Lens Technology for the Security Cameras

2. Imager Compatibility

 

The sensor size for a particular camera is referred as 1/2” or 1/3” or 1/4” CCD/CMOS.

The lens should be compatible to the size of the imager. For example, if camera images is 1/2” and lens is compatible up to 1/3”, then the image at the corners will be cropped or black in nature.

3. Focal Length
The Focal Length of the lens determines the angle of view covered by the camera. As the focal length is increased, the angle of view decreases as we are able to concentrate on a specific object at a distance from the camera. Below images show how focal length impacts the image formation:

So the user must be clear of his requirements. For example, if a user wants to do a general surveillance, then a lens with focal length of 2.8-8 mm may be sufficient and if the purpose is to recognize the face/number plate of the car, then 8-50 mm lens will suffice the requirement.

4. F Number

The F Number is a measure of the light-gathering power of the lens. A lens with a smaller F Number has higher lightgathering power. F Number is also called the lens speed (fast lens/slow lens).

The F Number is calculated by dividing focal length by the lens effective diameter.

Common F Numbers

With the above image, it can be seen that lower the F Number, brighter the image will be in low light conditions. But the constraint with lower F Number is that depth of field will be lower. This can be analyzed with the below images:

So there is always a trade of on depth of field and brightness required in the image. For example, we find normally 2.8-8 mm lens with F/1.2 at wide end as the purpose is to view a small enclosed area and 8-50 mm lens with F/1.6 at wide end to view long distances in city surveillance environments.

5. Iris

The popular types of iris are as follows:

• Manual Iris – low cost, requires constant lighting or shutter speed control

• DC Iris – camera converts light level into an analog drive signal that controls the lens iris using a galvanometer

• P-Iris – a stepping motor is used to control the iris position, allowing precise control of iris over the amount of light.

P-iris is the new technology and camera OEMs are now opting for this technology because they are able to get better depth of field with these lenses.

6. Resolution

Resolution is the ability of a lens to resolve details. It varies in different locations in the image (highest at center). Three common units of measurement are line pairs/mm, TV lines, Megapixels.

It has been seen that people normally buy megapixel (high resolution) cameras but they are ignorant that mega pixel lenses should be used with such cameras. The above two examples can help you to understand the difference in images by using a SD and HD lenses.

7. IR Correction

Different colors are represented by different wavelengths. The visible spectrum is between 400 and 750 nm. Infrared (IR) rays are beyond 750 nm, and are invisible to the human eye. CCD and CMOS imagers are sensitive to IR rays. Each wavelength has different chromatic aberration (dispersion characteristic).

Refraction of light through a lens depends on wavelength and IR has a longer wavelength than light, and hence focuses on a different plane (focus shift). A Day & Night camera needs an IR-corrected lens to capture clear night time images. A Tamron 3MP 8-50mm IR corrected lens delivers 3MP resolution in day mode and night mode.