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About fixed focus lens

1. What is the sensor of a lens

     The senor is an imaged photosensitive element. There are aeveral types of imaging sensors with different image size for cctv camera.


2. The relationship of Focal length and angle

focal length(Fixed focal length)

Focal length is divided into short focus (f<3mm), medium focus(3<f<10mm), long focus(f>20mm)

As the focal length increases, the angle of the lens becomes smaller. For instance,

Short-focus lens = wide-angle lens (FOV> 90 °)

Medium focus lens = mid-angle lens (FOV=50-60°)

Telephoto lens = narrow angle lens (FOV <10 °)

3. About the F/NO.(Fixed aperture)

Aperture is an index for the amount of light that passes through a lens.  The value of the aperture is represented by the F No.

the smaller the F/NO, the greater the amount of light, and the brighter the image generated by the lens.  The F /No. is inversely

proportional to the entrance pupil diameter of the lens and directly  proportional to the focal length.  It’s formula is as follows:   

F No. = f / D (f: focal length, D = Entrance pupil diameter)

4. What is distortion?

Distortion is the ratio of the change between the center and adge areas of a captured lmage.  Due to the aberration of the lens, 

the distortion is more noticeable at the edges of a captured image. There are two types of distortion: barrel distortion and pincushion 

distortion. The general rule is that when the absolute value of the distortion value is smaller, the lens offers higher accuracy.


TV distartion (Dtv) = h/2h x 100 (%)

A postive value of TV dstortion indicates

pincushion distortion and a negative value

indicates barrel distortion.

About zoom lens

The M.O.D. (minimum object distance) is the closest distance from the vertex of the front lens to the nearest object at 

which an image can be focused.

1. CS and C Mount

The CS-mount lens has the flange back distance of 12.5mm. TheC-mount lens has the flange distance of 17.5mm. 

The CS mount lens is only applicable to the CS mount camera, but the C mount lens is fit for both C mount and 

CS mount cameras as long as a 5mm adapter ring is used to match the CS mount camera.


2. Depth of Field

The depth of field (DOF) 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.  A small depth of field means only a small section of the field 

of view is in focus. The depth of field is of following properties:



1) The larger the F No. is, the wider the depth of field becomes.

2) The shorter the focal length is, the wider the depth of field becomes.

3) The longer the distance to the object is, the wider the depth of field becomes.

4) The backward depth of field is wider than the forward depth of field.


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