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osv2 wrote:
no doubt it's hard to understand for a sony newbie such as yourself, who just joined fm a month ago... and there are things that the manual doesn't tell you, which doesn't help either.
modern e-mount bodies focus in two ways:
1) the camera body thinks that it's an e-mount lens(20fps max a9/a9ii or ~10-11fps max a7/a6 series, both are sony limits)
2) the camera body thinks that it's an a-mount lens(10fps max a9/a9ii or ~2.5fps max a7/a6 series, both are sony limits)
so e-mount af adapters have to emulate either of those but nothing else... they either allow you to choose which one to emulate(metabones), or the adapter decides for itself which lens type it emulates(mc-11)... for instance:
supported sigma-brand glass on mc-11 is #1, the best-case scenario
canon-brand glass on mc-11 is #2
3rd-party e-mount lenses will of course be #1, because the camera thinks that they are e-mount lenses, which they are.
so why don't we see any 20fps 3rd-party e-mount lenses? is sony limiting it? probably not, there are many cases of 15fps af-c sigma lenses for example.
sony itself can't make all lenses do 20fps af-c on the a9:
25 sony lenses that'll do 20fps max
26 sony lenses that'll do 15fps max
http://support.d-imaging.sony.co.jp/support/ilc/products/ilce9/continuousshooting/en/index.html?id=spt
note that most of the 15fps af-c lenses are older designs, i doubt that there are any 15fps lenses that have the latest sony xd linear af motors? xd linear af motor = 20fps lens, and in some cases even faster than that with future bodies, according to hints that sony dropped with the fe400(?).
it's not just sony, focus motors limiting af speed is a problem with the r5, where the manual states that many 1st-gen canon lenses can't even do 12fps with the r5... and then there is nikon putting slow stepper motors in z-mount lenses, it's so slow that the dpr z50/1.8 review said that: "it's at least as fast as most of the company's older primes" :-0
beyond all that, sony doesn't provide any standardized method for measuring af-c speed... i usually reel off a really long af-c burst with the a9, while the target is changing distance to the camera, so the camera has to constantly re-focus, then i read the shot times in the exif.
i don't know how accurate that is, and unfortunately people never specify how they determine af-c speed in their own testing... i've literally never seen anyone else reference exif times, so i don't know how other people measure it.
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The burst tracking rate doesn't seem to have anything to do with AF motor speed. The slowest focusing Sony FE lenses officially support 20 FPS: 50mm F1.8 and 50mm F2.8. It is likely something similar to the change made on A-mount. Sony claimed the A-mount lenses that support 10 FPS on the LA-EA3 had 4x faster tracking compared to the older ones that only support 8 FPS. They talked about a new LSI circuit. The change that was made was likely the lens communication rate. Sony may eventually release II versions of the older FE lenses adding 20 FPS support and perhaps aperture drive support.
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