Post on 04-Apr-2018
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Under s tan din g. .. Foca l Lengt h
Focal Length Def in ed
When parallel rays of light strike a lens focused at infinity, they converge to a point called
the focal point. The f oca l l eng th of the lens is then defined as the distance from the
middle of the lens to its focal point.
[Note that since most camera lenses are made up of more than one lens elements, the
definition "middle of the lens' is adjusted slightly.]
The focal length of a lens is usually displayed on the lens barrel. Pictured below is a Nikon
lens with a focal length of 50mm. The maximum aperture is f/1.8 (also often written as,
F1.8).
Nikon 50mm f/1.8 AF Nikkor
Norm a l , W ide-Ang le , Te lepho to , Zoom
We usually categorize lenses as having a w i d e - a n g l e, n o r m a l or t e l e p h o t o focal length.
A normal 35mm lens covers a 24x36mm film with a field of view that corresponds
approximately to our normal vision; a lens with a focal length of 50mm (55mm is also
popular) is considered as normal. Any lens with a focal length less than 50mm (or 55mm)
can be considered as wide-angle; any lens with a focal length greater than 50mm (or
55mm) can be considered as telephoto. A zoom lens offers a range of focal lengths. The
table below lists some of the more popular focal lengths.
Lens Popular Focal Lengths for 35mm cameras
Wide-angle 18mm, 20mm, 28mm, 35mm
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Normal 50mm, 55mm
Telephoto 90mm, 135mm, 200mm, 300mm
3 5 m m F i lm
Do not confuse '35mm' lens with a focal length of 35mm. The former refers to traditional
35mm film camera format (also known as 135 format). In other words, '35mm film
camera' refers to the 135 film format -- not the lens format. [35mm is the width of the
film including the sprockets.]
3 5 m m Eq u i v a le n t
Nikon 28-105mm
f/3.5-4.5D AF-Zoom Nikkor
Nikon Coolpix 5400: 5.8-24mm
(28-116mm, 35mm equivalent)
f/2.8-4.6 Zoom Nikkor ED
In the example of the two Nikon lenses illustrated above, the lens on the left is a 35mm
film camera zoom lens with a focal length of 28-105mm. It is a popular zoom lens because
it provides a good wide-angle (28mm) as well as enough telephoto reach for a goodportrait lens (with 105mm, you can fill the screen with a face without getting 'in your face'
too close to your subject).
The lens on the right comes with the Nikon Coolpix 5400 and has zoom focal lengths of
5.8-24mm. Believe it or not, this has a 3 5 m m e q u i v a l en t of 28-116mm!
Because the image sensor size used in digital cameras are of different sizes, the same
focal lengths may be expressed using different numeric values. Smaller image sensors
require smaller lenses; larger image sensors require larger lenses to ensure all the surface
of the image sensor is covered. Other factors, such as amount of optical zoom provided,
may further affect the distance between lens and image sensor.
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By using the 35mm equivalent, we simply make it more convenient for everyone to use a
handy reference point. Most, if not all, camera manufacturers will list the 35mm equivalent
in the specifications of a digital camera. Interestingly, some digital cameras are now even
engraved with the 35mm equivalent on the lens barrel to express the focal length of the
lens. It makes more sense in a way, though it is important to bear in mind that 35mm
equivalent does not mean 100% compatibility with the real 35mm lens. This is especially
true in the area of depth of field.
Focal Length Mul t ip l ie r
If you use digital SLRs (dSLRs) that use lenses made for 35mm film cameras, you need to
be aware that the focal lengths expressed on the lenses must be multiplied by a factor.
That factor, the Focal Length Multiplier (FLM), depends on the image sensor size used.
Of course, if the image sensor is full-frame, i.e. it is the same size as 35mm film, then the
multiplier is 1, and the focal length of the lens is accurate.
However, only a few dSLRs use full-frame image sensor, with most using a smaller image
sensor, usually APS size (or roughly half-frame). That is why you will read that a focal
length multiplier of, say, 1.6 needs to be applied to the focal length of the lens to obtain
the "true focal length."
So, for a dSLR using an APS-sized image sensor, the focal length of a 55mm lens becomes
approx. (55 x 1.6 = ) 88mm lens. This is both good news and bad news. The good news is
that you can now achieve super telephoto coverage without buying costly and unwieldly
lenses. For example, a 100-300mm zoom lens, with a focal length multiplier of 1.6, now
acts like it was approx. 160-480mm. The bad news is, of course, that your 18mm superwide-angle lens is now acting as though it were approx. 28mm, making it almost
impossible to obtain the kind of super wide-angle coverage that most professionals desire.
Some readers have objected to our use of the term "focal length" in our explanation above
and have insisted that the technically correct term is "field of view." They are right, of
course. So here's the technically correct version: "The focal length of the lens used on a
full-frame sensor or an APS-sized sensor is the same (i.e. achieves the same
magnification) but since the smaller sensor only captures a smaller central portion of the
image projected by the lens, the field of view obtained in the latter case gives the
impression that the lens is acting as one with focal length times the FLM. Another way to
think about the FLM in our example above is to say that we would need an 88mm lens on
a dSLR using a full-frame sensor to obtain the same field of view that we obtain with a
55m lens on a dSLR using an APS-sized sensor."
On second thought, just multiply the focal length by the FLM, and think of this as your
"true focal length." ;o)
Dig i ta l Lenses
Digital camera manufacturers are now working on lenses that are made specially for digital
cameras ['Digital' lens is really an unfortunate misnomer]. Film can be properly exposedwith light incident on it at any angle. An image sensor, on the other hand, consists of a
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matrix of light-gathering photosites set in "wells" and pretty much requires that light be
incident on it at right-angle. This means that lenses made for 35mm film cameras may not
be always adequate to the job.
At t r ac t i veness of S tandards
Some digital camera manufacturers have joined together to propose and follow a standard
in image sensor size for dSLRs. By also standardizing on the lens mount, they can
concentrate on manufacturing digital lenses that are interchangeable on any dSLR
following the standard. This is an incredible attractive proposition for consumers who will
not need to worry about being tied in to a particular lens system. Buy any lens and any
camera body, and interchange at will!
One such example is the Four Th i r ds system supported by Olympus, Kodak and Fujifilm.
However, a proposed standard will only obtain the required traction when most of the
major digital camera manufacturers join it. Unfortunately, we do not yet see this
happening. The first 4/3 system dSLRs are now reaching the market and every camera
manufacturer is watching to see if there is adoption by a large enough base of enthusiasts
to provide the momentum required to attract other competitors to the 4/3 system fold.