Lens Information

For once, let's put camera lenses right at the top of the list. They are absolutely vital to the picture results that you will achieve. Their performance is key in controlling the amount of light that actually reaches the CMOS or CCD imaging chip in the camera, and the optical quality will determine just how clear (or high-resolution) an image you are able to achieve.

Furthermore, the lens will determine exactly what field of view (FOV) your camera is able to cover, and the size of the scene which you try to cover is absolutely essential to the level of detail that you will be able to see within the images..

OK, there's a bit of ground to cover and explain, so let's break it down ito bite-size pieces:.

Auto-Iris (AI) or Direct Drive (DD) Lenses

The lens iris is the adjustable aperture (hole) at the front of the lens which is adjusted to vary the amount of light passing through the lens to reach the camera's imaging chip.

If you are installing cameras to monitor any scene where the light levels are likely to vary, then you should be using AI or DD lenses. Just about the only places where light levels do not vary are windowless rooms!

Fixed-Iris lenses are cheaper, and for this reason alone you will see them used fairly often.

Obviously, if light levels vary, it is better to be able to adjust the iris to allow only a useful amount of light to fall upon the imaging chip.

With an auto-iris lens the camera and lens are connected by a cable and varying light levels are automatically compensated for; the iris opens if more light is needed, or closes if too much light is reaching the chip.

Field Of View (FOV)

The focal length of the lens determines the field of view which the camera will be able to cover.

As an indication, a lens with a low value focal length, say 3mm, will have a wide field of view, perhaps 90°.

Transversely, a lens with a high value focal length, say 50mm, will have a narrow field of view, perhaps just 5°.

For best results it is vital that you select a lens which is able to closely cover only the field of view (or scene if you prefer) that you need to cover.

Bear in mind that your camera and lens can only deliver a fixed aspect ratio - currently 4:3 format is still the most common format for the types of cameras we are discussing. This means that for every increase in scene width that you try to cover there will inevitably be a corresponding increase in scene height. Put simply, the wider that you make the view the shorter and smaller people will appear within the scene.

A few years ago it was necessary to calculate the lens focal length which was the closest fit to monitor the required scene. Nowadays vari-focal lenses have become very cost-effective and are therefore the default choice for all new applications. The vari-focal lens is adjustable between two values e.g. 3-9mm and enables you to exactly adjust (or 'clip') the view to only the area which you are interested in.

To use an analogy with hand-held digital cameras, what we are talking about here is optical zoom. If you want to achieve great results you use your camera's optical zoom function to zoom-in on the terget area before depressing the shutter and taking the picture. You could actually take capture the image of a larger scene and then use software to clip the actual area of interest, but hopefully you will realise and understand that you are wasting a proportion of the camera's resolution capability.

Transmission of Light (F-Stop)

The ability of a lens to allow light to pass through to the imaging chip is indicated by the F-stop value. The lower the value, the more light the lens allows through. F1.0 is a good value, some lenses with a high focal length value (narrow field of view) may have a rating of perhaps F1.8. As an indication of the significance of this value, each whole stop (say from F1.0 to F1.2) indicates only half as much light getting through.

A fairly recent development is the aspherical lens which is specially shaped to capture as much light as possible and achieve a low F-stop rating.

Night-time and Infra-Red (IR) lighting

Plainly, the best starting point is to use a lens with a low F-stop, one which is able to allow as much light as possible through the lens to fall upon the imaging chip.

Sometimes additional artificial lighting will be needed. You should always consider white lighting first. White lighting will allow the camera to render true-to-life colour images, is a visible deterrent by illuminating the scene and is helpful to users of the space. Yellow (SON or sodium type) lighting will cause a yellow colour cast effect within the resulting images.

Infra-red lighting is often selected for night-time usage because it is invisible to the human eye. This means that the camera is able to monitor an area and provide useful (black < white only) images, but there is no light pollution impact upon the scene.

If you must use IR lighting, make sure that the lenses selected are optically corrected for IR lighting. IR light is at a different wavelength to white light and this may result in a focus shift between day < night images. Some inferior lenses are merely coated to correct for IR lighting at a pre-determined wavelength, this is not as effective as optically corrected lenses.

Optical Quality

This is becoming ever more important with the development of megapixel cameras. Obviously, there is little point in paying for an expensive megapixel camera capable of delivering a very high resolution image and then fitting a cheap lens which will not allow the camera to resolve the required detail. Be sure to select a lens capable of matching the performance capability of the camera.

In Summary

There's more to lenses than meets the eye!

The points explained above are provided to assist you with lens selection. There are even more parameters and physical design matters which could be explained. Suffice to say that lenses are vital to system performance. These points should help you to understand why. The lenses presented in our web shop have been pre-selected to deliver best performance. You will need to choose the appropriate lens for your requirement.