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PostPosted: Sat Feb 01, 2014 3:27 pm 
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Just stumbled across a news release from Sigma Japan: "From now on, we are going to release “Geometrical MTF” Chart in addition to “Diffraction MTF” starting from the new products that we announce this time."
http://sigma-photo.co.jp/english/new/new_topic.php?id=409
There's an example for a 50mm f1.4 (obviously the old version) where i can hardly find any differences between both MTF-charts.
As they explain further down, geometrical MTF-charts should show better values.
For me it looks like a marketing trick, because they also admit: "Diffraction MTF data [...] is very close to the actual image data."
Hmmpf...

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PostPosted: Sat Feb 01, 2014 4:20 pm 
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Hi Thomas,

Ah, the dreaded MTF charts! :lol:

Maybe now would be a good time to refresh the links to a couple of explanatory (PDF) papers on the Carl Zeiss website:


As for Sigma, on the page you link to they state that 'the advantage of using “Geometric MTF” data is that it is easy to measure and calculate since it does not consider the diffraction quality of light'. While I'd agree that it is easy, or at least easier :!: , to calculate Geometric MTF I'd love to know how Sigma manage to measure such an MTF chart! Do they have a lens testing laboratory in some alternate universe where the laws of optics, and in particular the effects of diffraction, no longer apply. :P

Bob.

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PostPosted: Sat Feb 01, 2014 5:25 pm 
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When I read that sentence from their announcement I momentarily was wondering whether geometric MTF is simply calculated from their design-software while diffraction MT was actually measured. But thinking again I believe this is the wrong conclusion.

On another note: as all their MTF charts only display the results at widest open aperture, diffraction should not play a big role with large aperture primes. That may well be the reason that both examples from the 50/1.4 show no visible differences. But perhaps it's relevant to (super)zooms that have a starting aperture of f3.5-f4.5. But then again: if Sigma really measures their lenses' MTF how can they avoid the effect of diffraction???

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