图解教你如何看懂MTF曲线共22页.docx

1、图解教你如何看懂MTF曲线共22页图解教你如何看懂MTF曲线观察内容的选择，我本着先静后动，由近及远的原则，有目的、有计划的先安排与幼儿生活接近的，能理解的观察内容。随机观察也是不可少的，是相当有趣的，如蜻蜓、蚯蚓、毛毛虫等，孩子一边观察，一边提问，兴趣很浓。我提供的观察对象，注意形象逼真，色彩鲜明，大小适中，引导幼儿多角度多层面地进行观察，保证每个幼儿看得到，看得清。看得清才能说得正确。在观察过程中指导。我注意帮助幼儿学习正确的观察方法，即按顺序观察和抓住事物的不同特征重点观察，观察与说话相结合，在观察中积累词汇，理解词汇，如一次我抓住时机，引导幼儿观察雷雨，雷雨前天空急剧变化，乌云密布，我

2、问幼儿乌云是什么样子的，有的孩子说：乌云像大海的波浪。有的孩子说“乌云跑得飞快。”我加以肯定说“这是乌云滚滚。”当幼儿看到闪电时，我告诉他“这叫电光闪闪。”接着幼儿听到雷声惊叫起来，我抓住时机说：“这就是雷声隆隆。”一会儿下起了大雨，我问：“雨下得怎样?”幼儿说大极了，我就舀一盆水往下一倒，作比较观察，让幼儿掌握“倾盆大雨”这个词。雨后，我又带幼儿观察晴朗的天空，朗诵自编的一首儿歌：“蓝天高，白云飘，鸟儿飞，树儿摇，太阳公公咪咪笑。”这样抓住特征见景生情，幼儿不仅印象深刻，对雷雨前后气象变化的词语学得快，记得牢，而且会应用。我还在观察的基础上，引导幼儿联想，让他们与以往学的词语、生活经验联系起

3、来，在发展想象力中发展语言。如啄木鸟的嘴是长长的，尖尖的，硬硬的，像医生用的手术刀样，给大树开刀治病。通过联想，幼儿能够生动形象地描述观察对象。 目前我们越来越多地接触到MTF值的概念，一些镜头的广告和说明书中也直接列出MTF值图表。很多影友表示不太理解其意义，下文可以会有所帮助。一般说来，“教师”概念之形成经历了十分漫长的历史。杨士勋（唐初学者，四门博士）春秋谷梁传疏曰：“师者教人以不及，故谓师为师资也”。这儿的“师资”，其实就是先秦而后历代对教师的别称之一。韩非子也有云：“今有不才之子师长教之弗为变”其“师长”当然也指教师。这儿的“师资”和“师长”可称为“教师”概念的雏形，但仍说不上是名副

4、其实的“教师”，因为“教师”必须要有明确的传授知识的对象和本身明确的职责。 注意:下文可能帮助你初步了解MTF值,但仅此而已.真正的MTF概念十分复杂,不是这样一篇短文能解释清楚的。与当今“教师”一称最接近的“老师”概念，最早也要追溯至宋元时期。金代元好问示侄孙伯安诗云：“伯安入小学，颖悟非凡貌，属句有夙性，说字惊老师。”于是看，宋元时期小学教师被称为“老师”有案可稽。清代称主考官也为“老师”，而一般学堂里的先生则称为“教师”或“教习”。可见，“教师”一说是比较晚的事了。如今体会，“教师”的含义比之“老师”一说，具有资历和学识程度上较低一些的差别。辛亥革命后，教师与其他官员一样依法令任命，故又

5、称“教师”为“教员”。 MTF = 模量传递函数MTF测试是目前最精确和科学的镜头测试方法.瑞典权威的摄影杂志对它的解释是：MTF测试使用的是黑白逐渐过渡的线条标板,通过镜头进行投影.被测量的结果是反差的还原情况.如果所得影像的反差和测试标板完全一样,其MTF值为100%. 这是理想中的最佳镜头，实际上是不存在的；如果反差为一半,则MTF值为50%.0值代表反差完全丧失,黑白线条被还原为单一的灰色.；当数值超过80%(20lp/mm下)则已极佳；而数值低于30%则即使在4X6英寸扩印片下影像质量仍较差。测试分径向和切向两种方向.如果两者相差较大，说明镜头遭受较严重的像散.较高的空间频率值(即l

