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Modulation transfer function MTF(二)
Views:2226 Date:2013-03-05【Back】

1, what is the spatial frequency

        In clarifying the MTF curve with the variation of the spatial frequency before, let us first understand what "spatial frequency." Spatial frequency (Spatial Frequency) The concept is very similar to the concept of the resolution, the unit is "line pairs / mm" (lp / mm). However, the resolution test target board is a group of a group of white sharp contour lines, the distance between each two lines, and the ratio of the width of the lines of a given value, the current requirements of the resolution of standard plate, as this the common factor is 20 √ 10 ≈ 1.122 geometric progression; the spatial frequency is called "raster" of the standard plate test, which lines are gradual transition from black to white, and the spacing and width of the lines but also by dilute to dense, wide to narrow gradually transition

070418_mtf_02

070418_mtf_04

40lp/mm curve (red) Edge> 20% (graph on the right) Center> 65% (left of the graph).
    · 20lp/mm curve (purple) edge of> 45% Center> 80%
    · 10lp/mm curve (green) are very close to 5lp/mm curve.
    · 5lp/mm curve (blue) across the X-axis> 95%


Note: In extreme lens aperture (ie, the maximum aperture and less than f/16) the relative poor performance! Their value does not mean that the lens can achieve the best optical quality! Optimum aperture is usually smaller 2-3 maximum aperture closing files .

    

     A spatial frequency performance of the photographic lens, the resolution is determined by its; and its modulation transfer function value (i.e. the value of MTF) level, is determined by its contrast. Therefore, this MTF curves, very concentrated reflection of the camera lens in the most important two basic parameters. If so far still on the "spatial frequency", "Harmonized System" and "MTF value" concept is not clear, then, might put these three concepts equivalently understood as "resolution", "Contrast" and "contrast" does not affect the right understanding of the meaning MTF curve.

Several typical shape of the MTF curves

    Here we are still in the horizontal axis is the spatial frequency of the MTF curve, for example, talk about the shape of the curve and the relationship between the quality of the lens.

 070418_mtf_16 As shown, A, B, C are three curves representing three completely different optical quality photographic lens. Wherein A, B represents common two lenses, the same resolution and contrast are not typical of the camera lens, the lens A is the contrast between the high and low resolution; lens B is just the opposite, high resolution and the contrast is low. The Lens C is a very rare, contrast and resolution are high-quality photographic lens.
          

Lets take care about the lens A and B properties.

      Represents A, B two lens MTF curves have a common intersection point P, the point P corresponds to the spatial frequency (or roughly understood as resolution) of 24lp/mm; while the MTF value at P 0.33. Because the human eye can distinguish the low modulation is 0.05, when the scene modulation 0.15 (contrast ratio of 1:1.35) is, MTF value = 0.05/0.15 = 0.33. At this point two numerical resolution of the lens, are 24lp/mm, when the scene contrast is less than 1:1.35, the lens A higher resolution than B, ie the lens A on low contrast details of the scene better than B good; when the scene contrast is higher than 1:1.35, the lens B, resolution is only 37lp/mm.

      Lens A and B have their own advantages. In the performance of weak luminosity contrast, small differences in shading and soft color changes, the lens A has obvious advantages, it photo shoot rich layers, shadow patterns and delicate, bright colors, strong texture; when shooting high-contrast shadow objects, the lens B extraordinary performance, it is like shooting black and white lines sharp knife carving chisels general.

      From the use of point of view, the lens A more suitable portraits and advertising; while photographing lens B is more suitable for text, painting, printing plate and microfilm. A lens is a clear German style; while Lens B more Japanese specialties. Some people say that the lens A landscape photos taken like painting; while the mirror B performance bucolic like prints. It is really wonderful analogy.

      Of course, only the lens C is more adapted to any subject matter are versatile photographic lenses, which have both clarity texture, both crisp another level, both crystal clear and delicate and soft. Is the cream of the crop of the boutique. However, lenses are generally more inclined to like A or B.

