 

Distance to Object / Width x Height in Feet 
Focal Length 
5 ft. 
10 ft. 
15 ft. 
20 ft. 
25 ft. 
30 ft. 
40 ft. 
50 ft. 
75 ft. 
100 ft. 
150 ft. 
2.8mm 
9 x 6 
17 x 13 
26 x 19 
34 x 26 
43 x 32 
51 x 39 
69 x 51 
86 x 64 
129 x 96 
171 x 129 
257 x 193 
4mm 
6 x 5 
12 x 9 
18 x 4 
24 x 18 
30 x 23 
36 x 27 
48 x 36 
60 x 45 
90 x 68 
120 x 90 
180 x 135 
6mm 
4 x 3 
8 x 6 
12 x 9 
16 x 12 
20 x 12 
24 x 18 
32 x 24 
40 x 30 
60 x 45 
80 x 60 
120 x 96 
8mm 
3 x 2 
6 x 5 
9 x 7 
12 x 9 
15 x 11 
18 x 14 
24 x 18 
30 x 23 
45 x 34 
60 x 45 
90 x 68 
12mm 
2 x 1.5 
4 x 3 
6 x 4.5 
8 x 6 
10 x 8 
12 x 9 
16 x 12 
20 x 15 
30 x 23 
40 x 30 
60 x 45 
16mm 
1.5 x 1.3 
3 x 2.3 
4.5 x 3.5 
6 x 5 
8 x 6 
9 x 7 
12 x 9 
15 x 11 
23 x 17 
30 x 23 
45 x 34 
25mm 
96 x .72 
2 x 1.5 
3 x 2 
4 x 3 
5 x 3.5 
6 x 4 
8 x 6 
10 x 7 
14 x 11 
19 x 14 
29 x 22 
50mm 
.48 x .36 
.96 x .72 
1.4 x 1 
2 x 1.5 
2.4 x 1.8 
3 x 2 
4 x 3 
5 x 4 
7 x 5 
10 x 7 
14 x 11 
75mm 
.32 x .24 
.64 x .48 
.96 x .72 
1.3 x .96 
1.6 x 1.2 
2 x 1.4 
2.6 x 1.9 
3 x 2 
5 x 4 
6 x 5 
10 x 7 












3mm
8mm 
8 x 6
3 x 2 
16 x 12
6 x 5 
24 x 18
9 x 7 
32 x 24
12 x 9 
40 x 30
15 x 11 
48 x 36
18 x 14 
64 x 48
24 x 18 
80 x 60
30 x 23 
120 x 90
45 x 34 
160 x 120
60 x 45 
240 x 180
90 x 68 
5mm
40mm 
5 x 4
.60 x .45 
10 x 8
1.2 x .90 
14 x 11
1.8 x 1.3 
19 x 14
2 x 1.8 
24 x 18
3 x 2 
29 x 22
4 x 3 
38 x 29
5 x 4 
48 x 36
6 x 5 
72 x 54
9 x 7 
96 x 72
12 x 9 
144 x 108
18 x 14 

FOCAL
LENGTH
The focal length of the lens is measured in mm and directly
relates to the angle of view that will be achieved. Short
focal lengths provide wide angles of view and long focal
lengths become telephoto, with narrow angles of view. A
"normal" angle of view is similar to what we see with our
own eye, and has a relative focal length equal to the pick
up device. Our online lens calculator is a simple to use
device for estimating focal length, object dimension, and
angles of view.


CAMERA FORMAT
The size of the camera's imaging device (CCD) also affects
the angle of view, with the smaller devices creating
narrower angles of view when used on the same lens. The
format of the lens, however, is irrelevant to the angle of
view, it merely needs to project an image which will cover
the device, i.e.: the same format of the camera or larger.
This also means that 1/3" cameras can utilize the entire
range of lenses from 1/3" to 1", with a 1/3" 8mm lens giving
the same angle as a 2/3" 8mm lens. The latter combination
also provides increased resolution and picture quality as
only the center of the lens is being utilized, where the
optics can be ground more accurately.


F STOP
The lens usually has two measurements of F stop or aperture,
the maximum aperture (minimum F stop) when the lens is fully
open, and the minimum aperture (maximum F stop) just before
the lens completely closes. The F stop has a number of
effects upon the final image. A low minimum F stop will mean
the lens can pass more light in dark conditions, allowing
the camera to produce a better image at night. A maximum F
stop may be necessary where there is a very high level of
light or reflection, as this will prevent the camera from
"whiting out", and help maintain a constant video level. All
auto iris lenses are supplied with Neutral Density spot
filters to increase the maximum F stop. The F stop also
directly affects the depth of field.


C or
CS MOUNT
Modern cameras and lenses are generally CS mount. With CS
mount cameras, both types of lenses can be used, but the C
mount lens requires a 5mm ring to be fitted between the
camera and lens to achieve a focused image. With C mount
cameras it is not possible to use CS mount lenses.


DEPTH OF FIELD
The depth of field refers to the area within the field of
view which is in focus. A large depth of field means that a
large percentage of the field of view is in focus, from
objects close to the lens often to infinity. A shallow depth
of field has only a small section of the field of view in
focus. The depth of field is influenced by several factors.
A wide angle lens generally has a larger depth of field than
a telephoto lens, and a higher F stop setting typically has
a larger depth of field than a lower setting. With auto iris
lenses, the automatic adjustment of the aperture also means
constant variation of depth of field. The small depth of
field is most apparent at night when the lens is fully open
and the depth of field is at its minimum. Objects that were
in focus during the day may become out of focus at night.


AUTO or MANUAL IRIS
Generally we tend to use auto iris lenses externally where
there are variations in the lighting levels. Manual iris
lenses are used normally for internal applications where the
light level remains constant. However, with the introduction
of electronic iris cameras it is now possible to use manual
iris lenses in varying light conditions and the camera
should electronically compensate. There are several
considerations to this option though: the setting of the F
stop becomes critical; if the iris is opened fully to allow
the camera to work at night, the depth of field will be very
small and it may be more difficult to achieve sharp focus
even during the day. The camera can maintain normal video
levels, but it cannot affect the depth of field. If the iris
is closed to increase the depth of field, the low light
performance of the camera will be reduced.


VIDEO DRIVE or
DIRECT DRIVE
With auto iris lenses it is necessary to control the
operation of the iris to maintain perfect picture levels.
Video driven lenses contain amplifier circuitry to convert
the video signal from the camera into iris motor control.
With direct drive lenses, the camera must contain the
amplifier circuitry, and the lens now only contains the
galvanometric iris motor making it less expensive. The
deciding factor depends on the auto iris output of the
camera. Most now have both types. 

