Every time information is provided about the lens of a digital camera,
the focal length mentioned is also given in 35mm equivalent. While the
reason for this may be obvious to some, it also occasions questions for
quite a number of people.
28mm focal length (wide angle)
35mm focal length (wide angle)
35mm film frames measure
36 x 24mm, and are the film frame size of the majority of
film cameras in use today.
This widespread use of 35mm film has caused many people to develop a general
idea of the field of view that will be visible with various 35mm focal
lengths. In other words, because of its popularity, the 35mm film format
has become a reference point, much like a unit of measure such as the foot
or the metre.
For example, a 28mm focal length will capture a fairly wide angle;
a 35mm focal length noticeably less; a 50mm lens will provide a near normal
field of view, a 380mm lens will fill the field of view (seecomparison
lower down) with a distant subject.
Put another way, the smaller the number of the focal length the wider the
field of view and, conversely, the larger the focal length number the narrower
the field of view will be; and this holds true whether for film or digital.
All focal lengths are measured in millimetres, whatever
the format of the camera: 35mm, APS, or digital.
The focal length number indicates the distance between the lens and the
focal plane—the position of the film or sensor. The precise definition
of focal length is "the distance between the focus (where the image
is sharp on the focal plane) and the optical centre of the lens". (See
graphic at right).
50mm focal length (normal view)
380mm focal length (telephoto view)
With digital cameras, these
focal length numbers are usually very small because the image
sensors most commonly used today are quite small—under
an inch when measured diagonally. To form an image on such
a small target, the lens needs to be quite close to the focal
plane, hence the short focal length numbers common to many
digital cameras.
However, the real reason a 35mm equivalent is given, is not because people
can't relate to the short focal lengths of digital cameras, but because
the "real" focal length on a digital camera—for example
a 6 to 18mm zoom—will not always corresponds to the same field of
view on different digital cameras.
At the root of this difference is the fact that different electronic image
sensors—the digital equivalent of a film size—come in a variety
of different sizes.
Let's take 3 different CCDs as example:
a 2.1 megapixel CCD measuring 0.5 inch diagonally
= (1/2")
a 3.3 megapixel CCD measuring 0.55 inch diagonally
= (1/1.8")
a 4 megapixel CCD measuring 0.66 inch diagonally
= (2/3")
As can be seen, each CCD has not only a different diagonal
measurement, but a different resolution, which is to say
the number of pixels that will form the image.
It is important to note that
the number of pixels used to form the image is not related
to the the focal length. In fact, a number of digital cameras
have been produced which, while having different sensor resolutions,
are in every other respect, similar: same lens, same body,
etc. And, if the sensors used are the same physical size,
the 35mm equivalent of the lens will be exactly the same.
On the other hand, if the lens employed for each CCD is exactly the same
focal length, i.e. 8mm, but the CCDs have different sizes, then their 35mm
equivalent focal length will be different, as each will show a greater or lesser field
of view. (See graphic at left)
So, using a "standardized" way to describe
the field of view of digital cameras helps to simplify
everything, irrespective of the size of the CCD in use.
And that need for a common way of expressing the field
of view, is why the "35mm equivalent" is generally
mentioned when a digital camera lens is described.
