Depth of Field Basics- What & How
Understanding what is Depth of Field, what are the factors affecting it and how to use it, are essential for photographers who want to make artistic creations, other than just photographs. Because it is the most effective way to keep the viewer’s attention to the objects in the frame which the photographer wants to highlight.
When you focus on a particular subject on the frame, objects in front of it and objects behind it should be out-of-focus, ideally. This is what happens inside the camera; light rays coming from the objects that are in front of the subject gets focused behind the focal plane (sensor or film). Light rays coming from the subject gets focused on the focal plane. And light rays coming from the objects behind the subject gets focused in front of the focal plane. Light rays that get focused on the focal plane will only create sharp images. Others will be out-of-focused.
What is Depth of Field?
But, if you pay enough attention, in practice, you will see some objects in front of the focus point and some objects behind the focus point are also focused and sharp. That is where this particular topic comes in.
See the below images. Three ducks were set up on a bench. And they were photographed with two different settings. Now see the resulting two images.
In both images, the focus point is on the head of the larger duck in the middle and the distance between ducks are not changed. In the first image, the larger duck in the middle is sharp and the ugly ducklings in the front and back are out-of-focus (blurred). Further, the strip of the bed on which the large duck is, is sharp. But, in the second image, all the ducks are sharp. And a larger area of the bed is also sharp. What can be understood by that is, when the focus is set to a certain point, some distance in the foreground and some distance in the background also come into the focus, and that distance can be changed by adjusting camera settings. That total range or distance in which the objects are in sharp focus is called the depth of field.
If we turn back to the above two images, the two duckings are not within the depth of field in the first one, but they are within in the second one. A narrow range like in the first one is called ‘shallow depth of field’ and a large range like on the second one is called ‘wide depth of field’, in photography.
Now, let us look at the definition.
Depth of Field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image.
Wikipedia
Acceptably Sharp??
Your attention should be given to the phrase ‘acceptably sharp’ in the above definition. It gives us the vibe that sharpness is not uniform throughout the range. Actually, it is the case. In theory, when a point is focused, only the points in the same plane gets focused. But in practice, the camera cannot differentiate that exact plane within a specific range due to a phenomenon called Circle of Confusion. And that range in which the camera cannot differentiate the exact plane which is focused is the depth of field.
But, when the image is enlarged enough after capturing, we can see the sharpness is not the same within the depth of field.
Use of Depth of Field
Depth of Field is a great tool, or should I say the greatest tool that can be used to direct the viewer’s attention to the subject(s) the photographer wanted to highlight. An out of focus background and/or foreground can drive the viewer’s attention right to the main subject. Other than that, an out of focus background can be very aesthetically appealing to the viewer. See the below images.
How Depth of Field is Calculated?
Depth of Filed can be calculated by the above formula where,
u = distance to the subject
N = f-number
c = circle of confusion
f = focal length
Depth of field can be obtained in meters (m) or millimetres (mm) using the above formula.
Factors Affecting the Depth of Field
After referring to the above formula, it is crystal clear what factors affect the depth of field. They are,
- f-number
- distance to the subject
- circle of confusion
- focal length
Circle of Confusion
Circle of Confusion needs special attention among these factors. It is a whole another topic, but let’s look into it briefly. Circle of Confusion is the sole reason for the presence of depth of field. Ideally, the focus is achieved when the light rays meet exactly on the focal plane (sensor). But in practice, the sensor can’t differentiate the exact point where the light rays meet. Instead, it can only identify a range as the focal plane. See the diagram below.
The focal plane and the sensor coincide exactly in the middle one, but due to practical limitations of the sensor, it cannot differentiate any position within the top and bottom situations. Therefore, the sensor identifies anything which falls within that range as focused. That phenomenon is called the circle of confusion. So when you focus on a certain point, light rays coming from objects in front of it up to certain distance and objects behind it up to certain distance falls within the circle of confusion, hence identified as focused. That is how the depth of field is created.
