It is unfortunate that there is little good zoom lenses for Sony SLR / SLT models. I was looking for a good "Always top-zoom lens" because I do not feel like getting a whole lens equipment to lug around with me. I tested everything Sony - Tamron - have to offer Sigma zoom lenses currently on, in the range 18-250 / 270mm. To make it short. I note in my tests, what all the experts know. From 200mm can forget the image results. Remained ultimately 18-125mm only the Sigma. On my Sony a65 it brought the best result of all, I tested, lenses. That may be different in other cameras. In my Sony is at least. The Sigma 18-125mm I found a really good plan always objective, as long as you have good lighting conditions. Until the dawn it gets even. After that, however ... Well, at the relatively low light intensity, you can actually no longer be expected.
Since I like to do with atmosphere soaked scene photos, I was looking for now still looking for a high-intensity optical zoom. With continuous F2.8 Aperture itself offered me the Tamron and Sigma 17-50mm F2.8. The Sigma won on my Sony, because of its better image quality on my Sony and due to its extremely fast and inaudible focusing. It's under heavy twilight and night scenes even out of his hand, sensationally good record. Sony freaks who have no money for full-format cameras and fixed focal lengths, I can recommend the two lenses from our own, practical experience.