CCTV Lens:

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Technical Words

LENS

FAQ (Frequently Asked Questions)

1. How come my Varifocal Auto Iris lens doesnt see picture? Varifocal Lens uses CS type camera. When using Auto Iris Varifocal lens with C/CS camera make sure in back of camera that the "DC voltage" button is downside to 24v even with a 12vdc power adapter. Some camera varies.

2. How come my Varifocal Auto Iris lens see fuzzy? Picture is fuzzy because most CS lens have a silver type ring around the lens. Simply take off the cap from lens, and take off the thin ring off lens. Then adjust lens nob in back of camera for focus. Its that simple. For fixed lens, leave as is.

Important

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 of smaller format size than the camera then the corners of the scene being viewed will be cut-off. You can usa 1/2" lens with a 1/3" camera. But you can not use a 1/3" lens with a 1/2" camera.

Lens Mounts

A CCTV lens will be specified as either C or CS Mount. Both types of lens look very similar and there is nothing that can be physically measured on a lens, e.g. thread diameter or pitch, to determine whether it is C or CS. The difference between the two types is the position of the focused image behind the lens. CS lenses focus 12.5mm behind the lens whereas C lenses focus 17.5mm behind.Most new lenses tend to be CS mount because they are similar 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 Mount Camera by using a 5mm spacing with most body cameras.

Lens Type

There are two main types of lens used in CCTV, these are fixed focal length type and zoom (varifocal) lens type.

Focal length of a lens defines its effective viewing angels both horizontally and verticall y. Hence the focal length of a lens determines the size of a particular image on the monitor screen or the area of the scene being covered by the camera.

1/3" format lenses on a 1/3" format camera, the following lens focal lengths are commonly available.

Lens Measurement - Sizes

The choice of focal length of lens fitted to a camera affects how big a particular object, etc. a person, will appear on the monitor screen or a video recording from the camera. Obviously, the bigger the image apperas on the screen, the better the chances of recognizing or identifying a person being viewed or recorded.

Guide: Smaller the focal length number = wider field of view

A. Warehouse or Office, recommendation to go 2.8 - 4.3mm lens (gives from wide to distance view)

B. Entrance or Lobby, recommendation to use 8mm lens

For best result on view angle use varifocal lenses to get flexibility to adjust to different angle/distance view.

Following Chart indicates rough estimate, viewing depends on the distance from camera to destination view.

Lens Focal Length

Horizontal Viewing

Vertical Viewing

2.5mm

85 - 95 degree

65 - 75 degree

2.8mm

75 - 85 degree

59 - 65 degree

3.6mm

59 - 69 degree

45 - 50 degree

4.0mm

56 - 60 degree

43 - 48 degree

4.3mm

53 - 58 degree

40 - 45 degree

5.0mm

40 - 50 degree

30 - 35 degree

6.0mm

15 - 25 degree

15 - 20 degree

8.0mm

10 - 20 degree

10 - 15 degree

12.0mm

6 - 11 degree

5 -10 degree

Below are more estimates for lens viewing and distance.

3.6mm Lens 1/3" Image Sensor		Distance from Camera
Lens Angle - Deg.		5 FT		10 FT		15 FT		25 FT		50 FT
W-Deg.	H-Deg.	W	H	W	H	W	H	W	H	W	H
74°	55°	7.5	5.2	15.1	10.4	22.8	15.8	37.7	26.1	75.4	52.1

6 mm Lens 1/3" Image Sensor		Distance from Camera
Lens Angle - Deg.		5 FT		10 FT		15 FT		25 FT		50 FT
W-Deg.	H-Deg.	W	H	W	H	W	H	W	H	W	H
42°	32°	3.8	2.9	7.7	5.7	11.5	8.6	19.1	14.4	38.4	28.7


8 mm Lens 1/3" Image Sensor		Distance from Camera
Lens Angle - Deg.		5 FT		10 FT		15 FT		25 FT		50 FT
W-Deg.	H-Deg.	W	H	W	H	W	H	W	H	W	H
32°	24°	2.9	2.1	5.7	4.3	8.6	6.4	14.4	10.7	28.7	21.3


