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Modulation transfer function MTF(一)
Views:6393 Date:2013-03-05【Back】

    Photographic lens photographic image resolution and visual clarity is determined by two factors together determine the Ming sharpness.
     Resolution (Resolution), also known as resolution, identification rate, discernment, analytical, resolving power and resolving power, is clearly reproduce photographic lens fiber micro-details of the scene being shot ability. Obviously the higher the resolution of the lens, the captured image more crisp and clear. Its unit is "lp / mm." The advantage is that you can quantify the data that make the results more intuitive, more scientific and more tightly.

     The contrast of the object is infinite time (that is, all images in the white and black) lens to record the details of the capacity of the object, when the lens is in the range of a millimeter, can be distinguished by 60 a pattern of parallel lines, then we said this lens optical resolution of 60lp/mm. In theory, the higher the resolution of the lens, it is more clear image.

    Ming sharpness (Acutance), also known as sharpness, sharpness, is a photographic lens camera scene vividly reproduce intermediate level, slough department level, low-contrast shadow pattern detail, weak luminance contrast and subtle color change. Ming high sharpness of the lens, the image formed by clean-cut, sharp edges, contrast normal, rich layers, delicate texture, tone clear, strong texture, color transition soft, supple, true color reproduction, and natural. Obviously these features are indispensable for high-quality photographic lenses. However, the bright camera lens sharpness, it is difficult to simply use the data that is difficult to test it out with ordinary instruments. People are usually just by subjective feeling, qualitatively be reviewed.

   In contrast microscopic performance. Because it is not a standard test board in high contrast, the lens determines the contrast more clarity of the image. People began to find the same high contrast resolution lenses tend to get a higher visual clarity, this lens people took it as "sharp" nikon in the professional field is to "sharp" known. Many experts said the nikon lens to shoot the film has "Daoke chisels" feeling what it means.



  Optical transfer function, referred to as OTF (Optical Transfer Function), is nearly 30 years since the optical field of a very compelling topics at the forefront, but also the people pay more attention to the last decade of a new discipline - the "information optics" important part. In 1948, Americans Scheider (O.Schade) first optical transfer function approach to a new point of view to evaluate the image quality of television camera system, and was a great success.

  Modulation transfer function (MTF) is the actual image and the ideal image of the ratio between the relative spatial modulation frequency.

Modulation of the scene is the image of the scene, or the maximum difference between the brightness and the minimum brightness, the maximum brightness, minimum brightness and the ratio.

   Modulation = biggest bright spot - minimum brightness / maximum brightness + minimum brightness

   A white paper modulation is 0,

   Namely: M King = [(100-100) / (100 +100) = 0/200 = 0].

   A piece of paper printed with black and white lines, if the contrast ratio of 1:100,

   Then M King = 0.98;

A contrast of the printed magazine about 1:15 to 1:20 (the darkest lines, text brightness is 1, then the brightness of white paper about 15-20 are relative reflectance values) is the modulation of approximately of 0.88-0.90 (14/16-19/21).

  Another example is a pair of black velvet dress with his back to the sun, and if we put him and the sun backlighting shot into the screen at the same time, the contrast of the scene can be up to more than 1:100,000, and its modulation of about 1 (99999:100001) .

      Normal human eye can distinguish the minimum value of the modulation system generally 0.05, so at this time the contrast is about 1:1.1.

    "MTF tested using a white standard plate lines of gradual transition through the projection lens. The measured result is the reduction of contrast situation, if the resulting image contrast, and the same as the test target, the MTF value of 100%, which is Ideally the best lens, in fact, does not exist; contrast as if half of the MTF value of 50% value of 0 represents complete loss of contrast, black and white lines is reduced to a single gray; when the value exceeds 80% (20lp / mm lower) already excellent; while values ​​less than 30% even in the 4X6 inch enlargements piece next image quality is still poor. "

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