What is Depth of Field
Photography has adopted some strange terminology which can be off putting for beginners but also for those who are quite experienced. One such term is “Depth of Field”. In this article we answer the question what is depth of field? We also examine how we can use it in our photography and most importantly how we can control it.
At this point I should also make a disclaimer in that the information in this guide may not be 100% scientifically precise. This is because it’s intended to be useful in the real world. What we need in the real world are concepts and rules of thumb that can be easily used adopted to help us take better photographs.
Defining Depth of Field
Let’s start this section by saying there is a formula for calculating Depth of Field but we aren’t going to use it. If you want to know what it is, try a search on Wikipedia. Instead I will give you a simple explanation that should help you.
Imagine you are taking a picture of a landscape scene. First you set the ISO, Aperture and/or Shutter Speed. You then carefully frame the composition of the scene before focusing and taking the picture. The key point to understand is that in this process you are focussing on a single point in the scene. This single point is the point of true focus. If you were to examine the resulting photo in detail you will find that elements of the scene that are away from the point of focus appear blurred. The further you move from the point of focus the greater the blur.
Now if you try the above you might well decide that this description is incorrect and that the elements of the scene appear to be in focus despite being some distance from the point of focus. This effect is the Depth of Field. We define it as being the area around the point of focus that appears to be in focus. You can see this illustrated by the diagram below.
The point of focus in this image is somewhere between 3 and 4 on the tape. The further you move from this point the more blurred the numbers. Another point worth noting is that the depth of field extends roughly twice as far beyond the point of focus as in front of it.
What Determines Depth of Field
There are a number of factors controlling the apparent depth of field and you can employ these to make your photography more interesting. They are:
- Lens Focal Length
- Point of focus
- Sensor size of film format
Many people already know to use the Aperture to control depth of field. The wider the aperture the less (or shallower) the area that appears in sharp focus. If you want to maximise the depth of field you would use a small aperture. Be careful though not to confuse depth of field and sharpness. Using the smallest aperture might help maximise depth of field but it doesn’t make the image sharper. In fact it will probably make it softer.
The lens focal length can also help control the appearance of depth of field. If you use a wide angle lens such as a 24mm will allow you to create the appearance of a greater depth of field. If you want to create a shallow depth of field you could use a longer lens such as 150mm. This is why wide angle lenses are favoured by landscape photographers where the need is to maximise depth of field and longer lenses are favoured by portrait photographers.
Point of Focus
The point of focus is also important to use in controlling the depth of field. The nearer we move the point of focus to the camera, the less the depth of field (all other factors being the same). If you have ever tried macro photography you might well have realised that even when you use a very small aperture such as f/22, the depth of field is so shallow that the entire subject isn’t in focus.
The final factor is one that is more difficult to change without changing camera and that is sensor size. The smaller your sensor dimensions, the greater the depth of field (all other variables remaining the same). Look at the image below of the New York skyline. The image is in focus from the nearest to furthest buildings.
This image was actually captured at what you might consider to be a wide aperture of f/2.8. Part of the reason the depth of field is so great is that the camera used to take the picture was a compact camera with a relatively small sensor.
How Can We Use Depth of Field
We have already touched on this subject above. Stated simply, a shallow depth of field will allow you to focus on a single subject and remove the distraction of the background. This is a favourite technique of the portrait photographer who will focus on the eyes of the subject and throw the background out of focus. When you look at the photograph you will immediately focus on the eyes and disregard the background. I have used the same technique in this photo of an alpaca so that your attention is drawn only to the animal in the foreground.
This effect was created by using a long lens, a wide aperture and moving in as close as I can to the animal.
In the next image a greater depth of field has been used. This allows the viewer’s eye to wander into the frame and examine the scene in great detail, so adding to the appearance of depth.
This effect was created because the camera I was using has a relatively small sensor, a wide angle lens was use and a relatively small aperture. I was also very careful to place the point of focus where it would maximise the depth of field, a technique known as hyperfocal focussing.
Sometimes, it's impossible to achieve the depth of field you need for a shot. When this happens, you should consider using a technique known as focus stacking. If you don't know how to use focus stacking you should read my article Focus Stacking using Helicon Focus. It will explain what you need to know.
Your role as the photographer when taking pictures is to decide how much depth of field you want to create and then use the tools described in this article to control this.
Having finished this article you should now feel confident in answering if someone asks you "What is Depth of Field?". But what about if someone asks what is an f-stop? Could you answer that one?