There are two indicators for evaluation criteria
As the pixels of mobile phone lenses become higher and higher, everyone begins to pay more attention to how to scientifically and accurately evaluate the performance of a lens. Before that, we first understand the resolution and contrast of the two important indicators of lens evaluation.
Resolution, also known as resolution, discrimination, and resolution, is the ability of the lens to reproduce the details of the subject. The higher the lens resolution, the clearer the image. Its unit is "LP / MM". Two adjacent black and white lines can be called a line pair. The number of line pairs that can be distinguished per millimeter is the resolution, which is like the fineness of the zebra crossing. The higher the density, the higher the resolution.
Contrast, also known as sharpness and sharpness, is the ability of the lens to vividly reproduce the mid-level, dark-level, low-contrast landscape details, weak brightness contrast, and subtle color changes of the scene. A high-contrast lens has a sharp contour, sharp edges, normal contrast, rich layers, and true and delicate color reproduction.
The comprehensive performance of resolution and contrast is called clarity. A high-resolution lens may not be rated as an excellent lens. Sometimes the resolution of some lenses is not very high, but the contrast is quite good, still a good lens. Of course, it is really the ideal lens to be able to take into account.
Three ways to evaluate the lens performance
So do we have effective methods to evaluate lens performance? Next, briefly talk about several evaluation methods;
Method 1: MTF (Modulation Transfer Function) test is currently the most accurate and scientific lens evaluation method. Because MTF excludes the effects of chips and software, and can objectively reflect the lens performance, it is widely used in the research and testing of optical products.
MTF is a function of the modulation ratio between the actual image and the ideal image with respect to the spatial frequency. In layman's terms, it measures the reduction degree of the lens contrast at different spatial frequencies. It can reflect the lens contrast and resolution at the same time. Because the actual optical system has aberration, reflection, absorption and other factors, the contrast between the image and the standard cannot be completely consistent, so the MTF is always between 0 and 1. The larger the MTF value, the clearer the image. As shown in Figure 1, MTF = 0.4 is the clearest; MTF = 0.05, the image is the most blurred.
So how to judge the quality of the lens by MTF? We will give a general introduction through Figure 2. The abscissa is the spatial frequency, and the unit of the spatial frequency is lp / mm, which is very close to the concept of resolution. The ordinate is the MTF value. The curve represents the MTF value of the lens at different frequencies. It can be seen from the figure that as the spatial frequency increases, the lens MTF gradually decreases. If the abscissa extends infinitely, you will find that the curve tends to zero infinitely. In order to better describe the MTF curve, we add three curves A, B, C and corresponding pictures to describe.
Curve A indicates that the lens has a moderate MTF in the low frequency band. As the spatial frequency increases, the attenuation process is slow, and the overall performance of the contrast and resolution is better. From Figure A, it can be seen that the contrast of the dark and light colors is obvious, the hair details are detailed, and the overall reduction is more it is good.
The low-frequency MTF of curve B is the best in the comparison, indicating that the contrast of the lens is very good, but as the spatial frequency increases. Its attenuation is very fast, indicating that the lens resolution is not very good. From Figure B, it can be found that the details of the fluff at the ears of the kitten are difficult to distinguish.
Curve C indicates that the MTF will decay quickly at low frequencies and the overall performance will be poor. It can also be seen from Figure C that the contrast of the shades of the picture is low and the sense of layering is poor.
The enlightenment given to us above is that the definition and development of the lens will not pursue the extreme of a certain aspect, so it is often too inadequate, considering a uniform and smooth transition between high and low frequencies, to make a lens product with excellent performance.
Method 2: SFR (Spatial Frequency Response) is the response of the test module to different spatial frequencies, similar to the MTF algorithm, but the test result is affected by both the lens and the photosensitive device and the processing program (SFR = lens MTF * sensor MTF * ISP MTF), so this algorithm is called spatial frequency response SFR. In fact, SFR is equivalent to the simplified version of MTF, which is widely used in module evaluation.
The SFR data processing process is expressed in Figure 3, but it is not described here. There are two types of hypotenuse for SFR calculation, one is horizontal hypotenuse and the other is vertical hypotenuse. As shown in Figure 4, two red boxes, the wider width is called the horizontal hypotenuse, and the larger height is called the vertical hypotenuse, where we use the vertical hypotenuse when testing the sharpness in the horizontal direction, and test the vertical direction For sharpness, a horizontal bevel is used.
In terms of SFR analysis, the domestic market tends to use the MTF value at a fixed frequency to judge the quality of imaging. For example, the greater the MTF value at a fixed frequency @ 112lp / mm, the better the image quality. The European and American markets are more inclined to measure the image quality with a fixed MTF frequency. In the case of MTF30, the higher the frequency, the better the image quality. MTF30 generally uses LW / PH as the unit, that is, how many lines are resolved per image height, LW / PH = frequency * narrow edge image height * 2. In fact, the purpose of these two methods is the same, only the difference of thinking mode at home and abroad.
In actual evaluation, most of the modules will sharpen the image to different degrees to improve the image clarity. In the SFR curve, the black and red curves will be used to distinguish the data before and after sharpening. The solid black line indicates the original data without sharpening, and the dashed red line indicates the data after removing the sharpening effect on the module image. .
Method three: CTF (contrast transfer function) is called the contrast transfer function, which means that the object-side fixed line width (ie fixed object-side spatial frequency) lens response contrast, ie: CTF = (Imax-Imin) / (Imax + Imin), So CTF is always less than or equal to 1, I: intensity (light intensity) Imax: the brightest light intensity Imin: the darkest light intensity. This parameter is useful for evaluating measured edge contrast. The CTF value is obtained by means of contrast calculation, which can only test the black and white contrast state of a specific frequency. This detection method is simple and convenient, but the stability and accuracy cannot be compared with the previous two methods.
We can also simply distinguish the difference between CTF / MTF / SFR by testing frequency, method, etc .:
Note:
①Modulation degree is actually the formula of brightness ratio: M = (Imax-Imin) / (Imax + Imin), the original modulation degree is a sinusoidal grating with M degree, and the modulation degree of the image that reaches the image plane through the lens is M ′, then MTF = M '/ M
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