1. Good image quality of the photos so that they appear when printed on a big Full HD TV or A4 good
2. High focal length range in order not to depend on an objective change in mountaineering
With respect to the first point, I can only say good things so far about the Tamron. From 18 to 270 mm I feel the power as well and perfectly adequate for my needs. In the area betw. 200 and 270 mm, the sharpness could be a bit higher, but that is for me rather 'whining at a high level'. The color and contrast are very good at the right camera settings. I see myself here as ambitious amateur photographers, and am familiar with the photo-specific parameters quite good, but I am not 'pixel sensationalist'. Images must primarily work well, so I also can not tell if any a bit more or less drops the sharpness in a corner. The landscapes in the mountains me so far nothing negative to the image quality is certainly noticed, and also macro shots of flowers, etc. are very good. In particular, the depth of field and the autofocus works very well (at telephoto and macro shots I focus manually, however).
A distorting effect on the edges at 18 or 270 mm I can not see looking at the pictures.
2. To point (subject changing lenses), I can say one thing very clear: If you already have a lot of luggage on the mountain tours here, one is glad not to have to take even a second lens. I had ordered and tested for comparison with the Tamron also a Canon EF 70-300 IS USM. The imaging performance in high Telephoto was the Canon again slightly better (color and contrast but I found very similar to Tamron), however, the objective change has very disturbed. The idea of having to do this at a difficult point on a mountain hike, has ultimately strengthened my decision for the Tamron, although I would have preferred the Canon for the high telephoto. The advantage of the slightly higher sharpness bought one with considerably more effort (at least for my requirements). The take home a small compact car for the wide-angle also is not an option, because the noise performance and the settings are too strong to be limited.
Finally, I can say that the Tamron is indeed a great solution if you want / have to do without the lens changes. A fast wide-angle lens I need while still optimal landscape shots, but as a recreational photographer one must not always looking for perfectionism be. A good motive anyway gets through a more powerful lens may still the icing on the cake, but no more! A very strong buy recommendation from me (a possible series dispersion of lens quality is not taking into account)!