I bought this 'lens for all situations' for go anywhere / everywhere use. Mainly because it's small enough to take with on bicycle rides. But also to pack for trips anywhere. Because, in decent light, it has such an extraordinary range it should be able to capture almost anything.
I thought long and hard about this one. Over many months. Didn't want to get it unless it was good enough. I've been blessed to use some very good lenses. No point in adding to the collection if it wasn't going to perform adequately and, consequently, never get used.
Well i've had it for two days and about 30km on the bicycle and so far I'm more than impressed. This lens is able to surpass my expectations both at 18mm (effectively about 27mm field of view on Nikon D7200 / D500); and at 400mm (600mm fov on D7200 / D500). Ok, it's not quite as sharp, or as contrasty and free of chromatic aberrations when shooting into the light, as my Nikon 200-500mm F5.6. But that thing is a huge brilliant beast of a lens by comparison; and couldn't accompany me on bicycle rides. And it sure beats my iPhone at 28mm (think the iPhone starts at about 28mm) for detail when you pixel-peep... as it should.
So a few practical observations. I'd heard the Vibration Compensation was good for about 2 stops and I'd say that's probably how it feels. It's not the best; but, for what it is, it feels stable enough. Sharpness is certainly adequate; even impressive considering the range they've achieved. I've not pixel-peeped the edges. But, for my bird photography, the centre's are sharp enough. Also the fall off from in-focus to out-of-focus areas is smoother and more pleasing than I had hoped for. Contrast and colour also seem better than I'd anticipated. The lens is prone to purple fringing edges shot against a harsh white sky. The hood is quite shallow and a longer one might help shield the front element more... who knows. But fringing is very easily corrected in post-processing. Yes one loses some colour/contrast/sharpness in the process but, in an ideal world, we wouldn't be shooting subjects against harsh white sky. Birds in flight will do that to you sometimes. Many lenses are prone to this issue in extreme situations. Some are better than others. This one is probably about average in this area. But in all other areas, for what Tamron have achieved in such a huge zoom range, it's very quite remarkable indeed.
And speaking of birds in flight I had no difficulty getting focus lock with this lens on the D7200. I haven't tested it on the D500 yet... but it goes without saying that focus will be effortless on the D500.
Lastly: build quality. Looks / feels pretty solid in the hands. I believe it's even fairly well weather-sealed. Wow how far we've come. Ten years ago I started with an 18-200mm. Now I'm playing with an 18-400mm and loving it. That's what this is about: having fun. Being able to take it everywhere and not worry too much. Ok, here are some shots from the first two days. All with the D7200, all handheld. All locations reached by bicycle with the exception of the Spotted Dove on my back fence.
Australian Black Swan : The ecstasy of bath time! (350mm 1/3200s F6.3 ISO500)
Australian Magpie : Why use two if one will do? (400mm 1/640 f6.3 ISO500)
Soon the leaves will begin to fall (18mm 1/500 F6.3 ISO500)
Autumn colours everywhere (55mm 1/160 F6.3 ISO100)
Cattle Egret (400mm 1/3200s F6.3 ISO320) - Yes I exposed for the bird, not the background. And it was very dark almost raining at the time. The bird was far away and am so impressed with the clarity here!
Spotted Dove on my garden wall photographed through glass window (400mm 1/250 f6.3 ISO500)
Masked Lapwings under an overcast sky (400mm 1/1000 f6.3 ISO320)
Cats and dog at the Knox Vet (18mm 1/400 F8 ISO100) You can see from the sky what the weather was like : Mostly cloudy with more than a hint of rain!