I understand that a lens with a large zoom ratio is considered to be of lower quality than a prime or lower zoom ratio. I've been considering a sigma 18-200mm OS lens which I've seen decent reviews for, but I'm wondering just how much of a difference it would make in terms of quality. I generally like landscape and wildlife, which requires both extremes and would rather not change lenses for every shot. I currently have the nikon 18-70mm which I'm quite happy with, but was wondering if anyone had pictures taken with a 18-200mm lens which would show a noticible difference in image quality. Thanks.