Saturday, December 13, 2008

camera lens

Lens Formats;The early CCTV lenses were designed for the 1" format tube camera and many of these are still available on the market. The lens screw thread on these cameras is called a C-mount. In recent years' lenses have been developed for the 2/3," 1/2" and now 1/3" format cameras. For this reason great care must be exercised when selecting a lens for a particular camera. Just as there are four sizes of camera so there are four sizes of lenses and they are not compatible in every combination. A lens designed for a larger format camera may be used on a smaller format but not the reverse. In addition the field of view will not be the same on different size cameras.

There is now a further complication in that there is a new range of lenses with what is called the CS-mount. In this case a C-mount lens may be used on a CS-mount camera with an adapter ring but a CS-mount lens can not be used on a C-mount camera. The difference between the two types of mount is the back focal length (not the same as the focal length). This is an optical change to the back focal length and is not a mechanical difference to the lens. The screw thread and shoulder length for each type of mount are identical making it impossible to see the difference except that the overall size of the CS-mount lens is smaller. The main problem is that either type of lens can be screwed onto both types of camera without damage.

The result is that if the wrong type is used it will be impossible to focus the camera.
Diagram L2 shows the sensor sizes to be used when calculating fields of view and angles of view.A chart is provided at the end of this section showing the relationships between different lenses and camera combinations and the associated angle of view. At this time the majority of lenses with a focal length of 25mm and above are still designed for 1" cameras. This means that special care must be taken when using this long focal length lens on modern cameras. For instance a 25mm 1" lens provides the following approximate angles of view on the different formats.

It can be seen therefore that there would be a significant variation in the expected scene content if this fact were overlooked.The focal length of a lens determines the field of view at particular distances. This can either be calculated from the formulae or determined from tables provided by most lens suppliers. Most manufacturers also provide simple to use slide or rotary calculators that compute the lens focal length from the scene size and the object distance. The longer the focal length

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