Glossary of terms/ LENSES

VARIFOCAL LENS: Optical assembly containing several movable elements to permit changing the effective focal length (EFL). Unlike a zoom lens a varifocal lens requires refocusing with each change.

FIXED FOCUS: Lenses that are not provided with a means of focusing operation regardless of the distance to the subject.

FOCAL LENGTH: The basic parameter to determine the image position, magnification, and angle of view of a lens. The focal length of the lens is measured in mm and directly relates to the angle of view that will be achieved. Short focal lengths provide wide angles of view and long focal length become telephoto, with narrow angles of view. A 'normal' angle of view is similar to what we see with our own eye and has a relative focal length equal to the diagonal dimension of the pick up device.

ZOOM LENS: A lens that delivers different focal lengths without creating a shift of focus regardless of the focal length setting.

DEPTH OF FIELD: The depth of field refers to the area within the field of view which is in focus. A large depth of field means that a large percentage of the field of view is in focus, from objects close to the lens often to infinity. A shallow depth of field has only a small section of the field of view in focus.

ZOOM RATIO: The ratio of the starting focal length (wide end) to the ending focal length (tele end) of a zoom lens. A lens with a 10X zoom ratio will magnify the image at the tele-end by 10 times.

AUTO-IRIS LENS: A lens with an electrically controlled iris. The circuit controlling the iris is set to maintain a constant video level in varying lighting conditions. Depending on the placement of the driving circuitry (i.e. on the lens side or incorporated on the camera side), there are two types of Auto-Iris; with a driving circuitry built in and DC meter (galvanometer) only. Make sure to identify, before ordering an AI lens, whether the camera outputs video signal or DC current to actuate the auto-iris. (ref. DC-Type Lens)

FLANGE BACK (Flange Back Focal Distance): The distance from the mechanical flange of the lens (rear edge surface of the lens mount) to the focal plane. C-mount lenses have a flange back distance of 17.526mm while CS-mount lenses have 12.5mm. Because of this, C-mount lenses can be used on CS-mount cameras with an adapter ring of 5mm thickness (however, CS-mount lenses cannot be used on C-mount cameras).

BACK FOCUS (Back Focal Distance): The distance from the rear-most portion of the lens element to the image plane. It is important to adjust the back focus correctly in order to obtain the best image. Certain lenses come with a back focus adjustment mechanism, while others do not. Also, most of the cameras incorporate back focus adjustment, if it is not available on the lens side.

MANUAL IRIS LENS: A lens with a manual adjustment to set the iris opening (F-stop) in a fixed position. Generally used for fixed lighting applications.

MINIMUM OBJECT DISTANCE (M.O.D.): The closest distance a given lense will be able to focus on an object. This is measured from the vertix (front) of the lens to the object. Wide angle lenses generally have a smaller M.O.D. than large focal length lenses.

PINHOLE LENS: Lens used for applications where the camera/lens must be hidden. Front of lens has a small opening to allow the lens to view an entire room through a small hole in the wall.

PRE-POSITION LENSES: Zoom lenses which utilize a variable-resistor (potentiometer) to indicate zoom/focus position to the lens controller. After initial set-up, this allows the operator to view different pre-set areas quickly without having to readjust the zoom and focus each time.

A guide to selecting your LENSES

Auto or Manual Iris

Auto Iris Lenses are generally external applications where the lighting levels vary.Manual iris lenses are internal, where the light levels are constant.
With the introduction of electronic iris cameras it is now possible to use manual iris lenses in varying light conditions and the camera will electronically compensate, however there are several considerations to this option. The setting of the F stop becomes critical if the iris is opened fully to allow the camera to work at night. The depth of field will be very small and it may be more difficult to achieve sharp focus even during the day. The camera can maintain normal video levels but it cannot affect the depth of field. If the iris is closed to increase the depth of field in low light, performance of the camera will now be reduced.

C or CS mount?

Modern cameras and lenses are generally CS mount, with CS mount cameras both types of lenses can be used but the C mount lens requires a 5mm ring(VM400) to be fitted between the camera and lens to achieve a focused image. With C mount cameras it is not possible to use CS mount lenses as it not physically possible to get the lens close enough to the CCD to achieve a focused image.

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 does iris do 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 does 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 cameras 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 the same plug is used for A/I and DC lenses how will the 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: Are 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 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.

Q: What Does that mean in practical terms?
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 the 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: 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.


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