A Post By: Andrew S. Gibson
I can remember when autofocus was relatively new and many photographers distrusted it. Now it’s the other way around – autofocus has become so good and so ubiquitous that manual focus lenses are a rarity. It’s easy to forget that sometimes it’s better to turn it off and use manual focus instead.
A good example of this is when you use a wide-angle lens to take a landscape photo. In this situation, you probably want to record the entire scene as sharply as possible. But how do you do this when the nearest part of the scene may be only centimetres (less than an inch) from your camera?
The answer is that you need to understand depth of field and be willing to take control and decide exactly where you want to focus the lens, rather than leave it up to the camera.
What is depth of field?
Depth of field is the area either side of the point of focus that appears to be sharp. The amount of depth of field depends partly on camera settings (the focal length of the lens and the aperture) and partly upon other factors such as the size of the photo and the eye of viewer. Sharpness is subjective – what might appear to be sharp to one person may not appear so to another. Keep this point in mind as we progress through the article.
This diagram shows the depth of field when you focus on infinity. You can see that most of the zone of sharpness (another term used to describe the areas that are in focus) extends beyond the point of focus and is effectively wasted.
Depth of field in action
The best way to understand how depth of field works is with a practical example. Imagine that you are using a 24mm lens (or a zoom lens set to 24mm) on an APS-C camera focused on a point exactly one metre (3.3′) from the camera. You want a nice sharp image so you have set the aperture to f/11 (the sharpest aperture settings on most lenses are f/8 and f/11). In this case everything between around 73cm (28.75″) and 157cm (5’1″) will be sharp.
This is what happens if you focus too closely to the camera. You will get the area around the point of focus sharp, but anything close to the horizon will not be sharp.
In reality, what is happening is that anything one metre (3.3′) from the camera (the point the lens is focused on) is sharp, and then everything either side gradually becomes less so until it reaches the point where it no longer appears to be in focus to the human eye.
Where did I get the above figures from? I used the depth of field calculator at DOF Master and selected EOS 7D as the camera. The camera is important as depth of field as affected by sensor size, as well as lens focal length and aperture.
We have already seen that focusing on infinity effectively wastes depth hyof field. You’ll get a better result using the hyperfocal distance.
What is hyperfocal distance?
The depth of field calculator throws up another figure – the hyperfocal distance. This is the closest point at which you can focus the lens at this focal length and aperture settings, and still keep everything from the focusing point to infinity in focus.
In this example the hyperfocal distance is 2.7m (8’11″). Focus on this point and everything from 1.34 metres (4’5″) to infinity is in focus. That maximizes depth of field and helps you get the entire scene sharp.
This diagram shows what happens when you focus on the hyperfocal distance. Depth-of-field is maximised, helping you to create a sharp image.
Note that the near focus point (1.34 metres or 4’5″) is half the hyperfocal distance of 2.7 metres (8’11″). This ratio holds true no matter the hyperfocal distance of your lens, aperture and sensor size combination.
Hyperfocal distance in action
Alternatively, there are several smartphone apps that will perform the calculations for you such as these (do a search for DOF or hyperfocal distance, there are plenty of others)
Once you have the information, you need a lens with a depth of field scale on the barrel to tell you the distance at which the lens is focused. Many lenses have these, but some don’t (including lots of kit and pancake lenses). Distance scales aren’t precise, so be prepared to work with approximate measurements.
An example of a lens with a distance scale.
A lens without a distance scale.
If your lens doesn’t have a distance scale, your only option is to guess the distance (or use a tape measure!). Luckily, it’s not hard to guess distances. In the above example, you would know that focussing on a point around 3 metres (approx. 10′) from the camera will maximize depth of field, so it’s not that hard to make a guess.
After you have taken a photo you can double check sharpness by playing it back on the camera’s LCD screen and enlarging it as much as you can. The usefulness of this depends on the clarity and size of your LCD screen, but you should be able to tell if the parts of the scene closest to you are unsharp. You will only really know for sure once you get home and look at the photo on your monitor, but checking the LCD screen reveals any major errors in your calculations.
Given all the variables and potential for mistakes, one way to apply hyperfocal distance is to use the calculations for one aperture setting (e.g. f/8) but set the camera to a smaller aperture (such as f/11). This gives you margin for error.
Notes on using hyperfocal distance
A couple of things to be aware of:
First, the depth of field calculations in this article are based on something called the circle of confusion. This is the maximum size at which an out of focus dot appears to be sharp to the human eye. Circles of confusion are necessary because they form the basis of depth of field calculations. Each camera and lens manufacturer makes its own judgement as to which size circle of confusion they will use for their calculations. That means the figures in depth of field calculations and hyperfocal distance tables vary according to the circle of confusion size the figures are based on.
If you didn’t understand that, don’t worry. This video explains it perfectly:
Second, you will often read that depth of field extends 1/3 in front and 2/3 behind the point being focused on. This isn’t true. Focus on something close to the camera and the split is closer to 50/50. Focus on something far away from the camera and the split may be 10/90. Advice that you should focus 1/3 of the way into the scene to maximize depth of field is well intended, but inaccurate.
More reading on this topic at Getting Landscapes Sharp: Hyperfocal Distances and Aperture Selection
Understanding Lenses ebooks
My ebooks Understanding Lenses Part I and Understanding Lenses Part II will help Canon EOS owners decide what lenses to buy for their cameras. They are both filled with lots of tips to getting the most out of your Canon lenses. Click the links to learn more.