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For a given lens focal length, the
field of view can very substantially
depending on the size of the sensor
or film frame in a camera. For
example, a 75mm lens for the
Pentax 6x7 (60mm by 70mm film
frame) medium format camera is a
wide angle lens, but a lens of the
same focal length
on a Sigma SD10
camera (20.7mm by 13.8mm
sensor) is a telephoto lens. There
are several film size standards, 110,
APS, 35mm, 645, 6x7, and even
more CCD sensor sizes in use. In
fact Kodak recently announced that
a new 45mm by 34mm sensor will
be used in a new Pentax medium
format digital camera, which will
use the same lenses and
accessories as the 60mm x 45mm
Pentax 645 film camera. This leads
to awkward ways of describing the
characteristics of lenses that are
used on the new system, in order to
put it into terms that can be
understood by someone who is
used to using the older film version
of the camera. We get phrases like
"Your 85mm Pentax 645 lens
becomes a 110mm lens on the new
D645" which is factually incorrect as
focal length is actually a physical
property of a lens and does not
depend on the size of the film or
sensor behind it. To clear this up, I
propose to define a characteristic,
called "length", for how tele a
telephoto lens is on a particular
camera as the ratio of the focal
length of a lens and the focal length
of a normal lens on that camera.
Thus, a 100mm lens on a 35mm
film camera would have a length of
2, 50mm being the normal focal
length on that format. I propose to
define "width" as the reciprocal of
the "length", that way, a 25mm lens
on a 35mm film camera would have
a "width" of 2.
Examples:
Canon PowerShot A95: "length" of
0.77 to 2.3.
(built in 7.8mm to 23.4mm zoom
lens, 7.1mm wide sensor)
Polaroid x530: "length" of 0.72 to
2.2, or you could say: "zooms from
as wide as 1.4 to as long as 2.2".
(built in 7.3mm to 21.9mm zoom
lens, 7.1mm wide sensor)
Sigma 10-20mm zoom on Sigma
SD10: "width" of 3.0 down to 1.5.
(20.7mm wide sensor)
Sigma 30mm on Sigma SD10:
"width" of 0.99. (pretty darn close to
normal).
Pentax SMC 165mm 6x7 lens on
Sigma SD10:
"length" of 5.58.
Pentax SMC 165mm 6x7 lens on
Pentax K1000 SLR: "length" of 3.3.
(35mm film)
Pentax SMC 165mm 6x7 lens on
Pentax 6x7 SLR: "length" of 1.65.
(60 by 70mm film)
Note: higher "width" numbers
indicate wider effective field of view,
higher "length" numbers indicate
narrower effective field of view (and
higher effective magnification).
[edited for clarity 3/17]
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I've seen blurb that describes a lens as "equivalent to a 80mm lens on a 35mm camera", but it's by no means standardized. If you know how the figures are derived, and you have the required background data, this isn't required, but generally people don't, so please focus on the 210mm pastry (Rollei fit). |
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Is this an example of the new enviro-friendly muntions? [see [scout]'s environmentally friendly mortar shell] |
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//don't tell me the focal length, tell me the angle of view.// I have three Tamron lenses (28, 70-200 and 500mm) from the 1980's and 90's and each is clearly marked with the angle of view. Baked.
aov = 2tan^-1 (s/2f), where "s" is the sensor dimension (usually the horizontal one) and "f" is the focal length - simple mental arithmetic.
Other than that, this idea assumes that people are familiar with 35mm focal lengths, which may not apply in all cases, where people may have started using APS-C digital cameras, or simply used point-and-shoot in the past. |
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Absinthe, the 35mm examples
were just examples. The idea is
that a "1" always means normal,
and a length of "2" always means a
telephoto lens with twice the focal
length of a normal lens. The
probelm with the angle of view
being printed on the lens is
twofold, first, many lenses actually
are designed to be used with
multiple image sensor sizes, and
secondly, the angle of view
specifications for lenses are
advertised as if they will be used
with a sensor that is exactly as
large as the image circle of lens.
The sensor is always smaller then
that, and in any case is not related
to that number, except that it
must be smaller. |
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