Choosing the Wrong Lens

From CCTV Information
Jump to: navigation, search

Most Internet Sales Companies will not give you the following information as you would quickly realise that many of the Cameras they are trying to sell would not produce pictures that are fit for purpose.

The biggest problem with CCTV installations is that the wrong lens is chosen. This normally results in people being too small to recognise and with CCTV pictures only being around 40k each, and not the 10 million you get on your photo camera, you cannot electronically zoom in. To be able to recognise a 1.7m person on a standard* CCTV system they must be at least 50% height of screen (the standards of Detect and Observe are not good enough to recognise person simply to observe what a group is doing or detect a person is present in the picture). These standards are set out in the Home Office Operational Requirements Manual for CCTV – page 8 and used as a reference by the police and the courts.

Detect-observe-recognise-identify.png

To observe a person at 50% you can only view an area the width of two car park spaces (4.3m). The most popular lens sold to customers is a 1/3” 3.6mm lens. This gives a wide angle view of 70% but can only provide facial recognition of 3.2 meters so if it is located at a height of 5m you are depending on an intruder bringing a ladder and climbing up in front of the Camera in order to be recognised. Generally a 3.6mm lens will only enable you to either observe or detect a person.

The table below gives the angle of view and recognition distance for some popular lens formats.

Angle-of-view-recognition-distance.png

Work out the distance from the Camera and then choose a lens that will provide Facial Recognition. The table will also show the angle of view you will achieve.

Where possible use a Varifocal lens (i.e. 5-50mm) as this makes it easier to adjust to the view at the time of installation.

To find out exactly what lenses you will need use our free Online lens calculator.

For more information see Lenses (article)

(*) the requirement for a person to be 50% height of screen is based on the standard PAL screen resolution. To understand the requirements for other resolution formats see the following section.

Back to 5 Fatal Mistakes When Buying CCTV

Click here to find out about a free service that will ensure your CCTV design is correct.