Depth-of-field

Autofocusing is not a new invention. It has been around for thousands of years as a feature of the human eye. Your eye reacts so quickly, and without conscious effort, that you probably don’t think of your eye as an autofocusing lens. The speed of focus of your eye also means that you are not really aware that much of what you see is out-of-focus. If you look at something in the foreground, it appears sharp. If you shift your eye to look at the background, this snaps into focus.

One way to be aware of this effect is to hold a finger up in front of your face, about half a metre away. Close one eye and look at your finger with the other. While concentrating on the finger, you will become aware that anything in the background is out of focus. Now look at the background and your finger will appear unsharp.

Much the same happens when you take a picture with your camera. The lens can only focus on one distance. Anything closer to or further from the camera will be unsharp. The difference is that the view is captured as a two-dimensional image. When you look at the image, only subjects at the focused distance will be sharp.

This effect can be very useful in isolating the main subject from its background. However, it is also possible to make some of the out of focus areas appear sharper. The area of apparent sharpness in an image is called the ‘depth-of-field’, and it is one of the main creative controls you have.

  

A camera lens can only focus on one plane of the subject. This is the only area of the scene that is really sharp.

  

However, a wider area of the scene may appear to be sharp. This area of apparent sharpness is called the depth-of-field.

Circles of confusion

Depth-of-field is possible because your eyes are not perfect. They can’t resolve the difference between a point and a very small circle of light. When a lens focuses, each point of the subject at the plane of focus is projected as a point on to the camera film or sensor. All these points give you a sharp image.

However, parts of the subject not in the plane of focus do not form image points on the film or sensor. The rays of light from these points on the subject come to a focus in front of or behind the film or sensor. This means that when these rays of light hit the film or sensor they form a small circle.

If this circle is so small that it appears as a point to your eyes, that part of the subject will appear sharp on the image. If your eyes see it as a circle, that part of the subject will appear unsharp. The largest circle that appears to be a point is called the ‘circle of confusion’ and is a key factor in defining depth-of-field.

  

When rays are focused on the sensor or film plane, a point of light is formed. However, when the rays come from an area of the subject that is not in focus, a circle of light (shown in red) is formed. This area is known as a ‘circle of confusion’.

Sizing the circle

So what is the diameter of this circle? Well, that is where some of the confusion begins. How good is your eyesight? And what distance are you viewing from? With perfect vision, under ideal lighting, and at a normal reading distance, a circle of confusion might be as small as 0.06mm. But these conditions are far too strict for the real world and a figure of around 0.17mm is used in photography.

But there is another factor to consider. If you are shooting with 35mm film, you rarely view the image at its original size. You might have the film negative enlarged to give a 5x7 inch print. This is a 5x enlargement of the original image, so the 0.17mm circle of confusion will be enlarged to around 0.85mm – easily visible as a circle to most people. So what we need is a circle of confusion that gives a size of 0.17mm after being enlarged five times. A quick tap on a calculator shows this size to be about 0.034mm.

If your eye was part of the EF system, it might be described as a 15mm f/4 lens. However, there are problems in trying to compare the eye to a camera lens, as the eye concentrates on the central area of the subject, leaving the edges to peripheral vision. The performance of a camera lens is much more even over its entire field-of-view.

A circle of confusion is based on perception – it is not something that can be calculated precisely. This is why different depth-of-field charts and tables often give different results – they are based on different circle of confusion values. Canon uses a value of 0.035mm in depth-of-field calculations for its EF lenses on 35mm cameras. Some other manufacturers choose 0.030mm. On EOS digital cameras with the smaller APS-C format sensor, the image must be enlarged more to produce a 7x5 inch print, which means a smaller circle of confusion is needed. Canon uses 0.019mm in its calculations.

Depth-of-field factors

There are five main factors that affect depth-of-field.

