Has anyone tried any of the "off brand" zoom lenses that sell on places like Amazaon (less than $200) ? My largest zoom lenses I have now goes to 300, and I would like one with more zoom ability. I know you get what you pay for. I know the aperture numbers are high on all zoom lenses of this length (and especially the cheap ones). I know you probably need a tripod to support the lens. I have all Nikon lenses now, but have used Sigma before and was happy. What are your thoughts on these lenses if you have used them?