What are the effects of different lens types on target size?
 

As has previously been mentioned, in order to get the best out of a security camera, it needs to be fitted with the correct lens, and placed in the most appropriate location.

 

But how do you decide which lens is going to be the most suitable for a given task?

Although lenses are possibly the most widely misunderstood aspect of CCTV work, in many respects they are perhaps one of the most important.

 

For the purposes of comparing the effects of different lens types, the following information will refer to "fixed" focal length lenses, which produce a set "Field of View" which cannot be changed.

 

Once the principles of these are grasped, the use of adjustable lenses such as "Varifocal" or "Zoom" ( which incidentally do the same thing but in a slightly different way ) can then be better understood - important to quickly mention here that 'optical zoom' is very different from 'electronic zoom' that does not in itself contribute towards achieving a "Golden Zone" (electronic zoom will be explained in more detail elsewhere ).

 

The diagram below is intended to demonstrate the change in size of a notional Golden Zone for identifying targets, using a variety of lenses .... from wide angle to telephoto.

 

Purely for comparative purposes, the lenses are based on use with a camera that has a 1/3" format imager.

 

Whilst a Golden Zone represents an optimal area for recognising targets, for the purposes of this exercise it is assumed that the camera is SD ( Standard Definition ), with HD ( High Definition ) capable of providing a much sharper image by comparison.

 

It should not be assumed that by using an HD quality camera, it can then automatically be deployed to cover a wider area, as this naive assumption may well have a number of negative operational consequences.

In most situations, the "Golden Zone" for target recognition should be produced when a camera is tilted around 5 to 20 degrees below the horizontal, which in a practical sense would suggest that lenses with a focal length of between 8mm and 25mm would provide the most useful options for the majority of everyday situations requiring fixed cameras ( 12mm & 16mm perhaps being the most agreeable in 70% or more of the most common applications ).

 

The diagram above shows that using a 'standard' focal length lens will require the camera to be further tilted into a less than agreeable position, whilst the 'wide angle' lens ( 3.6mm which is the default optic fitted to many low cost mini dome and bullet type cameras ) is really only suitable for imaging areas quite close to the camera, but at the expense of a much more pronounced tilt angle, and not many images available on recording ( see details below ).

 

One quick point worthy of mention, is that interchangeable 'C' or 'CS' mount lenses will normally be fitted with an adjustable iris, that permits some control over matching the 'Depth of Field' to the Golden Zone. Given that iris adjustment is very much dependent on available lighting in the target area, this further re-inforces the notion that mid range lenses ( 8mm - 16mm usually provide the best overall balance of target size and image quality ).

 

Where a wide angle lens is fitted, most 'barrel' type lenses do not have an iris, so although the depth of field is often generous and adequate to cover the zone, in practice the optical performance is generally not very good, particularly in terms of edge resolution of the image.

 

The following guide information on the maximum available number of images captured, for targets moving through the Golden Zone, is based on a standard record setting of 25 images per second, and an average walking speed of 1.35 metres per sec:-

 

50mm Telephoto lens - 12 metre Golden Zone - 175 images over 9 seconds

25mm Telephoto lens - 6 metre Golden Zone - 112 images over 4.5 seconds

16mm Telephoto lens - 4 metre Golden Zone - 75 images over 3 seconds

8mm Standard lens - 2 metre Golden Zone - 38 images over 1.5 seconds

3.6mm wide angle lens - 0.9 metre Golden Zone - 16 images over 0.65 seconds

 

In practice, given the way that modern digital record systems compress and encode individual 'frames', the likelyhood is that far fewer usable images will be captured, then is suggested by the raw figures above.

 

Please Note: The diagrams showing lens angles and coverages are intended for general illustrative purposes only, and as such are not intended to provide a guarantee of accuracy or suitability for every individual situation.

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Doktor Jon's Guidance on .. Determining "Golden Zones" using different lenses - original content August 2008 - updated 12th April 2015

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