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+--- Thread: MTF50 figures in lens test. (/showthread.php?tid=893)
MTF50 figures in lens test. - Polkaha - 02-19-2016
Very much do I like the lens tests that Photozone is making available to us, they are very helpful and easy to understand.
However, looking at some lens tests with a 20Mp Canon full frame camera, I have a problem with the MTF50 figures.
The used 21Mpixel 5D MKII to be seen in some pictures with the test lens mounted has 3,744 pixels vertically.
One would expect a maximum MTF50 of something like 3,744/sqrt2=2,647 LW/PH.
But as an example. the test for a Canon 24-70mm F/2.8 II shows a figure of 3,732 LW/PH at F/4.0 in the centre, almost exactly the full amount of 3,744 pixels from the 5D MKII.
In case the resolution of the lens would be an extremely high 6,000 LW/PH (=125 LP/mm) combined with an ideal 21Mpixel sensor, the MTF50 figure would become (1/3744^2 + 1/6000^2)^-1/2= 3,176 LW/PH, the MTF now already being reduced to 85% of the total amount of pixels.
​To achieve the figure of 3732 with a sensor of 3744 pixels, the lens should have a resolution of 46,700 LW/PH or 972 LP/mm, and that just with a sensor that has its MTF50 up to the full amount of pixels, which seems just as impossible because of Bayer demosaicing.
Am I overlooking something?
Could it be that this particular lens has been tested with another camera with (much) more pixels?
MTF50 figures in lens test. - Brightcolours - 02-19-2016
The sharpening applied probably accounts for the inflated numbers.
MTF50 figures in lens test. - Polkaha - 02-22-2016
You are right, sharpening can blow up MTF50 figures quite a bit, but not to all the way up to the amount of pixels.
MTF50 figures in lens test. - Polkaha - 02-23-2016
Sharpening can indeed improve MTF50 coniderably.
But it cannot improve that much that this figure lands on the amount of available pixels.