I'm looking for the cheapest possible suggestions for acceptable 500mm lenses. I'm aware that options such as mirror lenses, those long, typcially fixed f8 lenses from companies like Opteka, and teleconverters are frowned upon in forums like this. But, living in Montana, I'm desperate to photograph some of the abundant wildlife that can be found in nearby wilderness areas. I recently bought a used Nikon D5100 and Tamron 70-300mm zoom and have been researching options to extend range. So, anyone with experience with the above (bad) products willing to weigh in (beyond the "don't waste your money, save up and get the ...") stock replies? I'm sure my options will be used, old, manual focus and so on, and I'm fine with that -- just wondering if anything is out there at all for less than $300. Thanks.