I want to use a Nikon 24mm f/2.8 AI Nikkor lens and/or Sigma 18-250 f/3.5-6.3 DC Macro OS HSM lens for macro images on a Nikon D5100.
B&H recommended the Opteka lens as an inexpensive way, but I don't know.
1. Which add-on do you recommend?
2. How can I find info on how to use these lenses and add-ons?
Neither of these lenses are true macro. You should not be buying either, just for that purpose.
Before making any lens purchases intended for macro-photography, you should read all of the FAQ threads listed here:
Introduction to True Macro-Photography Forumhttp://www.uglyhedgehog.com/t-22447-1.html
i bet if you reverse your 24mm f2.8 at f/11 you will get some nice shots at about 2x
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