Lense Guide

Camera formats
Cameras are often quoted as 1/2", 1/3" or or 1/4". This refers to the format of the camera(chip size). In itself, this gives no measure of a camera's performance although generally the smaller the format size, the smaller the light gathering area of the sensor. It is provided as information to enable the installer to match the camera to the correct lens.

The format size of a lens must be equal to or greater than the format size of the camera it is being used on. If the lens is of smaller format size than the camera, then the corners of the scene being viewed will be cut off. You can use a 1/2" lens with a 1/3" camera. But you cannot use a 1/3" lens with a 1/2" camera.

C or CS Mounts
There are two standard mountings used for connecting lenses to cameras. Both types of lenses look very similar and there is nothing that can be physically measured on a lens to determine whether it is C mount or CS mount. The only difference is the distance between the lens and the CCD image sensor. On a C mount the distance is 17.5mm while it is l2.5 mm on a CS mount.

Most new lenses tend to be CS mount because they are smaller and therefore cheaper to manufacture. Ideally, the camera mount and the lens mount should be the same but it is possible to use a C mount lens on a CS Camera by using a 5mm adapter ring. Check your camera manual for details.

Focal Length
The focal length of the lens is measured in millimeters (mm) and directly relates to the angle of view that will be achieved. It determines the size of a particular image on the monitor screen and the area of the scene being covered by the camera.

Short focal lengths (e.g. 2.5 mm) provide wide fields of view which is good for close-ups or for seeing a large area. As focal length increases, the field of view narrows and distant objects become more defined. A long focal length (e.g.16mm) provides a narrow angle of view that is ideal for observing a limited area such as entrances and hallways.

Auto or Manual Iris
The simplest type of iris control is the "manual iris". The lens is equipped with a ring on the body that can be turned to alter the aperture directly. Manual irises are used in fixed lighting conditions or where it is convenient to continually adjust the lens for correct picture brightness. For example, they would be ideal for an illuminated store or office.

Auto-iris lenses have electronic circuits fitted to the lens which enables the iris to control the amount of light falling upon the image device. An auto-iris lens is used in changing light conditions (e.g. cameras positioned outdoors).

The iris controls of a lens should be adjusted when the lighting levels are at or near the highest that the camera will be subjected to. Never adjust an iris setting when light levels are low. If you do, it will almost certainly be necessary to re-adjust them in daylight.

Video Drive or Direct Drive
There are two main types of auto irises: Video and DC irises. Video Drive Lenses contain circuitry which converts video signal from the camera into iris control. A DC iris lens, often referred to as Direct Drive, does not have this circuitry and simply requires a DC input from the camera.

There are no active electronics in the Direct Drive lens and they are simpler, smaller and cheaper. Direct Drive lenses have no adjustment controls other than focus (and angle of view in the case of vari-focal lenses). Picture brightness is now controlled by a level setting on the camera.

F Stop
F-stop is the ability of the lens to gather light depending on the aperture and focal length. An f-stop relates to the f-number, which is determined by the following formula.

F-number= Focal Length of Lens / Diameter of Aperture

Most lenses are marked with f-numbers with each mark equaling one "stop" that halves the area of the aperture through which light can pass. The standard f-number series is f1.0, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22 and each successive mark represents a halving of the aperture area from the one previous.

The smaller the f-stop, the more light the lens can collect. The lens usually has two measurements of f-stops or aperture with the maximum aperture (minimum F stop) being when the lens is fully open, and the minimum aperture (maximum F stop) just before the lens completely closes.

Depth of Field
The depth of field refers to the area within the field of view that is in focus.

A large depth of field means that a large percentage of the field of view is in focus, while a shallow depth of field has only a small section of the field of view in focus.

Depth of field increases when the focal length is short, the F number is large, or the object distance is longer. The depth of field is greatest at smallest aperture of the lens and vice versa.

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. It is important when focusing a camera and lens to ensure that the iris setting is as large as, or larger than, it is ever going to be when in normal use.

Focusing of CCTV cameras should be done either at night under worst case lighting conditions or the lens should be fooled into thinking it is night by placing an optical filter (ND2 or 3) over it to open the aperture fully and then focus the camera on the scene to be viewed.