Lens focal length, Perspective Depth of ﬁ eld

LENSES: FOCAL LENGTH AND FOCUS MODES

No matter what digital SLR you own, there will be a range of lenses available for it that will be able to cater to almost any subject, from close-up or macro work, wideangle shooting to telephoto shots. It will also have a range of focusing options and controls at your disposal

Lens focal length

Focal length is the name for the distance between the ﬁ lm plane and the focal point (or the optical centre of the lens) when the lens is focused at inﬁ nity and expressed in millimetres and shown on the lens. In plain speak it is the name given to an indication of the angle of view of a particular lens, where a shorter focal length lens – such as a 28mm lens – will provide a wider angle of view than a longer one – such as 100mm.

It stands to reason therefore that you can ﬁ t more of a scene into a shot using a wideangle lens than that from a telephoto lens, which is why you use longer lenses to get ‘closer’ or get greater magniﬁ cation of a scene. A wideangle lens is ideal for landscape work or for shots where you will need a lot more room to ﬁ t everything in. Telephoto lenses are ideal for getting close in wildlife photography, for example.

TELEPHOTO

WIDEANGLE

Zoom lenses and ﬁ eld of view

Zoom lenses have a range of focal lengths built into one optical device enabling you to carry one lens that offers a broad range of uses and ﬂ exibility – the reason they are so popular in fact. However, zooms are often not as good optically as a prime lens (lenses with one focal length) because they are an optical Jack-of-all-focal lengths, rather than master of one.

Lenses and aperture control

Lenses use a controllable aperture, analogous to the pupil in a human eye, altered to vary the amount of light entering a lens and reaching the sensor. Aperture control also affects the depth of ﬁ eld, with larger apertures (smaller F numbers such as F/2.8) providing a shallower depth of focus than smaller apertures (larger F numbers, such as F16).

The technical side of why this happens

is that a smaller aperture will straighten light more (even non-focused light rays) through the smaller aperture on its path to the focal plane (the sensor) than larger apertures. The closer the light is to a point at the focal plane the more ‘in focus’ it will be. Conversely, a wide aperture allows non-focused light to enter more diffusely, thus appearing more blurred.

A zoom lens with a ﬁ xed maximum aperture throughout its range (typically around F/2.8)

Focus and focus control

A DSLR will have inside it a system to measure the distance from the plane the sensor sits on inside the camera to the subject you’re trying to shoot. Usually a combination of mechanisms in the camera, it will include some or all of the following, usually working in conjunction. There will be a range-ﬁ nding device, an infrared beam (or several), colour and focal length information and a phase difference or contrast detection system. The information gathered by these systems is combined with one aim: to focus light onto the sensor (the focal plane) to ensure you get a sharp image.

The viewﬁ nder and focus control

When you look through a DSLR viewﬁ nder you’ll see a series of autofocus (AF) points that will usually illuminate for a moment in red when the focus is achieved, head-up display fashion. Some DSLRs have three such points, some can have over 30 grouped around the viewﬁ nder, but all are used to indicate that focus is achieved and in which part of the scene.

The reason there are multiple AF points is to make focusing more accurate for off-centre subjects, or for subjects that are more complex where an array of focus points might be employed to get a best overall focus position, ensuring all the elements selected are sharp. Using the camera controls, you’ll be able to override these systems if you wish, selecting separate

or groups of AF points to help further tailor the focus position for the subject. In practice, the type of subject will determine the focus points that you wish to use. In a portrait, where focus on the eyes is important, you may deﬁ ne a single AF point. For a large building with architectural projections, a group of AF points might work best to keep everything in the ﬁ nder sharply rendered.

Focus modes and when to use them

You will also have various focus modes to play with, each providing advantages. The two main focus modes are Single AF and Continuous AF. As their names suggest, the former will lock onto a subject and remain

ﬁxed there until after the shot is made, even if you or the subject move, making it best for static subjects. The latter provides a focus system that continually looks for a subject once it’s locked onto it and changes the focus position if it (or you) move, always keeping it sharply focused.

Another type of AF system, Predictive AF, is similar to Continuous in that the focus can track a subject, but this mode does not lock onto it until the shutter is ﬁ red. The AF actively monitors the position of the subject right up until the shutter exposes light onto the sensor. It predicts the subject’s position in the frame at the point it will be when the shutter starts to move and focuses there. With Predictive AF, it is quite possible the subject does not look sharp in the ﬁ nder

at any point until the shutter ﬁ res, so it can take some getting used to.

Finally, there is Zonal AF (what Canon calls A-DEP or ‘automatic depth of ﬁ eld control’), which is ideal for keeping larger or more complex subjects in focus where the depth of ﬁ eld is critical to keep a zone of the scene sharp. It is achieved by the camera checking all the active AF points, and then calculating the shutter speed and aperture required to keep all in focus, making it fast and simple to use because you don’t have to worry about adjusting apertures and checking depth of ﬁ eld yourself.

Perspective

and depth of ﬁ eld

Differences in focal length can alter what you see in an image. A wideangle lens will render all elements smaller in the frame (unless the subject is very close to the lens) and can distort perspective, as the optics used ‘bend’ light to ﬁ t it all into and onto a camera’s sensor. It is for this reason optical distortions can appear in an image such as barrel distortion, which gives the appearance of curling the image corners down and round at the top and vice versa at the bottom.

Another wideangle lens consideration

is the way perspective distorts if the camera/ lens is tilted backwards; say when shooting a tall building. This will make the verticals appear to converge towards the top of the shot. While this can be used to your advantage sometimes, or corrected in software, the easier alternative is to keep the camera perpendicular to the subject

or use a (rather expensive) tilt-and-shift lens designed to help correct for such problems.

Telephoto lenses get you closer with a smaller ﬁeld of view and in so doing they foreshorten perspective, appearing to bunch everything closer together in the frame. They also reduce depth of ﬁeld – the amount of the scene in front of, and behind the main subject, that is sharp. Again, this can be used to great advantage if there are distracting back- grounds. It is also why portrait lenses typically have focal lengths of about 90mm to 135mm to both give the most ﬂattering perspective to a face and help reduce depth of ﬁeld.

Depending on the camera you own, you may have a ﬁ eld of view multiplier to add to the focal length of your lens. This is because the lens has its focal length shown in relation to 35mm ﬁ lm frame size and your camera uses a sensor smaller than a 35mm frame of ﬁ lm (unless, that is, the camera you are using is a full frame – 35mm-sized – DSLR, in which case the focal length shown on the lens is the correct one).

Typical ﬁ eld of view multipliers are 1.5x (or 1.6x) and 2.0x. In the former group, a lens with a 50mm focal length will become a 75mm focal length lens (this includes APS-C sized sensors such as those in the Nikon D40). In the latter group (cameras using the FourThirds format eg. Olympus’s E-400) it will be a 100mm focal length.

is expensive, yes, but provides some real advantages in that you can get more creative control of depth of ﬁ eld at any given focal length. A zoom lens whose aperture reduces as the focal length increases (a variable maximum aperture lens) is less ﬂ exible since you have a reduced maximum aperture to play with, thus reducing control of depth of ﬁ eld and the amount of light entering the lens. Therefore a wide maximum aperture lens, zoom or otherwise, offers advantages over variable aperture lenses, both in terms of

the amount of light entering the lens (and so shutter speeds at your disposal) but the amount of control over depth of ﬁ eld at a given focal length (in zooms), making them far more

ﬂexible. Typically, they are much better optically speaking as well.