Resolution, contrast, MTF
HD z"i pwF])uf*{\ By Lars Kjellberg
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lNk There are different ways to achieve sharper pictures. One of them is to choose a large format film, another is to simply use a sharper lens. In this article I will try to explain what a modulation transfer function (MTF) is and the difference between sharpness and contrast. I will also explain how an MTF test graph should be interpreted.
Xj:\B] v] 1z@ ncqe Resolution, contrast, MTF By Lars Kjellberg
0f;|0siTAm t,kai6UM There are different ways to achieve sharper pictures. One of them is to choose a large format film, another is to simply use a sharper lens. In this article I will try to explain what a modulation transfer function (MTF) is and the difference between sharpness and contrast. I will also explain how an MTF test graph should be interpreted. The terms sharpness, resolution, contrast, brilliance and MTF are legion and not always easy to differentiate. What is the difference between sharpness and contrast? At first glance this seems an elementary question. Sharpness is how well-focused or blurred our pictures are, contrast is a measure of how great the difference is between their lightest and darkest parts. Think about it further though, and you will find that this description is incomplete. It all depends on what you intend to depict. Sharpness and resolution are two ways of expressing the same thing. Brilliance is a word that pops up here and there, although what is meant by it is more doubtful. MTF stands for Modulation Transfer Function and will be explained in detail in this article.
lO}I>yo}\ KnK\X>: The object
ff aMF~+ Taking a picture of an object with a completely black or completely white surface is not difficult. It can be done using the bottom of a bottle as a lens. The surface will come out black or white in the picture. But what about the sharpness and the contrast? Well, the sharpness is perfect and the contrast is close to 100%.
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n@ But if we imagine photographing a more complicated object, true reproduction immediately becomes more difficult. Imagine an object that consists of black lines on a white background. We want the lines reproduced so that they are 1/2 mm thick on the negative, that is, each millimetre of the negative contains one black line and one white line. We now have a line frequency of one pair of lines per millimetre (1 line-pair/mm, or 1 lp/mm). This will appear sharp even if a simpler lens is used, but how do we judge the contrast? Contrast is the difference in darkness between the black and the white lines. By measuring the difference between a white and a black line on the subject, and comparing this to the contrast in the picture produced by the lens, we get an idea of how well the lens reproduces contrast for this line frequency.
?Uhjyi Now imagine thinner and thinner lines (higher and higher line frequencies) and consider the contrast between the white and black lines. The thinner the lines become, the harder it is for the lens to reproduce them sharply and with an acceptable contrast.
"2T* w~V&y You might well wonder what the point of all this is, since you do not normally take pictures of black lines on a white background! But it is the case that photographic subjects are made up of finer or coarser details that generally are darker or lighter to different degrees. The actual optical part of photography is about projecting finer or coarser details and their different colours and grey tones onto film, and making it look as lifelike as possible.
Owh:(EJ"d lW]&a"1$ The pedestrian crossing
2}5@: cwR+ Imagine taking a photograph of a pedestrian crossing, consisting of a black, metalled road with wide white lines on it. We photograph this from not too far away and take a look at the negative. We have chosen our distance and focal length so that the black and white stripes will be 2 mm wide on the negative. The contrast between them will be high because even a bad lens is capable of reproducing such broad lines clearly.
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