In its simplest definition, focal length is the distance between
optical center of a lens and the sensor/film, when focus is set to infinity.
It is generally expressed in terms of millimeter(mm).
Focal length is a factor of lens, in fact, the principal factor
of a lens. When we want to buy a lens, we first decide what focal-length
should the lens have. 16mm? 50mm or 300mm prime? Or a zoom 16-35mm?
Or 18-300mm?
Focal length of a zoom lens
is changed physically by rotating the
zoom-ring on the lens. When we zoom in, focal length gets longer
(100mm, 250mm, 300mm ... ), and we get a magnified view
of the scenario/subject. The more we zoom in, nearer
the subject approaches. When we zoom out, we get a shorter or
wider focal length, such as 24mm, 16mm, 14mm ... etc. The more
we zoom out, farther the subject moves, and lower the
magnification we get.
Focal Length and Field of View
Focal length of a lens does not change, when used with differently
sized sensors. A 70mm lens is always 70mm, regardless of the
size of the sensor is used.
In combination with the sensor-size, focal length
determines what we call the field of view; how much of the
scenario can be seen by the camera, through the lens.
A short or wide focal length can produce a large field of view,
and a long focal length produces a narrow field of view. Again,
with long focal lengths we can get highly magnified image; in case of short
focal length opposite happens. Please check the following:
Observe how different
focal lengths change the field of view of a scenario.
Field of view also changes when different sensor-sizes
are used.
Focal Lengths Wide to Long
When a very wide focal length is used with a close foreground,
the lens tends to exaggerate the foreground element(s), and
farther elements tend to be smaller in size. On the other hand,
a very long focal lenth tends to compress the background elements
nearer to each other, which is known as Lens Compression.
For a visual representation of the same,
check this page.
Following are the examples shot in the
field. The same scenario was captured using a tripod with different
focal lengths and multiple results are displayed one by one.
When a lens has very short focal length, say 14mm... 16mm... 18mm...,
we get a very wide field of view; so a
large portion of the scenario is captured by the sensor
(with a very low magnification).
Those lenses are called
wide angle or ultra wide angle lenses,
and generally used
for landscape, architecture and astro photography.
Wide angle focal length
f/17, 71 sec., iso 100, FL 15mm eqv.,
Nikon d7000, Sigma 10-20mm f/4-5.6,
Sonbeel, Assam, India 2014
The above image is an example of wide-angle photography shot
at 15mm focal length.
On the other hand, by using a very long focal length, say 200mm ...
300mm.. etc., a very narrow field of view is produced; so very small
portion of the scenario is photographed (with a magnified view).
Lenses having very long focal lengths are also called
telephoto/super telephoto lenses,
and are suitable for wildlife, birding, sports etc. Following is
an example of a 300mm shot:
Using a long focal-length
requires a faster shutter-speed to avoid recording of any camera-shake.
Generally, if you shoot a 300mm shot, set your shutter-speed
1⁄300 sec. or faster.
Focal length of a lens is very closely related with depth-of-field of a photograph.
Medium telephoto lenses, such as 85mm, 105mm, 135mm, 200mm...
etc. produce shallow/narrow depth-of-field, and are suitable for
portrait photography. Following is a 127mm(eqv.) shot which blurs
out the background almost completely with the help of the used focal length
and aperture.
Medium telephoto focal length
f/2, 1⁄640 sec., iso 100, FL 127mm eqv.,
Nikon d7000, Lens Nikon 85mm f/1.8
Srinagar, Jammu & Kashmir, India 2018
Note: Aperture is also very closely related with depth-of-field. The page
aperture and depth-of-field
is a must read to understand the contribution
of aperture towards the depth-of-field of a photograph.
Normal focal length
f/6.3, 1⁄800 sec., iso 320, FL 52mm eqv.,
Nikon d810, Lens Tamron 24-70mm f/2.8,
Varanasi, UP, India 2015
A 35mm to 50mm focal length is said to be
standard/normal focal length.
Such a focal-length can record images similar to what human-eyes can see.
This range is also a good match for street photography.
Focal-length and aperture are the two principal characteristics
of a lens. Before buying a lens, we must ask ourselves:
which one, prime lens? or zoom lens? What should be the
focal-length? what will be the maximum aperture? constant or variable
aperture?
The other properties of a lens are selected then, such as :
auto-focus or manual, VR (IS/OS) or non-VR, filter-size, number
of aperture-blades, size, weight, price etc.
Prime and zoom lenses
A lens is a piece of glass (or a group of glass) that
converges light to the focal point to form an image on
the film or digital sensor.
It is said that if camera is the brain, lens is the eye.
