Digital SLR Handbook. John Freeman
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and continuous modes
In essence, the Single AF mode sets the AF system to find the main subject in a scene, focus upon it and then lock. It’s ideal for static subjects. Continuous AF, on the other hand, finds the subject and (depending on your camera) tracks its movement. A special predictive AF set-up ensures that when the shutter eventually fires, the subject is sharp.
Both these focus modes can work in conjunction with another powerful AF tool found on DSLRs, the AF Area Mode. Here, the type, number and shape of the AF zones that will be active are controlled. Typically, there is a Single Area AF, where just one AF zone works at a time, and a Dynamic AF (sometimes referred to as Servo AF), which allows just one, a whole group or all the AF zones to work together simultaneously – this helps when shooting complex or moving subjects and adds to focus accuracy.
Some DSLRs have a Closest Subject Priority AF, which will always try to focus on the main part of the scene closest to the camera. This is useful when there is a cluttered or possibly distracting background.
Switching off the AF
If you want to take complete control of the focusing chores on a DSLR, you can. Most AF lenses allow you to override the AF manually at any time, or you can turn off the AF, if required. To do this, there will be an on/off switch on your camera body and/or on the lens.
On this Canon DSLR, you can see the AF controls on the top plate (to the left) that control the focus modes and a switch on the lens barrel marked AF/MF to turn the AF on and off (manual focus).
Shutter or aperture priority
With a DSLR you can choose to have either aperture priority or shutter priority. Which you choose depends on the type of shot you are going to take and the effect you wish to create.
How and when to use
The mode dial of a DSLR has a range of automatic settings and a manual setting (‘M’). There are two other controls: aperture priority (‘A’, or ‘Av’) and shutter priority (‘S’, or ‘Tv’, which denotes time value). When one of these is selected, the camera automatically adjusts the other to keep a properly metered exposure.
Simply, if you need to control the depth of field but shutter speeds are less of an issue – for example, when the camera is tripod-mounted for a macro shot – use aperture priority. You then select the best aperture for the required depth of field while the camera takes care of the shutter speed. If shutter speed control is paramount – for example, if you need to ensure a fast shutter speed to freeze an action shot – then select shutter priority. While you ensure the shutter speed is fast enough, the camera sets an aperture to keep the shot correctly metered for the scene.
This image shows the mode dial on a DSLR with the various automatic settings on view, plus the all-important ‘Av’ and ‘Tv’ (aperture and shutter priority) settings. The ‘M’ setting is the manual control where the user controls both the apertures and shutters, as required.
Depth of field preview
DSLRs have a small button on the camera body, usually adjacent to the lens, that, when pressed, shuts the aperture to the current setting – this is the depth of field (DOF) preview control. With the DOF preview button depressed, the aperture of the lens closes to its current setting (if it’s smaller than the maximum aperture), allowing you to look through the viewfinder and assess how much of the scene will be sharp from in front of the lens to the distance.
A large aperture, that is a low f/number, such as f/2.8, gives a very narrow DOF, which means that the subject will be sharp, but everything behind and in front of it will be blurred. A higher f/number, say, f/22, will give a deep DOF, resulting in a sharp foreground and background.
The button at the bottom left corner of this DSLR is the DOF preview button. Pressing it will stop the lens to the metered or selected aperture, enabling accurate assessment of the DOF.
What the eye and camera see
Many people find they’re disappointed with their photographs because they never seem to come out in the way the shot is remembered. The reason for this is that the camera doesn’t see things in the same way as the eye.
Interpreting information
When you notice a subject, the eyes send a signal to the brain, which, in turn, interprets the visual information. The brain is very good at editing scenes and pushes some of the angle of view into the background, removing any distractions. It does this by keeping the part of the scene you are focusing on sharp, while letting the other parts go out of focus. This is a great asset, as it means you can concentrate on the important information and discard the rest. Of course, you are still aware of these indistinct parts but they do not impinge on your main focus. Perhaps the easiest way to understand this is to draw your attention to what you are doing right now: reading this book. You can see the typeface easily but if you think of the surrounding area, you realize that you are only aware of it and can’t see it properly.
Cameras have an advantage over what the eye can see because you can either zoom in or out, or change lens. This shot was taken with a 50mm lens and gives an angle of view roughly the same as the eye.
By changing the lens to 24mm, you have a much wider angle of view from the same viewpoint without having to scan the scene, which would be the case with your eyes.
Now when you look through the camera lens and take a shot, the camera interprets the scene differently. The chances are that not only will the book be in focus but the rest of the room will be sharp as well. This makes it difficult when you look at the finished photograph to concentrate on anything in particular because the scene is a jumble.
This shot of Nelson’s Column in London was taken with a 50mm lens. No matter how hard you try, it’s impossible to make out all the detail at the top of the column.
With a 400mm lens, all the detail of the statue has become clear.
Getting the desired shot
With a DSLR camera there is a plethora of lenses and accessories, together with a range of different features built into the camera, that will help you capture exactly what first caught your attention. Knowing what all these features are capable of is essential if you are to make full use of the DSLR system. You can then start editing in-camera in the same way as your brain edits what your eyes see: you can keep all the scene sharp or just a part of it; you can include a wide-angle or zoom in; you can use the shutter creatively and learn to be selective with your viewpoint.
When you watch television or read a book, your eye automatically concentrates on what is important to you. Although you might be aware of other detail in your peripheral vision, it is, in effect, put out of focus by your brain.
When you look at a similar scene through the camera lens, especially a wide-angle