Choosing the Right Lens

Choosing the Right Lens

Two Main Mounting Standards

There are two main mounting standards. Both have a 1" thread and look quite similar. If you are unable to focus, it might be because the wrong type of lens was chosen.

• CS-mount: The distance between the sensor and the lens should be 12.5mm.
• C-mount: The distance between the sensor and the lens should be 17.5mm.

A 5mm adapter ring (C/CS adapter ring) can be used to convert a C-mount lens to a CS-mount lens.

Sensor Size

The lens must create an image large enough for the sensor. The larger the sensor, the more expensive the lens. A 1/2" lens is suitable for 1/2", 1/3", and 1/4" sensors, but not for 2/3" sensors. If a lens made for a smaller sensor is used on a larger sensor, the image will have dark corners.

The figure above is an illustration of a lens. A lens usually consists of 4-10 lens elements.

Focal Length

The focal length, combined with the sensor size, determines the field of view. A shorter focal length results in a wider field of view. A longer focal length provides a narrow telephoto view.

Wide-angle lenses have a better depth of field compared to telephoto lenses. This means you can focus both near the camera and at a distance. Focusing adjustments are more critical when using telephoto lenses.

Three Main Types of Lenses:

• Fixed Focal Length: The focal length is fixed, such as 4mm or 8mm.
• Zoom Lens: The focal length can be adjusted within a range, such as 4-10mm. The focus remains constant as the focal length changes.
• Varifocal Lens: A cost-effective zoom lens. The lens must be refocused when the focal length is changed. Common types are 3.5-8mm.

The figure above shows the formula for calculating the focal length.

Example 1

If you want to view an object 5 meters wide from 10 meters away, what focal length do you need? The camera uses a 1/3" CCD sensor.

F = 4.8 x 7 / 5 = 6.7 mm (Choose a 6mm lens or a 3.5-8mm varifocal lens)

Example 2

If you use a camera with a 1/4" CCD sensor and a 4mm lens, how wide of an object can you see from 100 feet away?

H = D x h / f = 100 x 3.6 / 4 = 90 feet

Adjustable Iris

An adjustable iris is a mechanical device used to regulate the amount of light passing through the lens. There are three types of adjustable irises found in lenses:

• Manual Iris: Rotate a ring to adjust the iris, or fix it at a setting such as F2.0.
• DC Auto Iris: The camera output controls the iris.
• Video Auto Iris: An analog video signal controls the iris. This type is rarely used today.

For outdoor applications, an auto iris lens is generally used. The iris automatically adjusts the amount of light reaching the camera, optimizing its performance. The iris also protects the image sensor from damage due to intense light.

A smaller iris opening reduces the amount of light, leading to better depth of field when focusing over longer distances. Conversely, a larger iris opening provides better images in low light conditions. The iris is defined by the F-number.

F-number = Focal length / Iris opening

This is an example of the depth of field of a telephoto lens. A large F-number allows for focusing over longer distances.

For a typical 3.5-8mm lens, you can focus between 10 ft/3 m and 50 ft/15 m.

For auto iris, it is generally best to set the focus in low light conditions. If you adjust it in bright sunlight, focusing is easy; however, at night when the iris opens wider, the image might no longer be in focus. Special black focusing filters can be used to reduce light by about 10 times, which can be very helpful when installing cameras.

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