6、p/mm值，可理解为分辨率)如30lp/mm与20lp/mm相比, 其MTF值通常较低。注：这里的反差表现在画面中的表现相当于我们所说的“明锐度”。如何解译MTF值：反差/明锐度：5(或10)lp/mm的读数反映镜头的反差表现.即使微小的差别(2.5% !)也能在画面中体现出来!你可以把它看作一种最基本的锐度.一枚好的镜头在光圈收小后应该在5lp/mm下径向和切向同时高于95% .低于90%即表明镜头表现不佳. 一枚明锐度好而锐度差的镜头通常比明锐度差而锐度高的镜头看上去更锐利!不过,锐度和明锐度两项指标通常相辅相成. 锐度：10至40(或更高)lp/mm表明一枚镜头的锐度即再现细节的能力.4

7、0lp/mm表明镜头再现物体非常细微细节（如人像摄影中的头发丝）的能力.此时即使MTF值的差距较大(如10%)也无法直接在画面中辨认出来.按照人眼的辨别力和35mm胶卷的片幅,如果要得到质量非常理想的7英寸的照片,镜头20lp/mm下的MTF值必须大于50%.而要想在16英寸下仍有非常理想的画面质量,其70lp/mm下的MTF值竟须超过63%!几乎没有镜头可以达到这样好的表现!辨别好镜头的简易法则(收小两档光圈)：教你如何看懂MTF曲线40lp/mm曲线(红色)须位于 边缘20%(图形右侧) 中心65%(图形左侧).20lp/mm曲线(紫色)须位于: 边缘45% 中心80% 10lp/mm曲线

8、(绿色)须十分接近5lp/mm曲线. 5lp/mm曲线(蓝色)须于整个X轴上95%以上的MTF曲线评价方法参考自德国的彩色摄影杂志. 其它杂志或机构的评价标准可能会不同.根据MTF曲线对镜头作评价时还须考虑到镜头的不同种类,如对超广角镜头的边缘成像质量不能苛求。大多数情况下切向曲线比上述标准要低一些. 而径向曲线的值相对则较高。注意:镜头在极端光圈（即最大光圈和小于f/16）时相对表现较差!它们的值不代表镜头所能达到的最佳光学质量!最佳光圈通常为最大光圈收小 2-3档。锐度和反差锐度与反差以下图表横轴代表视场半径(距中心距离),纵轴代表MTF百分数值,实线虚线分别代表径向,切向值。Canon

9、EF 1.2 85mm L 于f/5.6 ：表现出色明锐度和锐度俱佳! Canon EF 1.2 85mm L 于f/5.6 Canon EF 2.8 20mm USM 于f/8 ：明锐度相对较好但再现细节的能力有限(切向于40lp/mm)Canon EF 2.8 20mm USM 于f/8Sigma AF 2.8-4.0 28-70mm 于70mm f/8 ：锐度好但明锐度不佳(5lp/mm差于90%) Sigma AF 2.8-4.0 28-70mm 于70mm f/8Sigma AF 2.8-4.0 28-70mm 于28mm f/2.8 (全开光圈) ：表现差无论是明锐度还是锐度! S

10、igma AF 2.8-4.0 28-70mm 于28mm f/2.8 (全开光圈) 浅谈MTF值的含义 MTF值（模量传递函数）是对镜头的锐度，反差和分辨率进行综合评价的数值。对于一个平面黑（白）色物体，它的线对频率是0。此时，任何一个最简易的镜头都可以完整的体现出这一反差。即MTF值等于1。而对于纯黑和纯白相间的线条（反差为100%）来说，随着线对频率的提高，通过镜头表现的反差就相应减少（反差小于100%）。当线频达到一个很高的数值时（例如1000线对/毫米），则任何镜头也只能把它们记录成一片灰色。这时镜头的MTF值就接近于0。因此，MTF值是一个界于0到1之间的数值。这个数值越大（越接近

