So, I bought the Smart Adapter II 2-3 months ago and of course the Speed Booster is announced shortly after (doh!). However, after ordering the Speed Booster, I'm glad I have both the adapters for the NEX series cameras - here's why.
First off, the Speed Booster is a very cool device. Not only does it increase the field of view by using more glass in your lenses (effectively turing your APS-C sensor into an almost Full-Frame) but it also lets you decrease the aperture around 2 stops. Here's the pros and cons of that IMO:
Wider lens = more content in frame
Smaller aperture competes with the DoF in much more expensive lenses (i.e. my Canon 1.8 is now a 1.3)
Low light situations are generally easier (since the iris can go lower, you don't have to crank the ISO)
A wider lens isn't always great - you are essentially using an entire new set of lenses with the speed booster so those telephoto shots you achieved at let's say 70mm APS-C will be dramatically more telephoto than the wider 70mm speed booster frame.
Chromatic aberration - is not terrible, but it's there. I wasn't able to make it too noticeable (which is a good thing) but I'm worried the more glass the image has to go through (let's say a 200mm lens) you would be able to see it more.
Zoom lenses under 16mm are hopeless. The Tokina 11-16mm I used caused severe vignetting as you can see. However, with anything below 16mm with the speed booster is practically a fish-eye, so I wouldn't want that effect anyways. This might not occur on primes.
Because of those reasons, I'd like to hang on to my Smart Adapter II so I can achieve more telephoto shots when I want to and I'm not limited on my wider zooms.
This is a great tool to have. If you have good EF lenses and a Sony NEX camera, I can't see why anyone wouldn't want this. Optically it just makes sense - It's just repositioning the flange distance with no strange components to mess with your image. If something was sacrificed to achieve this, I would be wary of it but nothing does :).
I'm not going to comment on the autofocus since I'm a video guy and I have little use for it with DSLR lenses.
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