Focus and Depth Of Field

Author Vern West
All images are copyright by Vern West

One of the questions that I get on various workshops is how to control Depth of Field (DOF) or sometimes understanding DOF. The explanation requires setting your camera to focus on a single point.

So let’s go over critical focus first. Most cameras have several ways of automatically focusing the lens, this is usually called autofocus or AF for short. I shoot the Canon System so my terminology tends to use their terms but the concept applies to all camera systems.

The first AF concept is what Canon calls One Shot and AI Servo. One shot means when you press the shutter button down half way the lens is focused on the subject. If the subject is moving the lens focus does not follow the subject while you have the shutter depressed half way. AI Servo is a continuous focus so that as long as you have the shutter depressed half way the lens will try and follow and maintain focus on the subject. Of course this means that you must keep the focus point on the subject as well as the shutter depressed half way.

This leads to the next AF concept. All camera systems have the ability to set a single focus point or multiple points these points are typically arranged in a diamond pattern or rectangular pattern. In the case of the single point the lens will AF on that point precisely. In the case of the multiple points the camera actually picks one or more of the multiple focus points to focus the lens.Focus-Image-03

This works well in some instances but not for our purposes of controlling DOF. Every camera has a different way of selecting the focus points so you will have to refer to you camera manual to find out how to set a single point. It seems like every Canon model in the last few years does it a little differently so I cannot even explain how to do it even on the different Canon cameras. Now, the first thing to do to control the DOF is to set your camera so that only one AF point is on and have it set to One Shot.

There is another focus mode on some Canon cameras called AI-Focus. It is explained as a combination of both One Shot and AI Servo in that it maintains focus on a stationary subject and tracks a moving subject. After some personal testing I don’t think that it works as well as either One Shot or AI Servo so I use those modes instead,

DOF is defined as the area in an image that is acceptably sharp. A lens can only be in focus at a specific distance but an area in front of and behind the focal point can be sharp enough to be acceptable to the eye. This acceptably sharp area or DOF is approximately 1/3 of the distance in front of the focus point and 2/3 of the distance behind it. If this seems too technical just bear with me and I will simplify it in a minute. I want to explain some of the concepts before going on. I use a free Smart phone APP to quickly determine my DOF in the field without having to calculate anything. The one I use is DOF Calculator but there are many and you need to pick one that you like and is easy to use.  Just search for DOF Calculator in your phone’s APP Store, there are lots of them.

There are three things that affect DOF. The first is sensor size but since we cannot change the size of our sensor we can ignore this factor.  Basically the smaller the sensor the greater the DOF which means that a point and shoot camera or camera phone will have a large DOF and there is very little that you can do control it. However, the other two things that we can control are focal length of the lens and F/stop.

The longer the focal length the shallower the depth of field. A 50mm lens will have a greater DOF than a 200mm lens at the same F/stop. You have probably seen images of a bird on a perch or animal where the background is just a blur of color such as image 01 . This is an example of longer focal length lens and a shallow DOF. The reverse would be a scene with everything in focus from the Ocotillo in the foreground all the way to the mountain in the distance such as image 02. This is an example of a shorter focal length lens and a greater DOF.

The next factor that we can control is F/stop or the opening that lets the light through the lens to the sensor. The smaller the lens opening, larger F/stop numbers, the more the DOF. I like to think of it as a bigger F/stop number gives me more DOF. These two examples show you the DOF of a 70mm lens at F/4 and F/16.


I said that I would simplify this so here goes! Looking at your Smartphone App. The DOF at F/4 in the example above is less than 1 foot but at F/16 it is nearly 4 feet when both are focused at a subject that is 8 feet away. So all you really have to do is let your App do the calculation for you. Adjust your lens F/stop to get the DOF that you want. If you don’t use a smartphone then you can print out DOF charts for the lenses that you use and refer to the chart.

Let’s look at a more real world example. Looking at image 05. Both the Cholla and the mountain are in focus. This was taken at F/16 and 24mm. Using the DOF Calculator App we see that at a subject distance of 8 feet everything would be acceptably sharp from around 3 feet all the way to infinity, the mountain in this case. Since the Cholla was about 8 feet I focused on it and knew that mountain and moon would be in focus. For a wide angle lens, stopping down to F16 will insure a large enough DOF most images.

DOFImage-05

Longer focal length lens require paying more attention to the DOF App and subject distance. The Saguaro flower was taken with 400mm lens at F/6.3 from an about 20 feet away.   The DOF in this case is only around a quarter of a foot.

DOFImage-06

Notice the bud behind the flower is out of focus and the more distant background is a very soft blur.

In the field armed with the DOF App or a DOF chart found on the internet you can improve your images by controlling the depth of field. I’ll save a discussion of Hyper Focal distance for another time.

Vern West is a nature photographer and trip leader for Arizona Highways Photo Workshops.

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