11、1），说明这个镜头还原真实的能力越强。例如在35mm底片上，10线对/毫米的线对频率时，优质镜头的MTF值为95%左右，而业余镜头的MTF值也在90%左右，这样在普通5寸片上的差别就几乎看不出来。而对于40线对/毫米的线频时，优质镜头的最高MTF值可达70%以上，而业余镜头此时的最高MTF值却只有40%左右。将底片放大到24寸时，被摄体的细节表现差别就充分反映出来。对于分辨率高而明锐度低的镜头，虽然能把一个黑发白肤女郎的眉毛（反差较大）拍的清晰可辩，而对于一个德国北部（灰发白皮肤）或印度（黑发棕皮肤）的女郎而言，由于体现不出来反差，其眉毛也只能是模糊一团了。举一个灰发女郎在白背景前的散发为例（

12、假定反差为0.45）：通过优质镜头后其反差仍然为0.4570%=0.315，而通过业余镜头后反差则降低为0.4540%=0.18。我们知道，在白纸上印刷灰字时，两者的反差小于0.2时读者就很难辨认了，更何况此时的发丝只有0.18的反差呢。 如何判读镜头的MTF值 图中给出了Carl Zeiss T 2885/3.34镜头分别在28mm、50 mm和85 mm焦距段，分别对最大光圈和光圈值为8时的MTF值线图。每个图上有三对曲线。最上面一对是10线对/毫米对应的MTF值，中间一对对应于20线对/毫米，下边一对对应于40线对/毫米。每对曲线中实线代表从图面中心向四周辐射方向的MTF值（径向），虚线

13、代表围绕图面中心的圆的切线方向的MTF值(切向)。两者是同等重要的。横坐标代表从被测点到图象中心的距离，0mm处即为图象中心位置，21mm处代表胶片四角端点的位置。纵坐标则是该点的MTF值。 让我们来看左边的两个图。此时镜头焦距是28mm。在光圈值为f3.3，线频为10线对/毫米时，在胶片中心处径向的MTF值是0.96，切向的MTF值也是0.96；在距中心15mm处，径向的MTF值是0.91，切向的MTF值是0.84。在线频为20线对/毫米时，在胶片中心处径向的MTF值是0.87，切向的MTF值也是0.87；在距中心15mm处，径向的MTF值是0.76，切向的MTF值是0.62。在线频为40线

14、对/毫米时，在胶片中心处径向的MTF值是0.69，切向的MTF值也是0.69；在距中心15mm处，径向的MTF值是0.47，切向的MTF值只有0.34。 再看在光圈值为f8时，各线频下的径，切向曲线数据都有明显提高，说明在光圈f8时，该镜头的光学成像大大好于光圈值为f3.3时的水平。 用同样的方法，就可以读出该镜头在50mm，85mm时两种光圈值下的MTF值。 在MTF曲线图上，还可以看出镜头的焦外成像质量。当径向和切向两根曲线越接近，镜头的焦外成像就越柔和。可以看出，该镜头在28mm焦距时，镜头的焦外成像要比85mm焦距时的焦外成像差很多。 Understanding MTFThe Modu

15、lation Transfer Function ExplainedDo I Really Need to Know This Stuff?Actually, yes, you do. Pay attention. Its important :-)You especially do if youre one of those photographers who enjoys thinking about, reading about and talking about lens sharpness. The reason is that the word sharpness is vague

16、 and can mean different things at different times to different people. MTF on the other hand is the terminology that lens designers use, and which you need to come to terms with if you really want to understand how to describe the complex interaction between resolution and contrast. For the optical

17、designer contrast and resolution are in conflict. Increase one and you reduce the other. Various lens makers have differing philosophies in the regard. Historically, for example, Zeiss was reputed to design their lenses for maximum resolution, while Leica apparently tended to favour maximum contrast