A lens sits in front of the camera sensor or film;
it controls the quantity of light allowed to the sensor
through aperture. A lens may be fixed to the camera; so that it cannot be
changed or detached. Cameras like Canon g7x, Sony Rx-100
and Nikon P1000 etc. have fixed lenses.
A Nikkor 70-300mm zoom lens
Most of the DSLRs and mirrorless cameras have the facility
to change its lens. The same camera can be mounted with
different lenses as per our requirement. Those are called
ILC (Interchangeable Lens Cameras). Those cameras can be
mounted with large variety of lenses.
There are two types of lens, prime lens and zoom lens.
A prime lens has a fixed focal-length. So, there is no
option for zoom-in or zoom-out. With such a lens, we
have to move our camera forward or backward to get zoom-in
or zoom-out effect. Even we may have to move ourselves forward or backward by
our feet (called zoom-by-feet). They are less complex in the
internal construction, small, light and less costly. By the
way, telephoto and fast primes are not cheap.
On the other hand, a zoom lens has different
focal lengths. An 18-55 lens has a widest focal-length of 18mm,
and longest of 55mm. Any focal-length inbetween may be used
for shooting. Say, at 18mm, we shoot a landscape scene;
we zoom in to range 30 to 35mm, and take a street shot. Again, by
zooming in to 55mm, we take a portrait shot. With an 18-200mm
zoom lens, we dont have to frequently interchange lenses, as
compared to a prime lens.
Here is an animation of the behaviour of a 35mm 1.4
prime lens with different apertures. Check that the
focal-length (sensor to lens distance) is fixed,
because it is a prime lens. For simplicity, the
little movements of the lens(group) due to focusing
are not shown here.
Note: The internal structure of a lens is not so simple as shown above.
On the body of a Nikon 85mm f/1.8
prime lens, the following text is written :
85mm refers to the fixed focal length of 85 millimeter.
The 1 : 1.8 part indicates that the lens has a largest aperture of
f/1.8. This lens allows to shoot with all the apertures in the
range of f/1.8 to f/16 at 85mm. Any 85mm lens with largest aperture of
f/1.4 or even f/1.8 is most suitable for portrait photography.
When a lens has very large aperture, it is also called a fast lens.
The smallest aperture-value(f/16 in this lens)
is generally not mentioned on any lens-body.
The "AF-S" part indicates that the lens
has its own auto focus capabilities, and the "G" part
indicates that this lens has no aperture ring.
In case of a
zoom lens, things get a bit complicated. The following text is
found on the body of a Nikon 18-55 lens :
The "DX" part indicates that this lens is meant for APSC sensor
or crop-sensor cameras. Those sensors have a dimension of 23.5 x 15.5 mm.
By 18-55mm we understand that the
lens has variable focal lenths. By rotating the zoom-ring,
we can zoom in and shoot at 55mm (longest FL for this lens, generally
used for portraits), and by zooming out we can shoot at 18mm(widest
FL, generally used for landscapes, cityscapes, astro photography).
The entire range of
18mm to 55mm can be used to shoot by zooming in and out.
The "1:3.5-5.6" indicates that this lens has largest
aperture of f/3.5. That means you can never shoot at f/3, f/2.8, f/1.8
apertures with this lens. The limitation does not end here.
The maximum aperture of f/3.5 is available at 18mm only.
When you zoom in to 55mm, you will be allowed a maximum aperture of f/5.6, not f/3.5.
Here is the behaviour of the aperture of an 18-55mm 3.5 to 5.6 DX lens
at different focal lengths. Set focal length to 18mm, and aperture to f/3.5.
But when you zoom in to 24mm, the aperture will automatically stopped down to f/4.
Note: The internal structure of a zoom lens is even
more complex than prime lenses.
Same is applied on minimum aperture.
At 18mm, you will be allowed a minimum
aperture of f/22; and at 55mm, you will get a minimum aperture of f/36.
Zoom lenses may have variable apertures or a
constant aperture. The above-mentioned 18-55mm lens has variable apertures.
A 24-85mm f/3.5-4.5 lens also has variable apertures.
At 24mm the largest aperture f/3.5
can be used; but at longer 85mm, largest aperture is f/4.5.
But in case of a 24-70mm f/2.8 lens, we get a constant
largest aperture of f/2.8 throughout the zoom range.
Constant aperture lenses are larger and heavier and
can allow more light. They are costly too.
Choosing an appropriate lens is a vital decision.
Lenses determine the actual image quality of a photograph.
New cameras are always emerging into the market,
but a costly lens produces good image quality and
may serve for a long period of time.
