Introduction
The lens is used by the photographer to change
the angle of view in the image. The choice of lens can have a major impact
on the resulting photograph. In this beginners guide I intend to describe
the technical terms and different properties of lenses.
Focal Length
A lens is considered to have a certain focal
length, measured in millimeters. The actual definition of focal length is
not important. What is important is the fact that focal length is directly
related to the angle of view of a lens. The shorter the focal length, the
greater the angle of view. For example, and focal length of 50mm has an
angle of view of about 46 degrees, whereas a 28mm lens has almost double
that, around 75 degrees. On the other hand, a 100mm lens has an angle of
view of around 23 degrees.
Aperture
As you will know from other Beginners Guides,
the choice of aperature affects the depth of field, how much is a scene
is in focus. A wide aperature (a small f number) results in very little
depth of field, whereas a narrow aperature (a large f number) results
in lots of depth of field.
Lenses have a maximum aperture, say 1.8 or
3.5. This is the widest aperture that the lens has. The lens can use narrower
apertures (to provide more depth of field), but not wider ones. This limits
the minimum depth of field possible.
The longer the lens, the less depth of field
is present for a given focal length and distance. So, a 50mm at f8 has
more depth of field than a 100mm lens also at f8 (This is good for portraiture
as little depth of field blurs any distracting background)
Shutter Speed
Due to the relationship between aperture
and shutter speed, a wide aperture requires a higher shutter speed. This
means that the wider a lenses maximum aperture the easier it is to work
in low light. For example, say at f1.4 the correct shutter speed is 1/60.
A lens with a maximum aperture of f4.5 requires a shutter speed of 1/15,
low enough to induce camera shake.
For this reason, an f1.4 len is considered
"faster" than an f4.5 lens.
As a general rule of thumb, using a shutter
speed slower than 1 / [focal length of the lens] without a tripod can
cause camera shake and result in blurred shots. So using a 50mm lens at
1/60 is OK, but you need a tripod for 1/15.
Focus
The photographer focuses the lens using a
collar at the front or autofocus. When the collar is turned different
distances are brought into focus. This can be seen through the viewfinder.
Zooms and Primes
There are two different types of lenses,
zoom enses and prime lenses. Prime lenses are as I have described above.
They have one focal length and hence one angle of view. A zoom lens on
the other hand has multiple focal lengths, say from 28mm to 70mm. The
zoom lens can change it's focal length to any value between 28mm and 70mm.
This allows the photographer to quickly choose the exact angle of view
required, and saves the extra weight of a 28mm, 35mm, 50mm and 70mm lenses.
They often have widest apertures that change over the different focal
lengths. For example a lens may be designated 28-70 f3.5-4.5, which means
that if set to a focal length of 28mm the widest aperture possible is
f3.5, but when set to 70mm this rises to f4.5.
However this comes at a cost. Zoom lenses
are slower than prime lenses. Their widest aperture is typically 3.5 or
4.5, compared to 1.8 or 2.8 for a prime. The are also heavier than any
one prime lens, due to there being a lot of glass in the lens. This also
means that whn shooting towards the sun, reflections can be caused within
the glass resulting in "lens flare", a loss of contrast and hexagonal
or octagonal shapes of light appearing in photographs.
Beginners' Guide to The
Studio :: Beginners' Guide to
Enlargement :: Beginners' Guide to Film
:: Beginners' Guide to Exposure
:: Beginners' Guide to Lenses
:: Beginners' Guide to
Wide Angle Lenses
| Article provided by: Edinburgh
University Photographic Society |
Date 01/05/03
|
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