How much are phase-contrast and dark-field microscopy used in biological research today? It seems these were invented a long time ago, so I am thinking would it be a good idea to learn these techniques. (I will start an undergraduate program in biology next spring.. but I am still mostly interest how much the researchers use these technologies: if little, I should use my time better)
I use phase contrast very often, as it is only a matter of switching a lens group from bright field. Instead of spending hours with staining, or, worse, immunostaining, I get the big picture (cell diameter) in a few seconds. I can even go back to the same cells next day. Switching to dark-field is also a matter of turning a dial or pushing a switch.
If you already use brightfield, there is not much to learn about them in terms of actually useful physics, or sample preparation. The things you could learn - say, that colors are altered in darkfield - will rarely be needed in practice. These methods are the sort of things you don't really need to know in depth unless your experiments require them. Even then, practice and elbow grease would be more helpful than advanced optics. Remember that in most cases, the scientific hypothesis is more important than the tools you can use to solve it.
If you're planning on doing any kind of cell biology work (and if you're going into biology, it is almost certain you will at some point), learning how to use a microscope is a must. You will use a scope frequently when growing cells to analyze their morphology, count them, look for signs of contamination, visualize differentiation, etc. Microscopes are also used when performing immunofluorescence and immunohistochemistry staining protocols.
So, even though the instruments and some of the techniques have been around for quite a while, they are still very relevant today.