By: Jo Chew
Date: 7/4/2005

C-Mount? CS-Mount? Auto Iris? Manual Iris? DC Drive? Video Drive? With all these confusing terms, it can be difficult to know what your looking for in a lens. Here we will cover different aspects of a lens so that you gain a better understanding, and in turn know what you are looking for!

Focal Length
Camera lenses have what is called a focal length. With CCTV camera lenses, the focal lengths usually start at around 2 to 4mm, and up to 50mm and so on. Lenses do come in discrete values, so you cannot order a particular size lens, ie: 6.25mm. Common focal lengths are: 2.8mm, 3.5mm, 4mm, 8mm, 12mm, 16mm, 25mm, 50mm, 75mm.

Lenses shorter in focal length (eg 4mm) give a wider angle of view and are called wide angle lenses. Lenses with longer focal length (eg 16mm), narrow the view and therefore they look as if they are bringing distant objects closer.

Angle of Vision
To put focal length in perspective, the focal length in the human eye is approximately 17°. This focal length gives an undistorted solid image angle of approximately 30°. This is why an angle of about 30° is considered a standard angle of vision.

Fixed Focal Length Lenses
Fixed focal length lenses, as the name suggests, provide a fixed focal length, ie give one angle of view. These lenses are usually designed to provide optimal resolution as there are not many moving optical parts.

Zoom Lenses (aka: Variable Focal Length/Varifocal Lenses)
Zoom lenses offer the flexibility to change the focal length of the lens. Therefore the angle of view can be adjusted. These lenses usually come in a range, ie 3.5-8mm, 8-48mm and so on. You are probably more accustomed to zoom being referred to as 2x, 6x, etc when referring to digital cameras and camcorders. In this instance a 8-48mm lens could be referred to as a 6x.

Manual Iris Lenses
The iris in a lens is what controls how much light is allowed through. A manual iris lens allows you to adjust the iris manually. This is usually done when the camera is installed. Because the iris will be adjusted initially when set up, if there are drastic changes in light levels, (eg sun shine), the picture could then be washed out because the iris is letting in too much light. These lenses are better suited to places where lighting levels remain constant, eg indoors.

Auto Iris Lenses
These lenses have an electronic circuit inside, which controls the iris by electronic feedback. The lens usually has a 4-pin connector which connects to the camera, where it gets its power supply(9vDC) and its video signal. The lenses electronics then analyses the video signal level and acts accordingly. If there are changes to light conditions, the iris then adjusts accordingly to provide the best picture possible. For example, if the sun comes out on a very clear day, the iris will close to be very small, and in the evening as it gets darker, the iris will open up to allow more light in. These lenses are ideal for all applications, as they provide the most flexibility.

There are two types of auto iris lenses:

Video Drive
With a video driven lens, the video signal is picked up from the AI connector, which is connected to the camera. Because the lens adjust the iris based on the video signal, the lens is referred to as a video drive lens.

DC Drive
DC driven lenses do not have the electronics for video processing, but rather only the motor that opens and closes the iris. The whole processing process in a DC driven lens is done by the cameras AI electronic section. The output from the camera is a DC voltage which opens and closes the iris.

In summary, the difference is that video drive lenses have the electronics onboard to analyze the video signal, whereas a DC drive lens opens and closes the iris based on the DC voltage being provided by the camera. A DC drive lens relies on the camera to analyze the video signal.

Video drive lenses cannot be used with cameras that provide DC AI output, and DC drive lenses cannot be used with cameras that provide video AI output. They are not cross compatible!

Difference between C and CS Mount
When referring to camera lenses and cameras, they usually refer to way in which the lens is mounted to the camera.

At the moment, there are two standards for the distance between the flange of a lens and the CCD imaging sensor. These two standards are:

C-Mount
This is represented with 17.526mm.

CS-Mount
This is represented with 12.5mm.

So basically, there is an approximate 5mm difference. Both C and CS mount use the same thread pattern of 1.00/32mm. CS Mount lenses are approx 5mm closer to the image plane, so to be used with a C-mount camera, a 5mm ring can be used to make the gap larger.