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Terminology
Focal Length
Exit Pupil Position
Angle of View
Brightness of a Lens (F and T No.)
Flange Back and Back Focal Distance
M.O.D.
●
The focal length will be
the distance from the back
principal point to the image
plane. Lower the focal length
wider the image.
The exit pupil is the image (virtual image) refl ected by the lens located at the
back of the lens diaphragm. The exit pupil position is generally represented
with the distance between the image
plane and the exit pupil. "- (minus)"
indicates closer to the object, and "+
(plus)" toward the camera.
The angle of view is the object size that can be
captured at a specifi ed image size, which is represented by
angular measure.
Normally the angle of view is measured assuming a
lens is focused at infi nity. When using a lens
of the same focal length with a different image
size, the angle of view will differ.
●
Flange back will be the distance between the mechanical
mount surface and image plane.
Back focal distance
will be the distance
between the rear end
of the lens part and the
image plane.
The M.O.D. (minimum object distance) is the
closest distance to the object at which an
image can be taken.
This is the distance from the vertex of the front
lens.
Image Sizes
C/CS-Mount
There are several types of imaging sensors for CCTV cameras, with
different image sizes.
The aspect ratio of a CCTV camera is normally 4:3 (H:V).
● CCTV cameras have either a C-mount or
CS-mount.
Product symbol
Image sensor
Image size (mm)
Horizontal: H
Vertical: V
Diagonal: D
C
1"
12.8
9.6
16.0
H
2/3"
8.8
6.6
11.0
D, S
1/2"
6.4
4.8
8.0
Y, T
1/3"
4.8
3.6
6.0
Q
1/4"
3.6
2.7
4.5
35 mm camera
lens (Reference)
35 mm
fi lm
36.0
24.0
43.3
C-mount
CS-mount
Flange back focal
length (mm)
17.526*
1
12.5*
1
Diameter of screw
thread (mm)
1-32UNF
C-mount camera CS-mount camera
C-mount lens
*
2
CS-mount lens
1 Length in air
2 Will need a C-mount adapter ring (5 mm) when fi tting a
C-mount lens to a CS-mount camera.
Standard
Interchangeability
Image circle
Diagonal: D
Horizontal: H
1/3"
1/4"
1/2"
2/3"
1"
Vertical: V
Back principal point
Focal length
Image plane
Image plane
Back focus (B.f)
Flange back (F.f)
θ
Ｈ
V
Ｄ
Example
The angle of view when the camera size is 1/2"and the focal
length is 12.5 mm:
Angle of view
Image size
Focal length
The F No. is an indication of the brightness of lens. The smaller the value, the brighter the image produced by the lens. The F No. is inversely
proportional to the effective diameter of the lens and directly proportional to the focal length. The scale on the iris ring of lens uses a ratio of
2, because the value of light incident on a lens is proportional to the cross section of luminous fl ux (square of diameter). In other words, the
brightness decreases by half each time the F No. is increased by one F stop.
The F No. is a value determined on the assumption that the transmittance of the lens is
100%. Virtually all lenses however, have different spectral transmittance, and thus, the
same F No. can have different levels of brightness. To eliminate this inconvenience, a
system has been developed to consider both F No. and spectral transmittance, the T No.
The T No. and the F No. are related to each other as shown in right:
F No. = f
—
d
T No. =
F No.
f: Focal length of a lens
d: Effective diameter of a lens
10
Transmittance
(%)
Image plane
Exit pupil
Lens diaphragm
Exit pupil distance
Zoom
Vari-Focal
HD Fish-Eye
HD
F
ix
ed
Fo
ca
l
HD Vari-Focal
Zoom Lens W
iring
Technical Information
