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Fixed focus lenses are the simplest type of Lens, and therefore the
cheapest. The presets focal length means a precise calculation is
required to select the Lens most suitable for the location, based on
the desired size of viewing area and its distance from the Camera.
Typical Lens sizes offer either 30 degree view - narrow to allow more
detail at distance - or 60 degree, which offers a much wider angle of
view.
Varifocal lenses offer more flexibility, allowing the field of view
to be adjusted manually. Although more expensive these lenses of
popular because the use it is able to get the view required rather than
the limited by the constraints of the fixed Lens. Finally, Zoom Lens
are the most complex type, offering the greatest functionality once
installed - unsurprisingly, Zoom lenses offer the widest choice of
associated features and technologies.
Zoom lenses can be remotely adjusted to allow variation of the focal
length. This means that a single Lens can be used to view a wide area
until an intruder is detected whereupon do it can be zoomed into
capture facial details. Generally Zoom lenses incorporate an Auto Iris
mechanism to permit 24-hour usage.
Lenses are also categorised according to size format. As Camera
technology has advanced, sensor chips have reduced in size, requiring
lenses to produce smaller images at the focal point. This has made
smaller lenses possible (less glass resulting in less physical size and
weight) although the requirements of precision manufacturing doesn't
permit a proportional price reduction - the component materials of a
Lens being a very small proportion of the overall manufacturing cost.
The quoted format of the Lens (1", 1/2", 1/3" and now even 1/4") is
derived from the ratio of diameter to the viewing image produced.
Whilst it is often most cost effective to match the lens format to the
camera sensor size, it is possible to use a larger lens on a smaller
size camera since the image only needs to be at least as large as the
sensor.
Using a larger lens can often be advantageous, since it offers
greater depth of field (the range of distances from the lens before
objects are too close or too far away to be in focus). Larger lenses
also mean that the area of the image that is used is taken entirely
from the central, flatter part of the lens causing much less corner
distortion and better focus.
Lenses have traditionally been shaped to the arc of a sphere, which
has the effect of causing some distortion of image at the very edges of
the lens, as well as reducing its light gathering capability.
A recent innovation in lens manufacturing, aspherical
technology, allows the edges of a lens to be less curved, producing a
larger area of accurate image and allowing transmission of a greater
amount of light. Aspherical lenses can therefore reduce distortion and
give a lower effective f-stop permitting camera to operate at lower
light levels.
To provide optimum performance neither too much nor too little light
should fall on the camera sensor. This can be adjusted by means of the
lens iris.
A smaller iris opening offers greater depth of field and
better focus, but the reduced amount of light admitted into the camera
results in poor quality images in low lighting levels. A fixed iris
lens offers no adjustment to different lighting conditions, so is
therefore limited in use and not suitable for applications where fine
detail is consistently required. A manual iris can be adjusted at the
time of installation, allowing an optimum picture to be obtained for a
fixed lighting level.
These lenses are best suited to indoor applications, where the
lighting level is controllable and consistent. Both manual and fixed
iris lenses can be used with cameras which offer a feature known as
'electronic iris' - an on-board technology to effectively reduce the
sensor exposure to compensate for the lack of iris control. This can be
cost effective, but does not provide the increased depth of field
offered by a correctly sized iris.
For external use (where conditions generally vary the most),
an automatic iris lens offers the best performance, as the iris
aperture automatically adjusts to create the optimum image by
monitoring the output signal from the camera. There are a number of
different lens types offering this method of iris control. The original
design for automatic iris (Al) lenses was wholly self-contained, with
the image analysing technology built into the lens and an iris that was
adjusted by servomotors.
Market demand to produce smaller, lower cost lenses led to the
introduction of direct drive technology which requires circuitry within
the camera, replacing that previously located in the lens. This
technique used a different iris control - galvanic drive. Subsequently
this technology has been introduced into the original style auto iris
lens where onboard camera circuitry is not required.
Today these are the choices for auto-iris control - traditional servo drive, galvanic iris and direct drive.
The final lens characteristic to take account of is the
light-gathering speed of the lens-expressed as an f-stop number. This
literally measures the amount of light captured by the lens in a given
period of time; the lower the f-stop range, the more light that can be
transmitted. |
