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Infrared light support

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Focus and light level

Knowing what will appear to be in focus and what will be out of focus is one of the most basic considerations when designing any CCTV system. Although exact focus occurs only at the precise focusing distance, depending on chip size, lens focal length, aperture size and focus distance, the apparent range of focus, or depth of field, can vary considerably. Additionally the size of the monitor will have an effect on this apparent depth of field

This apparent depth of field is due to an optical phenomena called circle of least confusion. When an object is at the exact distance the lens is focused, every point on the object will focus to a point on the chip plane. When an object moves out of focus, on the chip these points begin to grow and become circles. The farther out of focus an object is, the larger these circles become. But up to a certain point (again depending on chip electronic sensor size, lens focal length, etc.), these circles of confusion are unobtrusive (unresolved by the human eye) and the image appears to be in focus over a range of distances and has 'depth of field'.

The same factors that affect depth of field in film cameras apply to CCTV cameras too: aperture f-stop, focus distance, lens focal length, the size of the light recording areas whether a piece of film or an electronic sensor, and monitor size.

Determining the Diameter of the Circle of Least Confusion parameter used in the calculations is a laborious and ongoing task for each and every system

Because CCTV lens have automatic apetures the apetures changes as the light level changes and this results in the depth of field changing as well.  Night time the depth of field in much smaller than during the day and sometimes a series of calculations is required to determine a cameras performance.  Before the calculations for depth of field the aperture of the lens has to be determined This involves a further set of calculations which requires a knowledge of the chosen camera sensitivity in lux the scene light level and the scene reflectance.

Blakeglow carry out these calculations on critical system designs providing the client with a set of drawings detailing the expected results.

FIELD OF VIEW EXAMPLES