I am very new to photography. I shoot with a Nikon D3100. Was looking at a couple of Sigma and Tamron lenses. The 18-270mm or 18-200mm to be specific. I like the convenience of the lens. The Nikkor 18-200 is twice the money. I understand the image quality is not as good as Nikkor lenses. However, I am definitely not shooting professionally, and I am shooting with entry level gear. In your opinion, are these ok buys? Is there a preference between Sigma and Tamron? Should I be patient and wait until I can afford the more pricey glass? Appreciate your opinions. Thanks!