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If you are not too concerned with image quality you may very well experiment with this lenses.
An a more specific note:
- a 500mm mirror lens can actually be quite good as my review
here reveals. But there are differences in image-quality from different manufacturers. And: this is a manual-focus lens only!
- close-up lenses that are cheap are almost sure to disappoint. Better get a so-called apochromatic close-up filter like the Canon 500D. It also pays to have a lens of 150-300mm focal length for using a close-up filter otherwise you will not see much of an effect. Btw.: sometimes those are called close-up
filters as you screw them to the front of your lens. Wwatch out for getting a close-up filter with a filter-thread matching your lens!
- fixed focal lenses are pretty easy to build so almost anybody can manufacture something decent. But you never know. There are always people/companies out there that only want your money and don't deliver.
So as a final remark: I'd only buy if you can give the items back without a prohibitive "restocking fee" if you're not satisfied.
Good luck!
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Thomas (beware: Nikon-fanboy and moderator!)
My Lens Reviews,
My Pictures,
My Photography BlogD800+assorted lenses