This FREE Camera Lens Calculator tool is intended to help users choose the correct camera/lens, and help users understand what the capabilities are of the cameras installed or proposed – with respect to ability to DETECT, OBSERVE, RECOGNISE or IDENTIFY.

The first and most important question to be addressed with any CCTV system is “What do I need to see?”. The Home Office has published guidelines to assist those designing and using CCTV systems to ensure that the camera system is fit for purpose. (CCTV Operational Requirements Manual 2009 Ref: 28/09).

Most camera systems are designed to observe human activity. The application, however, can range from crowd control / public safety (where the movement of large numbers of people needs to be monitored over a wide area) to access control (where close-up, high quality imagery is required to enable individuals to be identified). The choice of CCTV camera in particular will depend on the nature of the activity to be observed.

To simplify the situation and provide guidance to a system specifier, four general observation categories have been defined by the Home Office, which are based on the relative size that a person appears on screen. As part of the Operational Requirement development, the user should decide which of these four categories best reflects the type of activity being observed. The CCTV designer will then be able to choose a suitable camera to meet the requirement.

Monitor and Control:

A figure occupies at least 5% of the screen height and the scene portrayed is not unduly cluttered. From this level of detail an observer should be able to monitor the number, direction and speed of movement of people across a wide area, providing their presence is known to him; i.e. they do not have to be searched for.


The figure now occupies at least 10% of the available screen height. After an alert an observer would be able to search the display screens and ascertain with a high degree of certainty whether or not a person is present.
Observe: A figure should occupy between 25% and 30% of the screen height. At this scale, some characteristic details of the individual, such as distinctive clothing, can be seen, whilst the view remains sufficiently wide to allow some activity surrounding an incident to be monitored.


When the figure occupies at least 50% of screen height viewers can say with a high degree of certainty whether or not an individual shown is the same as someone they have seen before.


With the figure now occupying at least 100% of the screen height, picture quality and detail should be sufficient to enable the identity of an individual to be established beyond reasonable doubt.


The Monitor or Detect categories may be suitable for covering a wide area such as a car park. The Observe category is useful in areas where it is necessary to monitor a group of individuals such as outside nightclubs and pubs, or in town centres, as it provides reasonable detail of the subject whilst simultaneously providing some context to their activity by monitoring the area around them. The Recognise or Identify categories would be used for the cameras providing close-up images at the entry and exit points. In scenarios where the purpose of the camera is primarily access control and identity verification, a figure much greater than the 100% Identify setting may be required.

The purpose of these categories is to suggest appropriate image sizes to aim towards when specifying a system to meet a particular requirement, rather than to define a minimum standard. It does not follow that it will be impossible to recognise or identify an individual if the image size is smaller than the 50% or 100% figures suggested. Equally, there is no guarantee that individuals will be identifiable just because they occupy >100% of the screen.
Other factors, such as lighting and angle of view will also have an influence.

ClearView manufacture a CCTV Test Chart in accordance with Home Office (HOSDB) guidelines to help users test the capability of their CCTV systems. This is intended as an alternative to the Rotakin system.

Contact us today for more information.