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.
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.
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.
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.