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> Tutorials
> Understanding... Focal Length
When
parallel rays of light strike a lens focused at infinity, they converge to a
point called the focal point. The focal length 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.]
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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
Normal, Wide-Angle,
Telephoto, Zoom
We usually categorize
lenses as having a wide-angle, normal or telephoto 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
Normal
50mm, 55mm
Telephoto
90mm, 135mm, 200mm, 300mm
35mm Film
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.]
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 good portrait 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 35mm equivalent 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 can use 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.
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
Multiplier
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 accurately represented.
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 super wide-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.
When you use a 55mm lens on a FF DSLR, the field of view you get is 55mm.
When you use a 55mm lens on a APS DSLR, the field of view you actually get is
not 55mm but (55 * FLM) = 88mm, if FLM = 1.6.
I.e., a 55m lens used on an APS DSLR gives the same field of view as an 88mm
lens used on a FF DSLR.
To get the same field of view of a 55mm lens as used on a FF DSLR, you will
need to use a (55 / FLM) = 34mm lens on a APS DSLR.
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."
On second thought, just multiply the focal length by the FLM, and think of
this as your "true focal length." ;o)
Digital 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
exposed with light incident on it at any angle. An image sensor, on the other
hand, consists of a 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.
Attractiveness
of Standards
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 incredibly 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 Thirds 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.
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