Choosing the correct lens for an application is an important decision that is often misunderstood, even among industry professionals. Confusing terminology and a lack of support from retailers contribute to a misinformed decision that can result in poor camera performance and customer dissatisfaction. It is with this in mind that we will attempt to disambiguate the various aspects of lens selection and allow you to purchase your lenses with confidence rather than confusion.

CCD SENSOR SIZE

The first step to choosing a lens is to determine the CCD sensor size of the camera you will be selecting a lens for. This is important because the lens needs to focus the image onto the surface of the sensor. Choosing the wrong sized lens will cause the image to either be too large for the sensor, resulting in a cropped image, or too small for the sensor which will cause the barrel of the lens to show in the image. CCTV cameras have sensors ranging in size from 1/5” on the micro scale up to a full 1” on the large scale. Most CCD cameras have a 1/3” sensor, though many smaller ones utilize a 1/4” sensor. We always publish the sensor size in all of our camera and lens specs, so be sure that the lens you are selecting has a size matching the camera’s specification.

A CCD Image Sensor

AUTO OR MANUAL IRIS?

The iris of a lens performs the same role as its anatomical namesake.

To further narrow your lens choices, decide whether you will use a manual or an auto iris lens. With a manual iris lens, the lens iris remains a fixed diameter and the camera adjusts the shutter speed to varying light conditions. An auto iris lens work more like the human eye, with the camera maintaining a fixed shutter speed and adjusting the size of the iris according to the light conditions. An auto iris lens is usually preferred, if your camera supports it, as it tends to perform better under dynamic lighting. Also note that there are two types of auto iris control - Video and DC. You will need to be sure and purchase the correct auto iris type for your camera (although most modern CCTV cameras will support both types). A manual iris does have one small advantage - no moving parts.  This allows them to work in much colder climates that would potentially freeze the motor or iris of an auto-iris lens.

LENS FOCAL LENGTH

The most important factor in choosing a lens for your application is the lens focal length, which is the distance (usually in millimeters) from the optical center of the lens to the focal plane in the camera (the sensor). The focal length will determine both the field of view and the depth of field, and is a crucial element for properly setting up your scene.
In order to determine the correct focal length you will need to know a few parameters of the scene and use the following formula: f = (d * s) / w, where d = distance from object, s = sensor size, w = scene width and f is the focal length. The sensor size should be expressed in millimeters (3.6 for 1/4” and 4.8 for 1/3”). For example, suppose you need to view a 4 foot wide area from 10 feet away with a 1/3” CCD: f = (10 * 4.8) / 4 = 48/4 = 12mm, so a 12mm lens would do the trick.
Focal length has a direct relationship to field of view and depth of field. Longer focal lengths create a narrower field of view and a shallower depth of field. See the descriptions of field of view and depth of field for more information. For maximum flexibility in scene coverage, you may wish to choose a vari-focal lens, also called a “zoom lens”. Vari-focal lenses allow the focal length to be adjusted within a given range, offering greater flexibility for setup.

To help illustrate the effect of focal length, the following images were taken with the camera at a fixed position 50 feet from the subject, and only the lens focal length was adjusted.
2.8mm	12mm	50mm	100mm

ANGULAR FIELD OF VIEW

Angular field of view (also called angle of view) describes the angular extent at which light can pass into a lens to create an image on the sensor. It is a function of the focal length of the lens and the dimensions of the sensor, and can be calculated with the following equation: a = 2arctan(d/2f), where d = sensor size in mm (see focal length), f = focal length and a is the angle of view. For example, to calculate the angle of view for 3.6mm lens on a 1/3” camera: a = 2arctan(4.8/2*3.6) = 2arctan(0.667) = 67.407 deg. Angle of view is sometimes described using the generalized terms “wide angle” (between 60°-100°) and “telephoto” (between 10°-15°).

Angle of View of a Particular Lens

DIMENSIONAL FIELD OF VIEW

Field of View of a Particular Lens

When most photographers use the term field of view, they are usually referring to the angular field of view described above. In the CCTV industry it is often more pragmatic to discuss the dimensional field of view, as it is better suited to aid you in choosing the correct lens for a given application. The dimensional field of view provides the spacial dimensions of a scene at a given distance from the camera. We can calculate this by first discovering the horizontal measurement with the following formula: w = d/f*s, where d = distance from scene, f = focal length, and s = sensor size (see focal length). For example, if we want to calculate the area of coverage for a 1/3” CCD with a 12mm lens positioned 10 feet from the scene: w = 10/12*4.8 = 4. That gives us a horizontal dimension of 4 feet. Using a standard aspect ratio of 4:3, we can calculate the vertical dimension to be 0.75 * 4 = 3. The dimensions of our scene will be 4 feet by 3 feet. Of course it’s simple to see how you can find the correct focal length needed based on scene size and distance with the given formula. For example, if you want to cover a 4’x3’ area from 10’ away: f = d/h*s = 10/4*4.8 = 12; so you would need a 12mm lens.

DEPTH OF FIELD

Depth of field is the distance in front of and behind an object that appears in focus. In essence, depth of field is a measurement of the spacial range of focus for a given lens configuration. A wider depth of field will have more of a scene in focus than a narrow depth of field. Although depth of field is affected by parameters such as the focal length, it is most drastically affected by the f-number (described below).

In this image only the area between the red bands is in focus, illustrating the depth of field.

F-NUMBER

Increasing the f-number decreases the size of the aperture. Actual aperture diameter will depend on the lens focal length.

The f-number of a lens (usually notated as f/#) is the ratio of the focal length to the diameter of the entrance pupil of a lens, and is used to describe the effective aperture diameter, also called the lens speed. The f-number is given as f/# = f/D, where f is the focal length and D is the diameter of the entrance pupil. As the f-number increases, the aperture diameter decreases, allowing less light to reach the sensor and increasing the depth of field. Most CCTV lenses do not have adjustable f-numbers, and this attribute is more useful in photography than in video surveillance.

CONCLUSION

This brief discourse has attempted to elucidate some of the ambiguities surrounding CCTV lens classification and selection. The importance of choosing the appropriate lens for a given application is often overlooked, resulting in sub-optimal equipment performance. It is my hope that with your newly obtained knowledge, you are better equipped to make an informed purchase, and maximize the value of your CCTV equipment.