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p.3 #1 · p.3 #1 · LensRentals: Planar T* FE 50/1.4 ZA is absolutely superb! | |
Fred Miranda wrote:
I bet the big tangential mid-field dip is due to the thickness of the glass cover skewing the results on that area. The 50/1.4 ZA has a mid-field dip but perhaps its effect was exaggerated. I have tested many lenses with a mild mid-field dip being grossly exaggerated by the thicker glass cover. (WATE as an example)
Perhaps Roger can weigh in on this.
I can speculate a bit. When we first started looking at cover glass thickness it was basically the SLR to micro 4/3 lens conversions. m4/3 has a 4mm cover so that 2mm difference was pretty huge. So was the Leica film to other camera (1 to 2mm difference).
We knew that SLRs from the same manufacturer varied a bit, maybe 1.8 to 2.2mm thickness so we figured that was pretty inconsequential and basically tested at 1mm (Lecia), 4mm (m4/3) and 2mm (everyone else) and that seemed fine. Just recently, though, it was pointed out to me by a really knowledgable person that with the short backfocus distance of a mirrorless (say 18mm) the difference might be more critical than on and SLR with it's 42mm or so distance.
Looking at some lenses like this one and the FE 85mm f/1.4, we see a bit of mid-range astigmatism that looks like what we saw with those big difference in glass thickness tests. Might be paranoia, but hey, could also be real. The only answer I have is to sacrifice an FE camera, get the actual glass, and use that instead of our 2mm wafers. Which will take a while. It's hard getting the full stack of glass off the sensor without cracking it :-)
Of course, if we sacrifice some cameras, then there will be no difference. That's how the scientific method works in my hands.
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