The aspect ratio of a standard CCTV picture is 4 units wide by 3
units high, typically complicated by being based on the diagonal
measurement of the tube or sensor. A 12" monitor would have dimensions
of about 220mm wide by 165mm high. Camera lenses have the vertical and
horizontal angles of view in the same proportions.
It is standard practice to set up cameras and monitors to view
in this normal plane, but is this always the best way to look at a
scene?
Many systems protecting a perimeter are looking along a long
narrow field of view as illustrated in the diagram 1. This shows the
view on a monitor in its normal orientation and with the camera mounted
conventionally. The field of view will be determined by the vertical
angle of the lens. It can be seen that there is a great deal of the
screen showing areas not necessarily important in relation to the scene
being monitored.
If the camera is turned through 90° and also the monitor, as in
diagram 2, the part of the scene being monitored is now represented in
greater detail. This is because the orientation of the required scene
is in a better relationship with that of the monitor.
(Believe it or not, the two screens shown are the same size and the corect ratio)
It is not just a question of rotating the camera and monitor,
because the field of view will now be determined by the horizontal
angle instead of the vertical angle of the lens. As previously stated
this is in the ratio of 4:3, therefore a lens with a longer focal
length will be required. This is quite straightforward. Having
calculated the required lens using the vertical angle, simply find a
lens that has the same horizontal angle of view. For instance a 12.5mm
lens has horizontal and vertical angles of view of28.4° and 21.3°
respectively. A 16mm lens has horizontal and vertical angles of 22.3°
and 16.8° respectively. Therefore the horizontal angle of the 16mm lens
is nearly equal to the vertical angle of the 12.5mm lens.
The longer focal length lens, though, produces a larger image
on the screen for the same scene content. This is illustrated in
diagram 3. The angle of view is the same in both diagrams.
Diagram 3
It can be seen therefore that more of the important part of the
scene is displayed when the camera and monitor are rotated through 90°.
This is obviously a somewhat controversial point of view and in
reality must take into consideration factors of other cameras to be
viewed which may require the conventional arrangement. However there
may be occasions when some lateral thinking may pay dividends. |
