Detailed explanation of optical lens parameters

August 7, 2021

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Detailed explanation of optical lens parameters (EFL, TTL, BFL, FFL, FBL/FFL, FOV, F/NO, RI, MTF, TV-Line, Flare/Ghost)

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Key predicates:

1. EFL (Effective Focal Length) effective focal length

Definition: Refers to the distance from the center of the lens to the focal point (pictured below).

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The focal length of the lens is divided into the focal length of the image side and the focal length of the object side (below):

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The image-side focal length refers to the distance from the main image side (back main surface) to the image side focal point (back focus).

The object focal length refers to the distance from the main surface of the object (front main surface) to the focal point of the object (front focus).

Precautions:

(1) If the focal length is too short, the field of view will be too large, which will cause distortion and the chief ray exit angle to be difficult to control, and the relative contrast is too low.

The lens is severely curved, and it is difficult to correct the phase difference, so it is difficult to design.

(2) If the focal length is too long, the lens will be too long, which is not conducive to the miniaturization of the system, and the field of view is too small to satisfy the user

Demand (FOV>60°)

2. TTL (Total Track Length) total length of the lens

The total length of the lens is divided into the total length of the optics and the total length of the mechanism:

The total optical length refers to the distance from the first surface of the lens in the lens to the image surface.

The total length of the mechanism refers to the distance from the end surface of the lens barrel to the image surface.

3. The definition of BFL (Back Focal Length) optical back focal length:

The distance from the last surface of the lens in the optical system to the image surface.

4. FFL (Front Focal Length) optical front focal length

Definition: the distance from the first surface of the lens in the optical system to the object surface

Note: It should be distinguished from the FFL of the mechanism back focal length

5. FBL/FFL (Flange Focal Length) mechanism back focus (Flange Focal Length)

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Definition: the distance from the last mechanism surface of the lens group to the image surface

6. FOV (Field Of View) field of view definition:

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Refers to the maximum field of view that the lens can capture.

The field of view can be divided into diagonal field of view (FOV-D), horizontal field of view (FOV-H), and vertical field of view (FOV-V).

The diagonal field of view is the largest, followed by the horizontal field of view, and the vertical field of view is the smallest. Generally speaking, the angle of view generally refers to

The diagonal field of view of the digital camera module.

FOV-H=2tan(H/2D)

FOV-V=2tan(V/2D)

FOV-D=2tan[sqrt(H2+V2)/2D]

7. F/NO. (F-Number) focal number (relative aperture)

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Definition: The ratio of the effective focal length to the entrance pupil aperture.

F/#=EFL/EPD (EPD: entrance pupil aperture)

Function: used to determine the brightness of the lens.

Note: Under the premise of ensuring the same clear aperture, the shorter the focal length, the smaller the relative aperture.

In general, F/#=2.8, but F/#=3.2 in single-chip design, N/B Lens F/#=2.0 F/# The smaller the F/#, the larger the clear aperture.

8. Distortion distortion

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Distortion is the off-axis phase difference. And it is the aberration of the off-axis thin beam. It is the height of the intersection of the off-axis point and the chief ray on the image plane

The height difference with the ideal (paraxial) image plane. It is a function of the field of view and has nothing to do with the aperture. The primary distortion varies with the field of view

(Image height) changes to the third power, and the percentage distortion increases with the square of the image height, so the distortion varies with the field of view. abnormal

The change of change is not linear, it is only the deformation of the image, and does not affect the sharpness of the image.

Distortion is divided into two types: TV distortion and optical distortion: TV distortion: TV-Distortion

Optical distortion: Optical-Distortion

TV distortion is divided into two types: pillow type and tube type.

(Vertical) TV-Distortion=[(V1+V2)/2-Y]/Y*100%

(Horizontal) TV-Distortion=[(H1+H2)/2-X]/X*100%

Precautions:

Distortion is the image quality that the user can easily perceive.

Distortion is a very strict lens index for users.

