如何解讀攝影鏡頭之MTF-data:∙對比度(Contrast): 以5 或10 本( lp/mm) 來評價該鏡頭之對比度性能. 在此頻率之MTF 縱然僅2~3 %之差異, 人眼可輕易分辨其差別, 所以這是一個重要的基本頻率.好鏡頭在頻率 5 lp/mm,小光圈時T 及S 方向MTF需達95 % . 若低於90% 就算是較差之影像品質..∙清晰度(Sharpness): 以40本(lp/mm) 來評價該鏡頭之清晰度性能.. 40 lp/mm代表一個鏡頭可以分辨多細的物體(譬如一根毛髮). 在此頻率之MTF 縱然達10 %之差異, 人眼不太能分辨其差別. 通常一顆傳統底片型攝影鏡頭在20 lp/mm 時需達50% MTF 可算是好鏡頭.∙一個鏡頭若有較佳的對比度, 但較差的清晰度, 其整體影像會比相反的鏡頭(較差的對比度, 但較佳的清晰度) 銳利, 感覺較好. 當然, 一般而言, 鏡頭若有較好的對比度, 其清晰度也較好∙好鏡頭的影像品質需求o紅色區域40 lp/mm▪>20% (邊緣)▪>65% (中心).o紫色區域20 lp/mm :▪>45% (邊緣)▪>80% (中心)o綠色區域10 lp/mm 與藍色區域5 lp/mm>95% (全部像平面)∙有些鏡頭不可能達到上述要求, 譬如較大廣角鏡頭等∙通常T 方向MTF 較差, 有時可用較高之S 方向某種程度之補償.優良之影像對比度相對良好, 但在40 本時, 離軸影像已無法分辨清晰度良好, 但對比度不良(5 lp/mm 低於90%) 不良知鏡頭以MTF vs. 像場曲線來解釋清晰度與對比度對影像品質之關係Note: 本文翻譯自“Klaus Schroiff”“How to interpret MTF graphs”Understanding MTF TestingBy Sam Sadoulet,Application EngineerWhen characterizing the resolution of an imaging lens, it is extremely useful to refer to the Modulation Transfer Function (MTF). The MTF of a lens is a measurement of its ability to transfer contrast at a particular resolution level from the object to the image (see Figure 1). In other words, MTF is a way to incorporate resolution and contrast into a single specification.An easy way to interpret MTF results is to think of imaging a target with black and white lines (100% contrast). No lens (even theoretically perfect) at any resolution can fully transfer this contrast to the image because of the diffraction limit. In fact, as the line spacing is decreased (i.e. the frequency increases) on the target, it becomes increasingly difficult for the lens to efficiently transfer this contrast (see Figure 1). Therefore, as the frequency increases, the contrast of the image decreases.An MTF graph plots the percentage of transferred contrast versus the frequency (lp/mm) of the lines. A few things should be noted:1) Contrast (also known as Modulation) is the image contrast expressed in terms ofa percentage of the object contrast (100% = white on black, 0% = gray on gray).2) The frequency in an MTF graph is measured in the image plane. Therefore, in order to define the object resolution (line-pair frequency), one needs to calculate it using the primary magnification (PMAG) of the imaging lens.The frequency of the lines is expressed in terms of line-pairs per millimeter (lp/mm). The inverse of this frequency yields the spacing of a line-pair in terms of millimeters.Why is MTF Important?In traditional system integration (and less crucial applications), the system's performance is roughly estimated using a principle of "the weakest link." This idea proposes that a system's resolution is solely limited by the component with the lowest resolution. Although this approach is very useful for quick estimations, it is actually flawed, because every component within the system contributes error to the image, yielding poorer image quality than the "weakest link" alone.Every component within a system has an associated MTF and, as a result, contributes to the overall MTF of the system. This includes the imaging lens, sensor, capture boards, and cables, for instance. The resulting MTF of the system is the product of all of the MTF curves of its components. For instance, we can compare a 25mm Fixed Focal length lens and a 25mm MVO™ Double Gauss lens by evaluating the resulting system performance of both lenses with a Sony XC-75 CCD monochrome camera. By analyzing the system MTF curve, we can make a prediction as to which combination will yield sufficient performance. In some metrology applications, for example, a certain amount of contrast is required for accurate image edge detection. If the minimum contrast needs to be 35% and the image resolution required is 30 lp/mm, the MVO™ Double Gauss lens is the logical choice (see Figure 2).Knowing the MTF curves of components allows an integrator to make the appropriate selection to optimize the system for a particular resolution.It should be noted that a theoretical MTF curve can be generated, given the optical prescription of the lens. Although this can be helpful, it does not indicate the actual performance of the lens after manufacturing. Manufacturing always introduces some performance loss in the design due to tolerances. For this reason, Edmund Industrial Optics has invested in an Optikos VideoMTF™ measurement system (pictured above) which has become essential in system integration and customdesigning. This MTF testing equipment enables characterization of the actual performance of both designed lenses and commercial lenses (whose optical prescription is not available to the public). As a result, preciseintegration-previously limited to lenses with known prescriptions-can now include commercial lenses.FIGURE 1: Effects of diffraction on the amount of contrast imaged as the frequency is increased.。
