Depth of field

It is an interesting property of the lenses … that I can’t describe. The first image that comes to mind is Portrait photography. Portrait photography is the type of photography where the subject is clearly separated from the background. Portraits are obtain best using long focal length and big aperture. So DOF depends of the distance to the subject, the aperture and the focal length used.

Let me star with a problem I have. I need to shoot running people at a marathon event with my Mamiya film camera. These are some friends of mine that participate and my target is to have pictures of them as they run towards the camera. I am using a film with ISO 400 and because people are running, I would also need a shutter speed of minimum 1/250 of a second, as not to have traces of movement and get a sharp picture.  Also, because is a sporting event, I do not have the possibility to get up and personal with each of my subjects. For this reason I will use my 135 mm lens, which is almost a telephoto lens.

Now let’s put together our data to be prepared for the shoot:

  • 135 mm lens
  • 1/250 seconds shutter speed
  • ISO 400

There additional data to think of, like the weather and subject distance. When working with a film camera, where everything is manual, you have to think in advance, before pressing the shutter. One of the disadvantage with the cameras, is that the focus is not automatic. Before shooting, you have to decide which is the subject of interest and do manual focus on it.

In our case the subject will always be a person running, so the way I do it, is manual focus on any subject at a precise distance, let’s say 15-20 meters. I would then plan my camera position to be 15 m away from the virtual line or position where I believe the subjects will pass, and I would just sit and wait for them to come into focus.

You might ask, what is with this focus? The answer is that the focus is well linked to the DOF concept (depth of field). DOF is the field in front of your camera where all subjects are in focus.

To get back to our preparations for the running people shoot, we have the following:

  • 135 mm fixed lens – 135 is the focal length
  • 1/250 shutter speed
  • ISO 400
  • 15 meters subject distance

Now let’s calculate the DOF to try to predict what part of the picture will be in clear focus, or more precise the distance for which the image is in focus. For this we are missing the aperture value.

To get a sharp focus, the rule of thumb is to use a f8, so will start with this value. Now, for the actual calculation there are plenty of applications. Here below an example for android.

Here you can see that for a 135 mm lens with subject at 15 meters and an aperture of f8, we will get a DOF of 6 meters. In this 6 meter of clear image, 40% of the distance is in front of 15 m mark, and 60% behind 15 m mark. What this means ? Starting from 18 m in front of your camera until 12 m, the runner will be in focus. In this space of 6 m you must press the shutter. This means there is about 2-3 seconds time to press the shutter, because the runner will do 2-3 meters per second.

Let’s make mental simulation of who the shooting will go:

  • Install the camera with viewfinder towards the track
  • Compose the picture: Expect people will run into the frame, towards the camera.
  • Focus 15m distance you will get a clear picture for 6m: 40% in front and 60% behind 15m mark, the rest will be out of focus.
  • Set the values: 135 mm lens, ISO 400, 1/250 sec, f8

f8 is an aperture suited for daytime, but what if the weather is shady and there are also trees that cast shadows on the track? You will end up with an underexposed photo. All cameras have an indicator that indicate if the light is sufficient or not to get a proper exposed photo. This indicator is the meter indicator.

The way to compensate for an underexposed photo is to have a slower shutter speed, or greater ISO, or greater aperture like f4 or f2. Since you can’t use a slower shutter speed to get more light, or use a higher ISO (because the film has fixed ISO), the only think you can do is to increase the aperture, from f8 to f4 to f2. My lenses have a maximum aperture of f2.8.

Let’s see how DOF behaves for 135 mm lens, 15 subject distance and various aperture values:

  • F8 > DOF = 6 meters
  • F4 > DOF = 3 meters
  • F2.8 > DOF <= 2 meters

When someone is running 1 km per 6 minutes, this is equivalent to a 3 meters per second. This means that when using f4 you have a depth of field of exactly 3 meters. Hence, you have one second to press the shutter to get the subject exactly when the runner is 15 meters away!

This is a practical example of when the concept of DOF is useful to prepare your shoot. There are also the artistic perspective, where you make an idea of the DOF and just go shoot beautiful bokeh photos. Most common uses are for photo portraits where the subject is clearly delimited and differentiated from the background.

There are a lot of applications to calculate the DOF. The ones I use are for android are:

For reference to Depth of Field, you can always check this wikipedia link:

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