Circle of confusion is measured in millimetres (diameter of the light spot)
For a given camera, the circle of confusion is a constant. It is based on the sensor size and some other technical aspects of the sensor. Sensor size is the major factor in deciding the circle of confusion of a camera. Larger the sensor size, larger the circle of confusion, hence wider the depth of field. That is the reason why some describe sensor size as a factor affecting the depth of field.
However, since the circle of confusion is a constant for a given camera, you can’t change it to adjust the depth of field. What you can change are the other three factors that affect it. Now let us see how they affect it.
f-number
The primary use of the ‘Aperture‘ is controlling the exposure. But it comes with a favourable side effect, the depth of field. Aperture (f-number) is the most used factor to adjust the depth of field. When you use smaller f-numbers (larger apertures) you will get a shallow depth of field. And when you use larger f-numbers (smaller apertures) you will get a wide depth of field. How can we explain that? See the diagram below.
When larger apertures (smaller f-numbers) are used, the light cone comes out of the lens is thick. On the other way around, the light cone is thin when smaller apertures (higher f-numbers) are used. Since the circle of confusion is a constant for a given camera, a larger distance comes under sharp focus when the light cone is thin, in other words, when the aperture is smaller.
Let us investigate the aperture factor in practice. See the below demonstration. Two photographs are taken with the same setup, with the same distance and same focal length (50mm), but with different f-numbers (f/1.8 & f/16). The focus point is on the ‘minion’ in both the photographs. Let us see the effect of f-number on the depth of field.
It is clearly visible that the depth of field is much higher when f/16 used than when f/1.8 used.
Distance to the Subject
Let’s turn towards the ‘distance to the subject’ now. Closer the camera to the subject, narrower the depth of field. Likewise, it will be increased when the distance between camera and subject increases.
See the below demonstration to get it clarified. Two photographs are taken with the same setup, with same f-number (f/8) and same focal length (50mm), but with the camera placed at two different distances from the subject. The focus point is on the minion in both the photographs.
These two photographs clearly show how the depth of field increases significantly when the distance to the subject (distance between the lens and the focus point) increases. In fact, you can see in the above-mentioned formula that the depth of field is directly proportionate to the distance to the subject ‘squared’. That means if the distance to the subject is doubled, the depth of field will be quadrupled.
Focal Length
The last factor in our list is the focal length. When the focal length increases, the depth of field decreases. And when the focal length decreases, the depth of field increases. Let us demonstrate this one also. Two photographs are taken with the same setup, with same f-number (f/5.6) and with the same distance to the subject, but with two different focal lengths (40mm & 140mm). Look at the results.
You can clearly see how shallow the depth of field is when shot with the focal length of 140mm and how wide it is when shot with 40mm.
Practical Application of the Above-mentioned Factors
Even though, four factors affect the depth of field, in practice there are limitations in using them to control the depth. ‘Circle of confusion’ obviously cannot be used to control it, since it is constant for a given camera. And when it comes to ‘focal length’ and the ‘distance to the subject’, they both cannot be used for the sole purpose of changing the depth of field, since they are the deciding factors of the frame of the photograph (the area that is captured). If the distance to the subject or the focal length changes, the frame is obviously changed.
Adjusting the f-number is the only way you can adjust the depth of field without losing the required frame. You can witness that in the above demonstrations also. That is why it is the most popular and most used method of adjusting the depth of field.
Last Words
Even though, there is a range in which the objects are in acceptably sharp focus, that does not mean that the sharpness is the same throughout that range. The highest sharpness is on the plane where the focus point is situated. And the sharpness decreased when gradually when going front and back. The only thing is, the sensor can’t differentiate it within a certain range due to the thing called the circle of confusion.
That should ring a bell in your head. If the sensor can’t identify the exact focal plane, the outcome of autofocus may not be 100% correct. Keep that in mind when you use autofocus. Manual focusing through the viewfinder may also not be 100% accurate. The most accurate way to focus on an exact point is manual focusing using the Live Mode with zooming in on the required point.
Feel free to leave a comment below if you have any doubts.