In addition to the basic requirements, conventional indicators such as focal length and field angle need to be tested, 

there are two key indicators to be tested:

1. Optical transfer function MTF

What is MTF?

Optical Transfer Function  is a function that takes spatial frequency as a variable to characterize the relative variation of modulation and lateral phase shift in imaging process. Optical transfer function is the filtering transformation of spatial spectrum by optical system. For an optical imaging system with incoherent illumination, the image intensity is also linear and meets the superposition principle. 

When we evaluate the quality of the lens, we should not only consider the size of the modulation transfer function (MTF) of low frequency, intermediate frequency and high frequency, but also comprehensively evaluate a series of transfer function curves corresponding to different fields of view. The modulation transfer function of the off-axis field of view for the cosine grating placed in the horizontal direction (meridian direction) and the grating placed in the vertical direction (sagittal direction) is not the same, so the meridian MTF and sagittal MTF should be investigated at the same time. Generally speaking, the higher the overall modulation transfer function curve, the better the imaging quality of the system.

MTF is the standard for comprehensively evaluating the imaging quality of a lens. The two-dimensional coordinate curve of MTF is usually the value of Y axis (0 ~ 1), and the X axis is the spatial frequency (LP / mm), that is, how many "line pairs". Low frequency evaluates the contrast (contrast) of the image after imaging. High frequency is to examine the clarity and resolution of the lens, that is, the ability to distinguish details. For example, for photographic lens, 10lp / mm is usually used to investigate the contrast effect. Generally, the MTF value should be higher than 0.7. For high-frequency investigation, 30lp / mm is usually greater than 0.5 in the half field of view and 0.3 in the edge field of view.

For some optical instruments or industrial lenses, there are higher requirements for high frequency, so how to calculate the high frequency we want to investigate? It's actually very simple:

Frequency = 1000 / (2) × Sensor pixel size)

If the pixel size of the sensor you use is 5um, then the MTF high frequency should be 100lp / mm. If the measured value of MTF is higher than 0.3, it is a better lens.

2. Distortion

MTF can not reflect the aberration of distortion, so the distortion is proposed separately.

Distortion is deformation, which is divided into pillow distortion and barrel distortion.

Distortion is related to the field of view. The larger the angle of view, the greater the distortion. For conventional camera lens and monitoring lens, the distortion is acceptable within 3%, for wide-angle lens, the distortion can be 10% ~ 20%, and for fisheye lens, the distortion can be 50% ~ 100%.

So how much distortion should I control if I decide what lens I want?

First, clarify the purpose of your lens. For example, if the above is photography or monitoring, the distortion of less than 3% is allowed. If your lens is used for measurement, the distortion should be less than 1% or even lower, depending on the system error allowed by your measurement system.

There are many lens evaluation methods, the content is for reference only!

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