Optical-Distortion<3% is not easy to detect by the human eye

Optical-Distortion<2% Distortion requirements of photographic objectives

General requirements:

CIF TV-Distortion<2.0%

VGATV-Distortion<1.5%

1.3M/2.0M/3.0MTV-Distortion<1%

9. RI (Relative Illumination) relative contrast

The definition of illuminance: the brightness of an object or illuminated surface illuminated by a light source is called illuminance.

The relative illuminance is the ratio of the central illuminance to the peripheral illuminance.

Note: If the relative contrast is too low, it means that the center of the image is brighter and the surroundings are darker, that is, vignetting, commonly known as dark

Angle (Shading). Too low contrast can also cause color distortion.

RI is proportional to COS4(semi-FOV): RI∝COS4(semi-FOV)

Semi-FOV=30°, theoretically RI<56%

Semi-FOV=35°, theoretically RI<45%

When the RI is less than 50%, the human eye can distinguish it. In severe cases, the corners of the screen will be completely black with "missing corners".

Therefore, the basic requirement of RI is: RI>50%

10. CRA (ChiefRay Angle) chief ray angle

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Definition: The chief ray angle is the angle between the chief ray and parallel rays.

The chief ray is the light emitted from the edge of the object, passing through the center of the aperture stop and finally reaching the edge of the image.

Note: The improper main light exit angle will cause serious vignetting, lower contrast, and color cast.

11. MTF (Modulation TransferFunction) optical modulation transfer function

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Definition of Modulation (M): Modulation is the Maximum of I minus the Minimum of I divided by the Maximum of I plus

The Minimum of I; that is, the ratio of (the brightness of light minus the darkest of light) to (the brightness of light plus the darkest of light).

So the result M is the contrast of light.

Modulation(M)=(Imax-Imin)/(Imax+Imin)

I: Intensity light intensity

Imax: the brightest light intensity

Imin: The darkest light intensity

Note: The demand calculation formula for Sensor MTF is as follows,

Sensor full frequency resolution:

1000/2.8/2=179lp/mm (2.8um Pixel Size) or 1/(2£) £: SensorPixel Size

12. Resolution (resolution)

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The definition of resolution: a line-pair that can be resolved per 1mm. Unit-based (lp/mm)

Sensor full frequency resolution (black and white): 1000/2.8/2=179lp/mm, but for color (RGB), the Sensor is based on

Every 4pixel resolves 1 point, so the number required by the Sensor is calculated as: Center 1000/2.8/2/sqrt 2=126lp/mm — 160lp/mm

(The center usually has a higher resolution)

Peripheral 1000/2.8/2/sqrt 3=103lp/mm — 100lp/mm (The outer periphery is 0.7F)

(sqrt 2 is a general calculation method, but the processing logic of different brands or models of Sensors is different, there will be differences)

13. TV-Line scanning line

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TV-Line is how many lines can be resolved in the horizontal image of the screen, which can be converted by the resolution:

TV-Line=lp/mm*2*Sensor width. Example 1/4’’ 1.3M Sensor:

Center 1000/2.8/2/sqrt2=126lp/mm —126lp/mm*2*1024*2.8/1000=722 — 700 TV-Line

Peripheral 1000/2.8/2/sqrt 3=103lp/mm —103lp/mm*2*1024*2.8/1000=590 — 600 TV-Line

14. Flare/Ghost stray light/ghost

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Definition: Refers to the opposite miniature or fog image formed by the random scattering of light on the image surface in an optical system, that is, the non-imaging beam in the optical system.

The scattered light from the lens surface, the bubbles in the element, the scattered and reflected light from the lens frame and the inner wall of the lens barrel, and other non-imaging beams incident on the image surface all become stray light.

Its direct effect is to form noise, reduce the signal-to-noise ratio of the image, and even make the signal light submerged in the noise.

Elimination of stray light: add a hood, auxiliary diaphragm, blacken the edge of the lens, blacken the lens holder or stick a matting cloth in the structure,

And plating anti-reflection film and so on.