Technical notes
Machine vision lenses
Glossary of main terms

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In this page you can find the main terms used when specifying CCTV lenses,
 useful to select the more suitable product for your application.
 Our technical service will be glad to help you, whenever further clarifications are needed.

To select the most suitable lens for your application, please use our optical calculator page.

 
Image format size

There are presently five main image format sizes, which are commonly used in CCTV cameras:  1", 2/3", 1/2", 1/3", 1/4" (these measures refers to the sensor diagonal).



A lens designed for a specific image format can always be used with a smaller format imaging device, but cannot be used with a larger format imaging device.

 

For instance: A 1/2" format lens can be used with a camera having a 1/2" or a 1/3" imager; however it cannot be used with 2/3" or 1" format cameras.

Telecentric lenses

Lenses in which the chief rays are parallel to the optical axis, providing a constant magnification regardless of the object distance.

The human eye suffer from a parallax error which causes the distant objects to look smaller than closer ones. Actually, this error is only a change in magnification within the range of depth of field of human eye. In a telecentric lens the system pupil is at infinity and this results in minimum perspective errors when changing the object position.

Therefore the size of the viewed object appears to be the same when changing the distance of the object (within the range of depth of field). These lenses are ideal for precision measurements applications.


Optical systems

 

Standard lens

Object side telecentric lens

Both sides telecentric lenses

Standard lens

Object side telecentric lensBoth sides telecentric lens
The size of viewed object changes when the distance between object and lens varies.
Lens has smaller dimensions.
The size of viewed object does not change when the distance between object and lens varies.
Diameter of lens is larger.
Cost is higher than standard lenses.
The size of viewed object does not change when the distance between object and lens and the distance between lens and sensor varies.
This lens is rather expensive
Resolving power

The ability of a lens to distinguish between two closely separated objects
If nothing is mentioned, the value indicated is the theoretical resolving power.

Resolution power

Indicates the number of pair of black and white lines which can be identified in the width of 1mm.

Example: 100/mm means that  a lens can identify a pair of black and white lines in 1/100 mm

Optical distortion

An aberration of a lens by which a square object is imaged with either pincushion or barrel lines.

Square object

Pincushion distortion

Barrel distortion

Square object

Pincushion distortion

Barrel distortion

  

TV distortion
Example of distortion on a TV monitor

TV monitor distortion

Depth of field

The range of distance within which a sharp image is mantained when a lens is focused at a specific object distance.
Images through a lens are formed from a composite of dots. As the image becomes less sharp, the size of the dots increases. The maximum allowable dot size for an acceptable clear image is called permissible circle of confusion.

Numerical aperture

It stands for the measure of the light gathering capacity of the lens.
A higher numerical aperture value shows a brighter and higher resolution lens.

F No.(at ∞)

It is the effective aperture of a lens divided by its focal lenght. Lower F no. shows a brighter lens.

Effective F No.

It shows the brightness of a lens at the finite.

WD=working distance

It is the distance between the front lens and object.

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Last modified 10-05-2013
 
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