Whichever type of microscope you will be using, choosing the right objective is very important. The magnification required is a big factor in the choice. The numerical aperture of the objective is also very important. High NA gives higher resolution and brightness, makes the depth of field smaller and generally makes the working distance less. Low NA objectives are dry, higher NA objective require some sort of immersion (eg water or oil). Matching the refractive index of the sample and objective immersant is generally a good idea (ie oil for fixed samples, water for thick living samples). Objectives have quite a lot of information written on them:

duke lmcf microscope objective markings