture. A large (or wide) aperture gives a shallow depth of field, while a small (narrow) aperture gives a deep depth of field. Remember that we said your default aperture should be f/8. To get a shallow depth of field you should us an aperture around f/2.8, while an aperture similar to, say, f/11, would give a deep depth of field.  The two photographs below show the same scene shot with a deep and shallow depth of field.  The difference is clear – with a narrow aperture most of the image is in focus, whereas with a wide aperture only the flower close to the camera is in focus.  Notice also how objects gradually become more blurred as the distance from the focal plane increases.  Some of the flower buds close to the flowers are almost in focus, while those a little further away are more blurred.

Aperture f/16 on the left and f/4.8 on the right. The difference between shallow and deep depth of field is clear to see. On the right the background is blurred, drawing attention to the flower. But on the left the deep depth of field means both the flower and the background are in focus, introducing distractions and reducing the impact of the flower.

Manipulating depth of field is a powerful tool. We can use it to isolate certain parts of an image, thus focusing the viewers attention on a particular subject. Often it is used to blur a distracting foreground or background, a technique often used in portrait photography.  In the two images above the one shot with a shallow depth of field is much more pleasing to the eye, and the viewers attention will quickly be drawn to the flowers.  With a deep depth of field we still see the flowers, but they are no longer the key subject of the photograph, and the viewers attention will be distracted by other elements in the image (for example the purple flowers in the background).

With depth of field we can introduce an important idea about composing a photograph. Part of a photographers job is to draw attention to certain subjects in a scene while eliminating distracting elements as much as possible. Using focus and depth of field means we can isolate parts of an image from an otherwise distracting scene. For example, portrait photographers often use a very shallow depth of field. The subject will be clear and sharp while the background is blurred and uninteresting. A viewer will naturally then focus on the subject of the portrait.

The two images below show how depth of field can help address a common issue in photography – taking a photograph through a fence or netting.  In this case the problem is in the foreground, and typically we may end up with a photograph like that on the left, where the background is obscured by a distracting and ugly fence.  However, by setting a narrow depth of field we can keep the background in focus but blur out the foreground.  In the example below aperture was increased from f/18 to f/3.5, allowing the photographer to almost completely hide the fence from the image.  A shadow of the fence can still be seen in the second photo, but the background is no longer obscured.

On the left – shot with aperture f/18. On the right – aperture f/3.5

Depth of field is all very well and good, but what if you want to have as much of the image in focus as possible? Let’s imagine we want to capture a landscape photograph and have both close and far away objects as sharp as possible. To do this you need to focus your camera at a distance called the “hyperfocal distance”. This is the focal distance at which depth of field reaches it’s maximum width, and so by focusing at the hyperfocal distance we will achieve the aim of having as much of the image in focus as possible.

Finding the hyperfocal distance can be quite a challenge, even for advanced photographers. In this course we will not go into any more depth on the subject, but feel free to check out this excellent tutorial on the subject.

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