12 mm Lens 1/3" Image Sensor		Distance from Camera
Lens Angle - Deg.		5 FT		10 FT		15 FT		25 FT		50 FT
W-Deg.	H-Deg.	W	H	W	H	W	H	W	H	W	H
22°	17°	1.9	1.5	3.9	3.0	5.8	4.5	9.7	7.5	19.4	14.9


16 mm Lens 1/3" Image Sensor		Distance from Camera
Lens Angle - Deg.		5 FT		10 FT		15 FT		25 FT		50 FT
W-Deg.	H-Deg.	W	H	W	H	W	H	W	H	W	H
17°	13°	1.5	1.4	3.0	2.3	4.4	3.4	7.5	5.7	14.9	11.4

Lens Lighting

Most CCTV lenses are equipped with an iris that consists of four or six opaque metal vanes which are arranged to give a roughly circular hole or apertaure through which light can pass and then fall on the camera sensor surface. The vanes can be driven together to alter the size of the lens aperture to control the amount of light falling on the sensor and hence alter the picture brightness.

Aperture Ratio

the size of the aperture is defined by an aperture ratio or f- number where:

F-number or Aperture Ratio =

Focal length of Lens

Diameter of Aperture

Example: A 6mm lens at f1.0 has an aperture of 6mm in diameter

The same lens at f1.4 has an aperture of 4.25mm in diameter.

The area has halved and so has the amount of light allowed to pass through the lens. Most lenses are marked with f-numbers; with each mark equaling one 'stop etc. halving of 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.

Note: The smaller the f-number, the larger the lens aperture and so the more light the lens can collect.

Depth of field

Depth of field is range of distances from the camera/lens for which the image obtained is in sharp focus. A large depth of field will have images of objects from as little as a 1m from the front of the camera through to images of objects at infinity all in sharp focus. The depth of field for a camera and lens is greatest at smallest aperture of the lens and vice-versa. If a lens aperture is set open etc.. at f1.4 this will let most light into the camera but the depth of field will be at its narrowest. It is important therefore, when focusing a camera and lens to ensure that the iris setting is at large as, or larger than, it is ever going to be when in normal use. If this is not done then the camera can be focused and give perfectly good pictures during the day but when night falls, the iris will open and the image to be viewed will go out of focus. Focusing of CCTV cameras should be done either at night nuder 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.

Iris Lens

Manual Iris Lenses - Recommend for area where light changes is not consistent

The simplest type of iris control is termed 'Manual Iris'. The lens is equiped with a ring on the body that can be turned to alter the aperture directly. In CCTV systems this can only be used generally in fixed lighting conditions or where the camera is readily accessible and it is not inconvenient to have to continually adjust the lens for correct picture brightness.

Auto Iris Lenses - Recommend for area where light changes consistently

A far more common arrangement is to use auto-iris lenses where the iris vanes are driven by a galvanometer or servo motor which is controlled by an 'iris-amplifier' circuit within the lens, the control input of which is the video signal from the camer itself. Hence an auto-iris lense is used where the lighting level is liable to continuous changes etc. outdoors where lighting can change from full sunlight down to near total darkness.Auto Iris Lens uses CS type video drive. To us Auto Iris Lens, u must first take off the silver type ring that is place on the C/CS camera. For fixed lens, the ring can be left there, because it uses C drive. After putting the Auto Iris lens on, on back of camera adjust DC to Down. Leave all nob option on back of camera as is.

As a scene gets brighter the video level increases. This is sensed by the lens and the iris is driven to give a samller aperture, letting less light onto the sensor and so causing the video level to be reduced. Damping is applied to prevent the lens from hunting, etc. causing the picture to continually change brightness.

