Автор: Lighter
Дата: 10-01-08 15:43
Understanding the MTF graphs, numbers and grades
Understanding the MTF graphs, numbers and grades we have been testing lenses with Hasselblads Ealing MTF equipment since 1991. All MTF reports are made with the same equipment and by the same operator (Per Nordlund at Victor Hasselblad AB).
So far we have tested lenses only at infinity.
We measure MTF at 10, 20 and 40 lp/mm, where 10 lp/mm (line pairs/mm) means 10 black lines with 10 white lines in between, for each millimetre.
The lens with the highest MTF in our tests (so far) is the Canon EF 200/1,8L USM.
Photodo test result: 4,8
Higher is better. 4,8 is the grade on a scale going from 0 to 5. The grade is based on the average weighted MTF for the lens. No other variables, such as distortion, flare, or ghosting are taken into account. We chose 0,88 for a grade of 4,8 to make sure that we will never hit the "roof" (>5). See more at Average Weighted MTF: 0,88 below.
Effective focal length: 195 mm
Often the real focal length differs from the one given by the manufacturer. 195 mm is the length we measured with the Ealing.
Weighted MTF for 200 mm: f1,8 0,82, f2,8 0,89, f4 0,90, f8 0,87
Higher is better. MTF is measured 0, 3, 6, 9, 12, 15, 18, and 21 mm from the image centre (on axis), at 10, 20 and 40 lp/mm, in two different directions (sagital and tangential). That is all together 48 measurements at each focal length and each f-stop. Rather than listing all these numbers we calculate a weighted average number for each f-stop of all lenses and each focal length of zoom lenses. The centre of the image is more important than the edges, and 10 lp/mm is more important than 20 and 40.
The maximum MTF is 1,0, but due to optical imperfections and diffraction, a perfect score is impossible to reach at these line frequencies.
The MTF at large apertures (f1,0 - f2,8) is limited by optical imperfections (with major differences between lenses). At small apertures (f11 - f32+) MTF is limited by diffraction (with minor differences between lenses). For example the Canon EF 200/1,8L USM has its peak at f4.
We don't measure beyond f8 because lens performance at small apertures is limited more by diffraction than by optical quality. At very small apertures all lenses are about equally bad.
Average Weighted MTF: 0,88
Higher is better. When we report an average MTF number for all f-stops at one focal length we use f4 (or wide open, if f4 is not attainable) and f8. Most lenses have their highest MTF at f8. High-speed lenses almost always have a very low MTF at full aperture. That is why we decided to use f4 and f8 as standard apertures. Since MTF for f8 is slightly more important than for f4, we assign weights of 60% for f8 and 40% for f4. In the case of zoom lenses, we report the weighted average MTF at each measured focal length.
Weighted MTF 10 lp/mm: 0,94
Higher is better. 10 lp/mm is important for evaluating the overall contrast and resolution when making small enlargements (10 x 15 cm or 4 x 6 inches).
Weighted MTF 40 lp/mm: 0,71
Higher is better. 40 lp/mm is important only when you intend to make large prints of high quality. If you are going to use the lens with fine grain film, the camera on a tripod, and make big enlargements, then it is important to choose a lens with a high MTF at 40 lp/mm.
Max distortion: -1,1 %
Closer to zero is better. Distortion is when a straight line near the edge appears curved in the image. If the line curves out in the middle, it is due to negative (barrel) distortion of the lens
Positive or pincushion distortion is when straight lines curve inwards in the middle.
The graphs
The graphs show MTF in percent for the three line frequencies of 10 lp/mm, 20 lp/mm and 40 lp/mm, from the centre of the image (shown at left) all the way to the corner (shown at right).
The top two lines represent 10 lp/mm, the middle two lines 20 lp/mm and the bottom two lines 40 lp/mm.
The solid lines represent sagital MTF (lp/mm aligned like the spokes in a wheel). The broken lines represent tangential MTF (lp/mm arranged like the rim of a wheel, at right angles to sagital lines).
On the scale at the bottom 0 represents the centre of the image (on axis), 3 represents 3 mm from the centre, and 21 represents 21 mm from the centre, or the very corner of a 35 mm-film image.
Separate graphs show results at f8 and full aperture. For zoom lenses, there are graphs for each measured focal length.
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