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Sony Diffraction-Free Computed Fantasy MTF Charts

Get Sony FE 85mm f/1.4 GM at B&H Photo.

I have little doubt that the new Sony 85/1.4 GM, Sony 24-70/2.8 GM and 70-200/2.8 GM OSS will be excellent, perhaps even outstanding (particularly the 85/1.4). The new lenses might even turn out to be superior to the equivalent Nikon and Canon lenses given the technology involved.

Sony’s press discussion places very strong emphasis on MTF of 50 line pairs per mm (lp/mm) in the new G Master (GM) lenses. So what’s with this 85/1.4 MTF 'picture' on the Sony web site?

  • Sony MTF is at 10 and 30 lp/mm... where is the 50 lp/mm stuff emphasized so strongly in the Sony press conference? What a strange disconnect. MTF at 50 lp/mm (or even 40) is far more demanding.
  • Sony MTF is computed from a design, not measured from a real lens. Real lenses have to be built, and can at best only approach the theoretical computed performance and only if built to perfection. When actual lenses are examined (like the 90/2.8 and 35/1.4), real images may paint a rather different picture.
  • Sony MTF does not take diffraction into account. While this has little bearing on the f/1.4 chart, the f/8 chart is pure fantasy (impossible)*. It tells us to expect strong performance at f/8, but it does not tell us what a real lens measured will deliver. At f/8, MTF of the Zeiss Otus 85mm f/1.4 APO-Planar (best 35mm lens ever made) even at 40 lp/mm falls short of 80% MTF. So this Sony chart hugely undermines Sony credibility—it’s not real. Furthermore, internal reflections can drop MTF at f/8 significantly in some conditions: let’s see measured results from a *real* lens Sony.

All that said, what I like about the MTF chart for the Sony 85/1.4 GM is that it suggests quite low field curvature. A consistent performance across the field, is a big plus. For example, even the Zeiss Otus 85mm f/1.4 APO-Planar needs stopping down with distance shots, because it has significant field curvature into the edge/corner areas.

It’s worth pointing out the Zeiss delivers MTF charts that are measured from real lenses on the K8 tester using the appropriate thickness sensor cover glass (while in Oberkochen I was shown how it’s done). But Leica to this day publishes computed MTF that does not even take sensor glass into account, which is why MTF for the new 28mm f/2 (and 28/2.8) does not look better than the prior version—yet the new Leica 28mm lenses are tweaked to perform assuming a digital sensor cover glass. Sigma does well, offering both geometric and diffraction MTF, but Canon and Nikon show fantasy MTF like Sony.

Observe (below) that the claimed MTF for the Sony 85/1.4 GM is on balance claimed to be nearly as good as the MTF of the Zeiss Otus 85mm f/1.4 APO-Planar.

* I first heard the term “fantasy MTF” from a senior optical engineer.

MTF for Sony 85mm f/1.4 GM
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