The relationship between acutal video level and corresponding aperture size can be adjusted by the installer using two control potentiometers on the lens. These pots are usually marked 'Level' or 'Gain' and 'ALC' or "P-A', abrreviations for average Level Control and Peak-to-Average respectively.

Direct-Drive Lenses (FCS)

These are a development on auto-iris lenses where the motor or galvanometer coil moving the iris vanes in the lens is driven directly by the camera, 'Direct-Drive'. There is no active electronics in the lens and hence they are simpler, smaller and cheaper than an equivalent auto-iris lens. They must be used with a camera capable of driving 'DD' lenses and this is normally identified by the presence of the characteristic 4-pin square 'Hi-Rose' (Panasonic) plug on the side of the camera.

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. DD lenses slightly less flexible and optimisable for outdoor use.

Setting auto-iris lens

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

Set the P-A pot to fully average. Point the camera at a bright scene or the brightest part of the scene to be viewed and adjust the level pot until the bright part of the picture just over-exposes and then adjust the pot down slightly until the bright areas are not overexposed. If it is required to make the lens stop down when a highlight enters the picture etc. a door opening, then adjust the P-A pot towards peak. If the average control is adjusted then the level control selling should again be checked and adjusted if necessary.

Other Lens Types

Vari-focal Lens

Often it is not possible to determine the focal length of a lens required to meet a particular application or perhaps the angle of view required is not provided by one of the standard lenses available. Narrow range, manually zoomed lenses have been developed to meet these application needs. These are known as range, manually zoomed lenses have bee developed to meet these application nees. These are know as 'Van-focal' lenses. (Variable focal length) and are available in a number of different configurations. The exact angle of view can be set on installation but is is important to ensure that the angle required is within the range available from the lens.

Pin-Hole Lens

These are primarily used for covert surveillance. They have very small front 'objective' lenses which mean that they can be put behind ver small holes and be virturally undetectable. Straight and right-angled models are available to facilitate the mounting of the camera and to reduce the depth requried behind the concealing surface. Due to teh samll objective lens, pin-hole lenses cannot gather as much light as conventional lenses and so their use dictates that the scene illumination is better than would otherwise be required.

Typically the maximum apertures available on pin hole lenses is F2.5 to F3.5 which is approximately 2 to 3 stops less than standard lenses. Between 4 and 8 times the normally quoted minimum scene illumination is required for the camera to product a useable picture.

Zoom Lens

For CCTV applications these are almost exclusively motorised types to enable zooming and focusing of the lens from remote control positions. Zoom ratios etc. the ratio of maximum to minimum focal lengths are most commonly either 6:1 or 10:1. The viewing angles obtained are shown below:

1/3" Zoom Ratio

Focal Length Range

Horizontal View Angle (wide)

Horizontal View Angle

6:1

5.7 - 34.2 mm

46 degree

8.1 degree

10:1

6.0 - 60.0mm

44 degree

4.7 degree

Understand Lens Concept

BLC - (Back Light Compensation) The cameras are fitted with a backlight compensator, this is used to compensate for very bright areas in the picture. In order to counter this problem switch the BLC switch to ON and the camera will try and adjust to compensate for this problem.

AGC - (Auto Gain Control) Can be switched off by selecting off on the AGC switch. This controls an amplifier that is used to boost the video signal; this may also amplify any noise and may result in a poor picture quality.

AES - (Automatic Electronic Shutter) is used when a manual or fixed iris lens is fitted this will allow the camera to adjust to varying light levels. The AES should be switched off when an Auto Iris Lens is fitted.

Video Drive Lens - If a Video Drive Lens is fitted connect the Iris lead to the four-pin conncetor on the camera and set the auto iris selection switch to DC Drive the level may be adjusted by turning the level adjust.

Level Adjust - The level adjust is used when a direct drive lens is fitted, the level adjust is used to set the size of the aperture for normal conditions. Turn the level adjust so that the picture appears very dark then turn the level adjust the opposite way until the picture is just to bright, then turn the level adjust back untile the desired picture brightness is achieved.