Aperture

The lens aperture is the easiest way to control depth-of-field. The rule is simple – the smaller the aperture, the greater the depth-of-field. For example, f/16 will give you more depth-of-field than f/4. The reason small apertures give more depth-of-field is shown in the diagram. A smaller aperture gives a narrower beam of light from any given point on the subject. Other things being equal, the circle produced by an out-of-focus point will be smaller. As a very general rule, use apertures between about f/2.8 and f/8 for portraits where you want the background to be out-of-focus. Use an aperture between about f/11 and f/22 for landscapes where you want everything from the foreground to the far distance to appear sharp.

  

A wide lens aperture produces a large circle of confusion (shown in red) from an out-of-focus area of the subject. A smaller lens aperture produces a smaller circle of confusion from the same area.

Focal length

You will usually choose the focal length to suit the subject rather than to suit the depth-of-field. However, the accepted rule is that you get more depth-of-field with wideangle lenses than with telephoto lenses. In fact, this rule is misleading. What actually happens is that a wideangle lens magnifies the subject less than a telephoto lens, which means that more of the image appears sharper. A simple test will show that if you take two photographs of the same subject from the same position with a wideangle and a telephoto focal length, then enlarge the centre of the wideangle image to match the view of the telephoto image, the depth-of-field will be identical.

However, depth-of-field is all about acceptable sharpness, and a wide-angle shot will give the appearance of greater sharpness across a scene. As a very general rule, wideangle lenses are good for landscapes where you want sharpness from front to back. A medium telephoto lens (around 100mm or 135mm) is good for portraits if you want an out-of-focus background.

  

A wideangle setting on the EF-S10-22mm f/3.5-4.5 USM lens, combined with a small aperture of f1/16, gives extensive depth-of-field in this photograph taken in Oxford, England.

Subject distance

The greater the distance between the lens and the subject, the greater the depth-of-field is. The reason is much the same as that for focal length. Reduced magnification of the subject means there is greater apparent sharpness. Anyone who has tried close-up photography will realise that the opposite also applies – increased magnification gives a narrower depth-of-field. At life-size magnification, little more than the subject in the plane of focus will appear sharp, and the point you focus on will be critical to the success of the photograph.

Print size

Depth-of-field appears greater in small prints than in big enlargements from the same negative or digital file. This is because as the image is magnified, the circles of confusion appear larger.

Viewing distance

However, print size is less important than it might appear, because you normally view big enlargements from a greater distance than you view small prints. As the viewing distance increases, so does the apparent depth-of-field because the circles of confusion appear smaller to the eye. In practice, the combination of print size and viewing distance can cancel each other out.

Although not quite the same principle, a good example of the effect of size and distance is seen in giant advertising posters. Up close, all you see are large cyan, magenta, yellow and black dots. Move back and a clear image appears in what appears to be fine detail. In fact, there is no fine detail – the appearance of a continuous tone image is an optical illusion.

Setting depth-of-field

As you can see, defining depth-of-field is a fairly arbitrary affair. So how can you hope to control the results produced by your camera? Here is a range of options.

The rough guide

If you want a lot of depth-of-field, set a small lens aperture, such as f/16 or f/22. A small aperture may lead to a slow shutter speed, so use a tripod to reduce the effects of camera shake. Use a wide-angle lens for maximum effect. If you want a narrow depth-of-field, set a wide aperture, such as f/2.8 or f/4. Use a telephoto lens for maximum effect. If depth-of-field is not a critical factor in your composition, use an aperture of around f/5.6, f/8 or f/11. Your lens will usually give maximum performance at these settings.

Basic modes

You might think that using one of the Basic modes settings available on some EOS cameras would save you time and trouble. After all, surely the Landscape mode will give wide depth-of-field, while the Portrait mode will give an out-of-focus background? Unfortunately not. PIC modes are designed to give foolproof settings for beginners, avoiding the extremes of apertures or shutter speeds which give true creative control. The best advice is to shoot in aperture-priority (Av) mode.

Depth-of-field tables

Very precise figures for depth-of-field limits are available. EOS magazine publishes a booklet giving details for lenses from 14mm to 1200mm at a range of apertures and subject distances.

