LENSES AND FOCAL LENGTH
Today’s modern zoom lenses open a whole
range of possibilities when photographing a scene.
Choosing a specific lens focal length is not just a
simple matter of bringing the subject closer or widening
the field of view but ultimately is used to determine the
relationship between objects at different distances.
Lens focal length is specified in millimeters (mm) with
the smaller numbers giving you a wider field of view
(wide angle) and the larger numbers giving you a
narrower field of view (telephoto). So for most digital
SLR cameras focal lengths of 17mm are considered
wide angle and focal lengths of 200mm are considered
telephotos.
Complicating this is the fact that most DSLRs use a
smaller imaging chip which creates an effective
magnification increase for any given focal length. This
is known as the Lens Focal Length Conversion Factor
or crop factor. Usually this is 1.5X (Nikon) or 1.6X
(Canon) depending on the camera. Any lens focal
length is therefore multiplied by this factor to get the
actual “effective” focal length. So for example a 200mm
lens on a APS-C/DX sensor camera would in effect
give you the magnification of approximately 300mm.
Wide Angle Lenses
Have the widest field of view and are commonly used to
photograph landscapes and other situations where you
need to capture a large expansive area.
Another use of the wide angle lens is to exaggerate the
change in size between foreground objects and those in
the background. By using it close to foreground objects
it results in giving the photograph more of a sense of
depth and stretches out that relationship.
Another major advantage is the increased “depth-offield” at any given aperture making it easier to keep
both the foreground and background in sharp focus.
© Paul Sparrow 2016
LENSES AND FOCAL LENGTH
Normal Lenses
A “Normal” lens is one that has a focal length that is
approximately equivalent to the diagonal of the image
sensor and gives the perspective of what the human
eye sees so objects in the photograph that are at
different distances appear relative to each other in the
same way as you see them.
Telephoto Lenses
Telephoto lenses are ideal when you’re at a great
distance from a subject and need to bring it closer or
increase its magnification to reveal detail.
This can give the image a “you are there” look so if you
compare the photograph to the actual scene they would
have a similar perspective.
Telephoto lenses also produce a flattening of the
relationship between any foreground objects and
what’s in the background creating a distinctive
“compressed” look to the image.
And fixed focal length normal lenses can also have a
much larger maximum aperture than zoom lens and
can be as low as F1.8, F1.4 or even F1.2. This
extremely large aperture can be used to capture
images under very low light or used to minimize “depthof-field” to a very narrow area helping to isolate the
subject out of the background.
“Depth-of-field” also becomes much narrower with
telephoto lenses and therefore can be used to isolate
a subject and make it “pop” from its surrounding
foreground and background.
© Paul Sparrow 2016
LENSES AND FOCAL LENGTH
Perspective
Perspective is the relationship between objects at
different distances based on the subject to camera
position. By changing both the distance to the subject
and the focal length of the lens used one can alter the
perspective dramatically and change the size of
background objects in relation to the foreground.
Portrait Lenses
The
focal
length
needed to do a head
and shoulders portrait
should be at least 1½
times the “normal” focal
length for your camera.
This makes sure there
are no distortions in the
facial features. Portraits
using large aperture
telephoto lenses have
the added advantage of
al lo wi n g
a
m ore
restricted
“depth-offield” which can focus
attention on the main
subject.
Macro Lenses
Macro lenses are specifically designed for extreme
close-up work that can reproduce intricate details of
very small subjects that would be impossible to get with
other lenses.
So by getting in close to a foreground subject with a
wide angle lens you distort the objects closest to the
camera and the background appears much smaller
than it would in reality. But by backing off from a
foreground subject and using a telephoto lens you
compress the background on top of the foreground
and it appears much larger than it would in reality.
A true Macro lens allows very large magnifications down
to 1:1. This means that an object that is just a little
under an inch across would fill the imaging chip from
edge to edge on a APS-C/DX sensor. Macro lenses
have much better optics than a regular zoom lens at
these close focusing distances and are also optimized
to give much better results when photographing a flat
surface for copy work.
© Paul Sparrow 2016