here is the first review of the most compact 900mm lens on this planet:

Tamron 17-70mm E at BHphoto, Adorama, Focuscamera, Calumet Germany and Wex UK.

Opticallimits reviewed the Tamron 17-70mm f/2.8 E lens and concluded:

The Tamron 17-70mm f/2.8 Di III-A VC RXD is a bit like Jekyll and Hyde – it’s a curious mixture of good and bad things making your purchasing decision a bit more interesting than usual. On the positive side, there is, of course, the very high resolution, and that’s straight from f/2.8. It is easily comparable to the best in class in this respect. 70mm may lack a little bit of contrast at f/2.8, though. Image distortions are quite normal for a modern zoom lens for mirrorless cameras – they are quite high at the extreme ends of the range without being excessive (unlike on the Sony E 16-55mm f/2.8 G at 16mm which is almost a fish-eye). Auto-correction will, of course, take care of fixing this. The amount of vignetting is very obvious in RAW files at 17mm f/2.8 and 70mm f/2.8, but, once again, auto-correction is giving a helping hand for this. The same goes for lateral CAs, which are slightly higher than average. The lens has been criticized for the quality of its bokeh, and we can see where this is coming from. Out-of-focus highlights are roughly rendered – but this isn’t unheard of among standard zoom lenses. The bokeh in the focus transition zone is actually quite decent. You may spot the occasional colored ghosting when shooting into extreme light sources, but in more conventional scenes, you should be good.

While we are quite impressed by the optical performance (after auto-correction), the build quality isn’t on the same level. This certainly “feels” like a consumer lens rather than targeting professional users. The plastic used for the lens body is a bit on the cheap side. Our sample also exhibited zoom creeping despite having a fairly lightweight inner lens tube. Tamron also omitted the usual switches for AF and image stabilizer, which is something you’d normally expect in a mid-level offering. The high AF speed and the efficient image stabilizer are on the plus side again.

If your only desire is high image quality, the Tamron 17-70mm f/2.8 Di III-A VC RXD is probably a winner. If you enjoy having tank-like build quality, you should look elsewhere, though.