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For video it is also very interesting on the S5ii(X) : You would use it instead of the 28-200mm.
With hybrid zoom the 28-200 would be 28-300mm, but this camera gives you a much larger 24-450mm reach FF equivalent (Without hybrid zoom ofcourse, you are already in APS-C).
It's reassuring to see that Sigma haven't left the super-zoom market but it would be nice to have a FF one - all they'd need to do is make it start at 24mm and I'm sure it would trounce the Panasonic 28-200 in usefulness (and probably IQ).
Correct. It will only use the central 50% area of a FF sensor. It will work on any L-mount camera but on FF it will only use that central part of the sensor.
I've never used a crop sensor on a L-mount body. Do the raws get smaller as well as the JPEGs?
With the smaller body of the S1RII, this certainly could be a compelling alternative to the 28-200 for travel, etc. Usefully wider, and much longer - even if you account for cropping in post from the 28-200.
Of course, it's significantly larger and heavier, but it's good to have options.