For the OpenFlexure microscope we recommend 40x-100x 160mm tube lenses with RMS threads for the high-resolution version. We have 3d-printable adapters for these to be mounted on the Raspberry Pi camera.
My colleagues @rbowman and @julianstirling would be able to talk more intelligently than me about trade-offs of using different lenses and their suitability for detection of Malaria. I also just came across these wiki articles explaining some of the trade-offs of different options that we have adapters for: -