Basics of CCTV lenses page by CCTV Lens Depot

Q: What are the common sizes of CCD imager in CCTV cameras?
A: It is 1/3", 1/2" and 2/3" There is also new format coming up and that is 1/4".
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 and usually has better F number. DC lenses generally cost less since they don't have amplifier, just driving coils for iris. However, Uni 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 Uni 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 Uni 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) or 3.6 (68 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 are always equipped with Auto Iris circuitry 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 Uni lenses wired?
A: Check this page and find out how are connectors wired, wire colors and other details.

More Questions and Answers coming soon!

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