      Simply look, an integrated optical lens quality level can be used with both horizontal and vertical axes MTF curves enclosed area under the line to determine the size. MTF curve line area of ​​the lens, the optical quality must be good, because it is certainly the contrast and resolution are high, or one in which a significantly high. So more lenses A and B, which only better, not simply the conclusion, the curve of the line as they are approximately the same size, contrast, and resolution are both off than B higher than A, so its the MTF curve of the line area ratio A, B both are large. Here we want to point out is that generally only high-end professional camera lens manufacturers, finally agreed to publish it in the MTF curve, so we see the MTF curve, the area under the line are larger (but generally are lower than if Lens C as big), but the actual lens, such as a class A and B is very common, and even some far better than them.

      Now, we can already see very clearly: a photographic lens optical transfer function modulus is how both accurate and comprehensive description of the integrated optical quality of the lens, and how to clear tax resolution and degree of these two important optical indicators combined together, learn to look to understand the amount of the transfer function curve, the effect of the pursuit of high-quality photographic images more necessary personnel.

MTF curve analysis of the basic essentials


    1.MTF curve the higher the better. The higher the quality the better description of the optical lens.
    2.MTF the area below the curve contains the bigger the better. Comprehensive contrast and resolution of view, MTF area below the curve contains the bigger the better.
    3.MTF curve becomes flat as possible. The more flat, indicating the edge and the middle and good consistency. If the curve right serious decline, indicating that the lens edges, especially corners contrast with the low resolution.
    4.S curve and the M curve closer the better. Smaller distance between the lens astigmatism small.
    5 The higher frequency curve, the greater the contrast lens. Low frequency (10 line pairs / mm) MTF curve represents the contrast lens characteristics. This curve is higher, the greater the contrast lens.
  

    6 high-frequency curve is higher, the higher the resolution the lens. High-frequency (30 line pairs / mm) MTF curve represents the lens resolution characteristics. This curve is higher, the higher the resolution camera.
    7 best aperture MTF performance curves. In general, to the MTF performance curves reflect F8 lens ideal conditions for optimum performance. It is any photographers are very strict valued properties. However, most manufacturers do not provide this MTF performance curve, the maximum aperture provides only the MTF performance curves.
    8 maximum aperture performance curves. Maximum aperture MTF performance curve, reflecting lens corners should at least reach performance. In a compromise between money and large caliber, it must be this performance as an important factor to consider.

MTF curve analysis should be noted that the problem
    1 do not do different focal length lens horizontal contrast. Telephoto lens, the edges of the image field is only equivalent wide-angle lens near the center position. Therefore, compared with the wide-angle lens telephoto lens MTF of the edge, wide-angle lens will come to the wrong conclusions are very poor shot. Wide-angle lens, especially ultra-wide-angle lens edge MTF value decreased a lot is a normal phenomenon. For wide-angle lens, especially ultra-wide-angle lens, it must be like the edge of the field values ​​of MTF quite tolerant.
    2 Do not put large aperture (eg F1.4 or F2.0) lens with a normal lens do horizontal comparison. Normal lens "maximum" aperture, large aperture lens aperture than twelve small block or more, between the "maximum aperture" totally not comparable. Large aperture lens, designed to cater to maximum aperture effect, and do a bit of compromise on other properties. Therefore, we must large aperture lens maximum aperture MTF somewhat forgiving.

   3 Do not zoom and fixed focus lens horizontal contrast. Compared with the fixed-focus lens, lens structure is much more complex, designed to take into account the factors that should be much more, so some features as fixed-focus lens is a normal phenomenon. In other words, the zoom lens is fixed focus lens dispersion ratio larger dispersion is normal.
    4 upscale upscale ordinary inexpensive lens and the lens is not directly economic contrasts. Buy cheap lens, generally focus on the optimal aperture (F8) MTF characteristics, well aware of the effect on the maximum aperture can be. In particular, note that: economy telephoto zoom lens at the telephoto end, even F8 "Best Iris", compared with the high-quality lens is also quite different, you must be careful when purchasing. If in addition to general family photography, but also want to engage in the creative, think twice before buying this type of lenses needed.

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