Glossary of terms/
Optical assembly containing several movable elements to permit
changing the effective focal length (EFL). Unlike a zoom lens
a varifocal lens requires refocusing with each change.
FIXED FOCUS: Lenses
that are not provided with a means of focusing operation regardless
of the distance to the subject.
FOCAL LENGTH: The
basic parameter to determine the image position, magnification,
and angle of view of a lens. 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
length 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 diagonal dimension of the
pick up device.
ZOOM LENS: A lens that delivers
different focal lengths without creating a shift of focus regardless
of the focal length setting.
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.
ZOOM RATIO: The
ratio of the starting focal length (wide end) to the ending focal
length (tele end) of a zoom lens. A lens with a 10X zoom ratio
will magnify the image at the tele-end by 10 times.
A lens with an electrically controlled iris. The circuit controlling
the iris is set to maintain a constant video level in varying
lighting conditions. Depending on the placement of the driving
circuitry (i.e. on the lens side or incorporated on the camera
side), there are two types of Auto-Iris; with a driving circuitry
built in and DC meter (galvanometer) only. Make sure to identify,
before ordering an AI lens, whether the camera outputs video signal
or DC current to actuate the auto-iris. (ref. DC-Type Lens)
FLANGE BACK (Flange Back Focal
Distance): The distance from the mechanical flange of the lens
(rear edge surface of the lens mount) to the focal plane. C-mount
lenses have a flange back distance of 17.526mm while CS-mount
lenses have 12.5mm. Because of this, C-mount lenses can be used
on CS-mount cameras with an adapter ring of 5mm thickness (however,
CS-mount lenses cannot be used on C-mount cameras).
BACK FOCUS (Back Focal Distance):
The distance from the rear-most portion of the lens element to
the image plane. It is important to adjust the back focus correctly
in order to obtain the best image. Certain lenses come with a
back focus adjustment mechanism, while others do not. Also, most
of the cameras incorporate back focus adjustment, if it is not
available on the lens side.
MANUAL IRIS LENS: A lens with a manual adjustment to set
the iris opening (F-stop) in a fixed position. Generally used
for fixed lighting applications.
MINIMUM OBJECT DISTANCE (M.O.D.): The closest distance
a given lense will be able to focus on an object. This is measured
from the vertix (front) of the lens to the object. Wide angle
lenses generally have a smaller M.O.D. than large focal length
PINHOLE LENS: Lens used for applications where the camera/lens
must be hidden. Front of lens has a small opening to allow the
lens to view an entire room through a small hole in the wall.
PRE-POSITION LENSES: Zoom lenses which utilize a variable-resistor
(potentiometer) to indicate zoom/focus position to the lens controller.
After initial set-up, this allows the operator to view different
pre-set areas quickly without having to readjust the zoom and
focus each time.
A guide to selecting your LENSES
or Manual Iris
Auto Iris Lenses are generally external applications where the
lighting levels vary.Manual iris lenses are internal, where the
light levels are constant.
With the introduction of electronic iris cameras it is now possible
to use manual iris lenses in varying light conditions and the
camera will electronically compensate, however there are several
considerations to this option. 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 in low light, performance of the camera will now be reduced.
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(VM400) 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 as it not physically possible
to get the lens close enough to the CCD to achieve a focused image.