18、. It is design decisions like these that account in part for the differing looks of different lens brands.Resolution & ContrastHeron Stalking Algonquin Provincial Park. June, 2019Canon 10D with Canon 500mm f/4L IS lens ISO 400Resolution and contrast are inseperably bound. Think of a series of altern

19、ating lines; black, then white, then blank, then white again. What differentiates them is their contrast. Make the black lines lighter and the white ones darker and eventually you wont be able to tell them apart. In other words, if you have a white line on white paper you have no contrast and theref

20、ore no resolution. The line will be invisible. Keep this in mind, because a discussion of resolution without taking contrast into account is meaningless.Resolution is usually measured in lines per millimeter, or line pairs per millimeter. (L/mm and LP/mm). Be careful not to confuse the two. These ar

21、e not the same. Typically engineering types refer to lines per millimeter, rightly assuming that to have a black line one must also have a white line. But, photographers tend to be less rigorous in their thinking and so line pairs per millimeter is commonly used. Just be aware of what is being discu

22、ssed, but also be aware that numerically L/mm is double LP/mm. 50 L/mm to an engineer means 50 line pairs, because as discussed above every black line must have matching white line. This is similar to the confusion that exists between DPI (dots per inch) and PPI (pixels per inch). Different, to be s

23、ure, but similar enough in common usage so you need to know what each is actually referring to.The next thing to realize is that resolution is not a quantifiable scientific absolute. It is totally subjective, and therefore is different between individuals as well as with the same individual at diffe

24、rent times. While one observer may be able to see a resolution of 50 L/mm on a particular test, another may see 45 L/mm or 55/ L/mm. A 10% variance with the same observer at the same time is not at all unusual. Add to this the other variables, such as focusing errors, and Ive been told by experts th

25、at a variance of as much as 30% in such tests is not uncommon.And, to make things even more indeterminate, it can clearly be shown that the actual image quality produced by a lens does not correlate at all well with its measured resolving power. For this reason resolution test results alone should n

26、ot be considered a valid measure of a lens goodness. This is why the optical industry as well as those that really want to understand how a lens is performing rely instead on its MTF.MTF A DefinitionFig. 1MTF of Canon 50mm f/1.4 lensOf course you want to know how to interpret an MTF chart, and well

27、get to that most important aspect in a moment. But first take a moment to understand how MTF is defined. An ideal lens would perfectly transmit 100% of the light that passes though it. But, no lens is perfect, and therefore there are losses. When these losses are measured in terms of contrast this i

28、s called the modulation of contrast. In other words, how much contrast is lost modulation simply being another word for variance.This modulation of contrast is measured at different spatial frequencies. What does that mean? Simply, the number of contrasting bars on a test target, ranging from 0 to 1

29、00 line pairs per millimeter. Look at Figure 1 above. It shows the MTF chart for a Canon 50mm f/1.4 lens. Ignore the squiggly lines for a moment and look at the vertical and horizontal axes. The vertical axis shows numbers from 0 to 1, which is really shorthand for 0 to 100%. Therefore .10 means 10%

30、 contrast, .5 is 50% contrast, and 1 is 100% contrast.The horizontal axis is in millimeters and shows the distance from the center of the image toward the edges. In other words, 0 on the horizontal axis is the center of the lens and 20 is 20mm out from the center. Since a 35mm frame is nominally 36m

31、m across this point is therefore at the edge of the frame.Are you with me so far? To summarize the higher a point is on the chart the higher the contrast transfer capability, and the further to the right one is the further from the center of the image.Now lets talk about the measurement lines. Youll

32、 notice in Fig.1 above that there are blue as well as black lines, and thick as well as thin ones. Each of these line types is also shown as solid and dotted, for a total of 8 different line types. Heres what each represents;The thick lines are measurements taken at 10 LP/mm (low spatial frequency, or low resolution) and the thin lines are at 30 LP/mm, at higher frequency / higher resolution.The black lines are measurements taken wi