| Basics
of CCTV lenses page by CCTV Lens Depot |
|
| FREQUENTLY ASKED QUESTIONS
AND ANSWERS ABOUT CCTV LENSES |
| |
| Q: What
are the common sizes of CCD imager in CCTV cameras? |
| A: It
is 1/3", 1/2" There is also new format coming up and that is 1/4" while
2/3" and 1" are disappearing. |
| |
| Q: Does
it mean that 1/3" camera needs 1/3" size lens, 1/2" needs 1/2"? |
| A: Yes,
but you can also take bigger lenses for smaller camera imager. Example:
1/3" camera will take 1/2" and 2/3" lenses. 1/2" camera will also take
2/3" size lens. |
| |
| Q: What
iris does in lenses? |
| A: Iris
is mechanism which closes and opens allowing more or less light into the
lens. |
|
| Q: What
is Auto Iris lens? |
| A: Auto
Iris lens has a little amplifier built in which responds to amount of scene
light and will open or close iris automatically to maintain same amount
of light coming to imager. Doing that picture will not be too bright or
too dark. |
| |
| Q: Where
A/I lens gets power from? |
| A: Power
for A/I lens comes from the camera. Plug in most cases looks like small
rectangular connector with 4 pins. |
| |
| Q: What
is DC Iris lens? |
| A: DC
or DD (Direct Drive) is same as Auto Iris lens but there is no amplifier
built-in the lens. Amplifier is in the camera and camera drives the lens
iris again through the cable plugged into the camera. Plug is same as in
A/I lenses. |
| |
| Q: Is
it hard to find camera with DC lens drive circuitry? |
| A: Almost
all modern cameras have a receptacle for A/I and DC lenses. It is small
rectangular connector usually in the back of the camera. When selecting
camera you should pay attention to select one which will accept both types
of lenses. |
| |
| Q: Since
same plug is used for A/I and DC lenses how will camera determine which
lens is connected? |
| A: There
is a little switch-selector labeled DC-A/I (or similar) and you have to
select switch to match type of lens you have. |
| |
| Q: Is
there any advantages with A/I or DC lenses? |
| A: A/I
lens costs more. DC lenses generally cost less since they don't have amplifier,
just driving coils for iris. However, all Raymax DC lenses have the same
aperture ratio as A/I lenses (and A/I lenses of other manufacturers) so
you can save some money by buying Raymax DC lens over A/I lens. |
| |
| Q: What
is F number in lens specifications? |
| A: That
number is called Aperture Ratio. It tells you a ratio between maximum and
minimum iris opening. Auto Iris or DC lenses respond to changing light
conditions and ratio between iris position when is completely open and
completely closed called aperture ratio. |
| |
| Q: What
numbers are desirable for F? |
| A: Lowest
F should be in range of 1.2-1.6 (lens opened in the dark to pickup as much
light as possible). Highest F should be at least 90 for lens you will have
in the closed area (like office) and there is a solid amount of internal
lighting. However, that would be a poor choice for lens you want to use
outdoor where there is a great difference in light range, going from darkness
during the night and bright, sunshine day (lens is closing iris as much
as it can to reduce amount of light to acceptable level. Desirable F number
should be at least 185 but over 300 is desirable. |
| |
| Q: What
are the F numbers for Raymax lenses? |
| A: For
90% of lenses it is 1.2, 1.4 and 1.6 and ALL of them got to 360 (iris closed). |
| |
| Q: What
is focal length (in mm) number telling about? |
| A: In
short, it tells you how much from the scene you will see using that particular
lens. It is called Field of View and is expressed in degrees. Example:
4mm 1/3" lens will give you 63 degrees of horizontal field of view while
8mm 1/3" lens will give you 34 degrees a field of view. Click
here to find out exact numbers. |
| |
| Q: What
that practically means? |
| A: That
means if you have a camera in the corner of the room and you want to pickup
as much of scene (field of view) as possible, you would select 2.8mm (82
degrees). In contrary, if your camera is in the lobby and you want to watch
the door, you would take something like 12mm lens (about 23 degrees). Higher
the number, smaller is field of view and there is a bigger zoom effect
(objects become closer). |
| |
| Q: What
is varifocal lens? |
| A: That
lens is kind of zoom lens where you have a range of focal lengths you can
take. Example 3-8mm lens will allow you to adjust for the field of view
you like (within that specified range). Varifocal lenses because of that
ability cost more. |
| |
| Q: Are
varifocals same as zoom lenses? |
| A: Close
to that but no motors built in. In real life you would do adjustment at
the moment of initial installation and you would leave it like that, with
possibility to change that field of view when you need it or at moment
when you need that camera to point somewhere else and you need to change
field of view for new scene. |
| |
| Q: What
are zoom lenses then? |
| A: They
are similar to varifocals but they have wider ranges of focal lengths,
they come with manual iris, DC iris or video (auto) iris and they have
motors for zoom and focus. Obviously, you can change field of view whenever
you want to zoom into object you want to see closer or go for wide angle
to see as much of the action you want. You need remote controller for zoom
lenses. |
| |
| Q: What
is C or CS? |
| A: That
is type of lens mount you have on your camera. |
| |
| Q: Does
it mean that you have to select a lens with mount type you have on your
camera? |
| A: Yes,
however if you have CS mount on your camera and you have lens with C mount
there is inexpensive C to CS mount adapter you can buy to put between lens
and camera. We sell those adapters. |
| |
| Q: What
kind of lens mount is more common? |
| A: It
is CS, however, almost all lens types we carry are available to C and CS
lenses. |
|
| Q:
How are Raymax lenses wired? |
| A:
Check
this page and find out how are connectors wired, wire colors and other
details. |
| |
| Q:
We have bright (direct) light coming into the lens, what to do? |
| A:
This question is coming up from time to time and customers are asking what
kind of lens they have to use. Aside of using video iris and DC iris lenses
only real help is to move camera to look at the object under different
angle so there is no direct light coming into the lens. |
