Welcome to the
All you need to know about lenses,
So far, weve managed to avoid getting too bogged down with unnecessary technicalities when it comes to interpreting lens performance. However, weve now reached that junction where the road splits off, towards the little town of Blissful Ignorance, or the loud in yer face city known as Detailed Knowledge.
Absolutely no prizes for guessing where were off to next!
When it comes to being totally comfortable with interpreting all the various bits of information available about CCTV lenses, it is actually essential to understand just the basic mechanics of how a lens works.
You will recall that a lens is actually made up from various different pieces of optical glass (elements) which are sometimes combined togethor to form a group. Now somewhere inside the elements at the back end of the lens, there is an optical point known as the Principle Plane (it's also sometimes called the Secondary Principle Point).
Quite literally, the distance from this abstract point to the location where the rays of light come into sharp focus, is called the Focal Length of the lens.
Bearing in mind that the focal length provides a clear indication of what the lens has originally been designed to achieve, (in terms of width of coverage on a particular format), the lens mount itself is actually arranged so that the sharp image can be absolutely aligned with the cameras imager.
For example, a 1/3 format 8mm lens is designed with a CS mount, which will provide a sharp image on the CCD, provided it is exactly 12.5mm from the front edge of the cameras lens mount.
Now the 2/3 format 8mm lens, will not only cost eight times as much to buy, but because it is a C mount optic, will actually have to be spaced exactly 17.526mm from the front of the cameras lens mount, simply to achieve a sharply focussed image.
Whilst you can use either lens on a CS mount 1/3 CCD camera (the C mount lens requiring an additional 5mm spacer ring to be screwed onto the back); both lenses have the same focal length, and will therefore produce exactly the same coverage (both standard on this format).
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