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All you need to know about lenses,
If theres one thing that irritates Doktor Jon more than most things to do with CCTV, its the suggestion that if you zoom in to a distant target and the picture goes out of focus, then the zoom lens hasnt been set up properly on the camera.
Time and again, Doktor Jon has been told by clients that the installer didnt set the cameras up correctly, or an article in a security magazine will point the finger of suspicion at some overworked and underpaid commissioning engineer; lets get real here, more often than not it just simply isnt true.
If you recall from our stroll through the meadow that is Depth of Field, the zone of acceptable focus is always smallest with telephoto lenses, and extremely long with ultra wide angle lenses.
So if you focus a zoom lens at the wide angle setting (particularly if it is an auto iris lens with the aperture half closed), which could of course be at a point anywhere inside the zone of acceptable focus, as soon as you zoom in to a target, the depth of field decreases dramatically, and the iris may well open slightly which in turn reduces the depth of field still further; wouldn't yer know it, the pictures gone out of focus.
The point here is that if you focus on a distant point using the Telephoto setting, as the lens is adjusted to a Wide Angle framing, the focus will be maintained. Zoom in yet again, and it still remains in focus.
So the moral of this, is to always focus at maximum magnification, and if the focus then shifts during zooming, you then know for sure that the 'back focus' hasn't been set up correctly.
Because of limitations in design, with some older lenses, particularly very powerful zooms used on 1 format cameras, there was a tendency that as you changed the focal length, the actual back focal point would change ever so slightly. This of course meant that no matter how well the camera /lens combination was set up, you would nearly always have to readjust the focus setting every time the zoom was moved.
The most important thing for any zoom lens, is that it should be capable of focussing at any zoom setting, from a couple of metres away (subject of course, to the lens minimum focus distance - youll need to check the individual lens specification sheet!) to infinity .... and beyond!!
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