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WIDEANGLE
TELEPHOTO
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
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
fi xed 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.
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.
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.
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.
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)
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
fl exible. Typically, they are much better
optically speaking as well.
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 fi eld of view and in so doing they
foreshorten perspective, appearing to bunch
everything closer together in the frame. They
also reduce depth of fi eld – 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 fl attering perspective to
a face and help reduce depth of fi eld.