If you have access to the internet, various online calculators are available which give the information you need after the frame size, camera or circle-of-confusion are entered, along with the lens focal length, aperture and subject distance. There is a good calculator at www.dofmaster.com/dofjs.html

These tables and calculators often give the near and far limits of depth-of-field to two decimals places, or more. That’s because the figures are calculated using a mathematical equation. But there is a gradual change to apparent sharpness – it does not suddenly change at a precise distance from the main subject. So although tables and calculators are very useful in helping you choose a suitable aperture, the figures they give are only an approximate guide.

Depth-of-field AE

Fortunately, technology comes to your aid. Most EOS cameras have a built-in depth-of-field calculator. Earlier models use the Depth-of-field AE (DEP) mode.

In DEP mode, you position the active autofocus (AF) frame over the nearest point of the subject you want to appear sharp and press the shutter button. ‘dEP-1’ will appear in the viewfinder. Next, place the active AF frame over the farthest point you want to appear sharp and press the shutter button again. ‘dEP-2’ will appear in the viewfinder.

Now recompose the image in the viewfinder and partially depress the shutter button. In an instant, the camera will calculate the aperture needed to give the depth-of-field you want. The aperture and shutter speed values will be displayed in the viewfinder. If you are happy with these settings you can fully depress the shutter button to take the picture.

DEP mode can’t perform miracles. If the aperture value in the viewfinder blinks, it means that the depth-of-field you want isn’t possible – you must try again with the near and far limits of depth-of-field closer together.

You can use DEP mode to set a very narrow depth-of-field by making the dEP-1 and dEP-2 readings from the same area of the subject. However, this is a little pointless – you will get the same result more quickly by setting the maximum lens aperture in aperture-priority (Av) mode.

Cameras with Depth-of-field AE EOS-1D, EOS-1Ds.

Auto Depth mode

Depth mode gives you the depth-of-field you want without the need to know anything about circles of confusion. However, Canon has made the process even easier.

Auto Depth AE (A-DEP on the command dial) offers one-shot depth-of-field. All you have to do is compose the image so that all the areas of the subject that you want to be in focus are covered by the camera focusing points. As you press the shutter button, the camera will automatically calculate the aperture needed, and also select the appropriate shutter speed.

The advantage of A-DEP is that you only have to push the shutter button once, rather than the three times needed in DEP mode, so you can shoot more quickly. The disadvantage is that you can’t select the near and far limits of depth-of-field manually – you have to rely on the points selected by the camera.

Unfortunately, you can’t select between DEP and A-DEP – earlier models offer the first mode, while later models have the second. An exception to this is the EOS 10. When the command dial is set to ‘DEP’ and a single focus point is select, the camera sets DEP mode and you selected the near and far points in the usual way. However, when all three focus points are selected the camera changes to A-DEP mode and operates with a single press of the shutter button.

DEP and A-DEP modes do not operate on EOS cameras with flash photography. If you set a DEP mode and use either the built-in flash or a Speedlite, the camera will automatically revert to program (P) mode.

Cameras with Auto Depth mode: EOS 10D, 20D, 30D, 40D, 50D, 300D, 350D, 400D, 450D.

Hyperfocal distance

What is the maximum depth-of-field you can achieve in a photograph? You won’t get it by using the maximum focusing distance (infinity) of the lens. Depth-of-field extends both in front of and behind the focused distance, but if you have focused on infinity, there is nothing behind.

What you need to do is focus in such a way that the far limit of depth-of-field extends to infinity, which means focusing on the ‘hyperfocal distance’.

Finding the hyperfocal distance is actually quite easy. It is the near limit of depth-of-field when you are focused on infinity. And the interesting thing is that when you focus on the hyperfocal distance, the depth-of-field extends from half the hyperfocal distance to infinity.

So just look at a set of depth-of-field tables, find the near limit when the lens is focused on infinity, and divide this by two. Then set your camera lens to manual focusing (there is an AF/M switch on the side of most EF lenses) and turn the focusing ring to this distance.

Hyperfocal distance is not a fixed value for a lens – it changes with the aperture and the focal length – so you really need to carry depth-of-field tables to make effective use of it.