How to calculate the best depth of field
A very important part of photography in general and landscape photography in particular is to ensure that your prime point of interest, your main elements are in sharp focus. Nothing ruins a shot more than the image being fuzzy and blurry when you need it to be pin sharp.
The problem however, is that in many cases in landscape photography you have a foreground element as well as a distant object that you want to be in as sharp focus as possible at the same time.
Shooting with a wide-angle lens and setting a very narrow aperture of f/22 or even f/32 will yield greater depth of field but you will also run into the optical phenomenon known as diffraction, which is a softening of the image due to the bending of light rays as is passes through the narrow aperture of your lens. This means you will normally be operating around the f8 – f/16 mark when setting aperture.
You may find that the depth of field provided at these apertures is not enough to render everything you need in sharp focus. This is where the hyperfocal distance comes into play. When you focus on an object, technically speaking, only that point of focus is sharp. Beyond that focus point, extending both in front of and behind, is a plane of focus running parallel to the camera sensor.
This area is known as the depth of field and it is this region that is deemed to contain an area of acceptable sharpness. The key to hyperfocal distance is knowing the closest distance at which a lens can be focused while keeping objects at infinity as acceptably sharp as possible. Everything from half the hyperfocal distance out to infinity will be in focus.
A very basic rule of thumb is to compose your scene and then focus roughly one third of the distance into the scene. This can be quick and reasonably helpful, particularly where your scene has excluded the horizon or near foreground, but rarely is it optimal to get the best out of the depth of field.
You could use your camera’s live view function to visually set focus on the most distant object in your scene and then slowly adjust the focus closer whilst keeping an acceptably sharp background.
Finally of course, there is the mathematical way to calculate the exact point. There is a bit of assumed knowledge when using the mathematical formula. You will need to provide the focal length of the lens you are using, the Circle of Confusion value for a given sensor size, which is the largest blurred spot that the human eye can detect (usually a value of around 0.03 – 0.02) and the f-stop you are using.
If you have a calculator you can do the following:
H = Hyperfocal distance
f2 = focal length x focal length
N = Aperture number (f-stop)
c = Circle of confusion
The result, in millimetres, will be the distance at which you need to focus to attain greatest depth of field. Thankfully there are plenty of on-line calculators and apps for your phone that can do all the heavy lifting for you. Just so we can say we understand the theory, let’s try out a couple of examples as shown here.
We’re using a full frame camera with a Circle of Confusion value of .029 as an average on a 16mm lens and a 50mm lens. Both are set at an aperture of f/16.