GHarris
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 p.5 #9 · Pre-order: Sony FE 50mm f/1.2 GM ($1,998) | |
DavidBM wrote:
Yeah. Partly, I think, to record good resolutions figures in the corners wide open, though why you would care is beyond me.
I think high optical vignetting can help give high corner resolution figures.
But a pleasing look into the corners is surely more important, corner to corner sharpness can wait a few f stops.
But it's a brave company that makes a nice looking 1.2/50 that gives bad numbers in the corners wide open in order to have a nice look.
I Hope Sony is that company, but I doubt it.
I think there's a lot in this. A lens' aberrations come more from the edges of the glass than the centre - that's why stopping down, and thus only looking through the central part of the glass, can help sharpness on most lenses.
If the slightly aberrated (higher CA, lower contrast) light from the edges of the glass is simply dimmer than the sharply-focussed, well-corrected light from the centre, then the aberration is not so visible. In a sense, you're softly stopping down even when wide-open.
It might be described as a difference between, for example, the Sigma Art 50mm f/1.4 and the Sony ZA f/1.4. The one doesn't weigh much more than the other. The Sony has little or no CA and a tiny tiny bit better sharpness, but far more vignetting than the Sigma.
The new and old Sigma 85 f/1.4s differ mainly in size, weight, and vignetting. The newer design being the tiniest bit sharper and aberration-free, too, perhaps thanks in part to that same vignetting? Though the optical performance of the old 85 was already at a very high level.
And all of these examples are flawed, there's a lot lot more going on in lens design, in these specific examples and in general, of which I am being recklessly ignorant. Still, I suspect that an added vignette is indeed a way to bring aberrations down, but am happy to learn more from anyone who can correct me.
Is this inherent? I think it's more a matter of precision and cost. To make a large set of lens elements, that is in sum wholly aberration-free without the commonly acceptable corner-cut of vignetting, must require very high standards of manufacturing indeed. The edges of a lens may have the strongest curvature and need to deflect the path of light the furthest from its original course, hence the edges of a lens are usually the hardest to figure perfectly and smoothly. The quality has risen a lot in the past decade but the cost of further progress might be prohibitive eventually.
"Big and heavy" in a lens body doesn't have to mean "low vignetting" - whether because of the near-unavoidable issue with wide-angle lenses on short-flange (mirrorless) mounts or, on both longer and shorter focal-length lenses, because some extra vignetting is a way to help make the lens sharp.
(Again, I apologise for waffling so much, I am ignorant of the subject really and just speculating more than I ought on a topic of no great importance)
The "high-vignetting, sharp, relatively small, relatively inexpensive" design is popular, hence even the older 50mm f/1.4 ZA - and the 55mm f/1.8 - which both go down that design route already, so yes, I'd have thought a possible Sony f/1.2, if the rumours are real, will also exhibit high vignetting in return for a smaller size and cost at a given quality. As someone who likes to do astrophotography I would prefer to have as high a true / usably "low vignette" aperture as possible, but I am a niche market, and should probably have bought the Sigma Art 1.4 already if I want that in 50mm (I imagined Sigma might one day make an f/1.2 that was even a little better than the f/1.4's slightly ageing design... but I'm less sure now - they have usually released first, and cheaper, than a subsequent Sony model, so their window in the market is narrowing a little with this news of a Sony f/1.2 beating them to it).
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