Resolution, contrast, MTF "s]
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By Lars Kjellberg 0QPY+6
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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. cfv:Ld m
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Resolution, contrast, MTF By Lars Kjellberg C@]D*k
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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. Wf5;~RJC?
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The object 4|5;nxkGm8
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%. ??P\v0E
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. -Q5UT=^
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. ZnAQO3%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. q27q/q8
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The pedestrian crossing Wtk|}>Pf
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. YryMB,\
But when we look at the white stripes with a loupe we find narrow black cracks in them. These are much finer lines that are a lot more difficult to reproduce with their contrast intact. The white next to the cracks blends in with the black. The contrast between black and white has decreased considerably. This is where we can sift the wheat from the chaff because a low-grade lens cannot handle such fine lines. {:_*P
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When talking about the contrast between such fine lines it is easy to understand that we are also discussing sharpness. The lower contrast with which a simple lens reproduces the lines can also be interpreted as poor sharpness. J"GsdLG.-
From this we can conclude that sharpness and contrast are really the same thing, but with the difference we perceive it as (and call it) contrast when looking at coarser details (low line frequencies), and as sharpness or resolution when we look at finer details (high line frequencies). q&&"8.w-
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Modulation Transfer Function }S')!3[G
Put simply, MTF-measurements assess the contrast between black and white lines of differing thickness or line frequency, and give an objective measurement of a lens' performance. xZ%3e
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The MTF gives us a measurement of how much contrast remains between white and black lines after they have been projected through a lens. How big was the difference, or contrast, in the original? What percentage of the original contrast is left after projection? If the MTF for a particular line frequency is 0.85, that means that 85% of the original contrast remains after projection. This decrease in contrast does not only affect white and black surfaces; the difference between dark grey and light grey surfaces also decreases by the same percentage. X@Zt4)2#
Common to all lenses, regardless of price or quality, is that they all have nearly 100% MTF at very low line frequencies, in other words when coarse details are reproduced. They also all have 0% MTF at very high line frequencies. No photographic lens in the world can reproduce 1000 lp/mm successfully. This means that all lenses start at 100% and end at 0%. The lens' performance in between can be described as a curve. %x@bP6d[
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The X-axis shows the line frequency from zero upwards. The further along the x-axis you go, the finer the detail. The Y-axis shows the MTF from 0 to 100 per cent. The further up the Y-axis, the better the subject's contrast is reproduced. If the difference in darkness between two surfaces is identical for the original object and the projected image, the MTF is 100%. This is rare. Most of the time you get a slight decrease in the contrast. By measuring the percentage of difference in blackness that remains after projection, you get the MTF-value. *\}$,/m['
This MTF diagram is not the same as the ones we present when testing lenses. In this diagram we are only looking at a small part of the projected picture, for example the centre of the frame, and give an MTF value for all line frequencies at this point. The diagrams we are presenting along with the tested lenses show the MTF value at three line frequencies but for the entire picture, from the centre (far left on the X-axis) to the corners (furthest right on the X-axis). ht6}v<x.eA
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Resolution and Contrast FYE(lEjxi
You often hear people make comments like: 'This lens is sharp, but gives a low contrast'; or 'That lens has a high contrast, but it isn't that sharp.' It is not always very clear what they mean. It can be a question of multi-coating. A lens with poor multi-coating can in a certain light, especially in direct light, give a lower contrast without a deterioration in resolution. NYg&