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I've found Nomarski optics to be better than either of those. Although I don't truly understand how the optics work, phase contrast does some kind of "doubling" of the image to make things more clear, but unfortunately, photographing through phase does not result in a good picture. Photos through Nomarski optics don't have these problems. Dark field can be good in some circumstances, like for very opaque structures. Anyway, I hope that helps a bit! I think that dark field is quite inexpensive.