What is Focal
While a photographic lens
comprises multiple lens elements, it can be regarded as a single convex
element. The focal length is defined as a distance from the center of such a
convex element (principle point) to the focal point (image plane) and it is one
of the most decisive factors that determines the characteristics of a lens.
* The focal length of a
photographic lens is established with the subject positioned at the infinity
Angle of View (Telephoto & Wideangle
The area size captured by a
photographic lens can be expressed as a diagonal angular field called Angle of
View. Generally speaking, a focal length range that provides a similar
perspective to the human eye is considered to be somewhere between 40-60mm.
With this established as a standard focal length, those with shorter focal
lengths are called "wideangle" and those with longer focal lengths are called
"telephoto". The shorter the focal length becomes, the wider the angle of view
(wideangle), while the longer it becomes, the narrower the angle of view
* The relationship between
focal length and angle of view is generally consistent regardless of the lens'
focal length. However, there are cases where different angles of view are
observed depending on the difference in the focusing system of a lens and
Perspective (Sense of distance)
A photographic lens
provides a visual effect, making closely located subjects larger while remotely
located subjects smaller. As the focal length becomes shorter in a wideangle
lens, this perspective difference expands making closely located subjects even
bigger and remotely located ones even smaller (exaggerated perspective).
In contrast, in a telephoto
lens, as focal lengths become longer, less difference is observed between close
and distant subjects, making it appear as if they are closer regardless of the
distance between them(compressed perspective).
Depth of Field
When focused on a subject,
there are areas in front of and behind the main subject where details are
sharp. This area is referred to as depth of field. When the sharp image area is
narrow, it is expressed as "shallow depth of field". When it is wide, it is
expressed as "deep depth of field". The depth of field becomes shallower as the
lens aperture goes toward a full open position (or the faster the lens' maximum
aperture becomes). It becomes deeper when the aperture gets closer to the fully
stopped down position (or the slower the lens' maximum aperture becomes). Also,
a wideangle lens delivers deeper depth of field compared with a telephoto