Microscopes are used to view the incredibly tiny structures that make up the world around us. From the intricate patterns in butterfly wings to amoebas, microscopes have enable scientists and hobbyist alike to a get a glimpse of what would otherwise not be visible. Microscopes delicately balance resolution and contrast to achieve the most appropriate image of very small objects and textures.
In microscopes, as in photography, contrast is the difference in light value between two objects next to each other. The higher the contrast, the more extreme the difference is and thus the more easily different objects can be discerned. Increasing the contrast makes it easier to see separate objects in a slide, but it differs from resolution in that it doesn't create greater detail than what is already present.
Resolution is the maximum level of detail available to a given microscope. Similar to a digital image, the resolving power of a microscope represents how many tiny details can be projected. However, high-resolution microscopes do not guarantee that the smallest of details can be detected. When used in conjunction with contrasting techniques, high-resolution microscopes can allow you to more easily view the movement and structure of tiny organisms.
A common way to produce a contrast is to view objects against a black background. This works similarly to the way a beam of light into a dark room can highlight dust floating in the air. Instead of light being presented directly behind the object being viewed, it hits the object at an angle. The result is that the items directly within the viewing area become lit while everything behind remains black.
Phase contrast is another way that microscopes can emphasize the difference between objects. Phase contrast utilizes normally undetectable difference in the phases of light that occur when light passes through two different objects. These differences generally don't affect the brightness or color detectable to our eyes. By using these phase differences and passing them through a "phase plate," objects in a microscope can be more sharply defined. Although effective, phase contrast only works with phase-ready objects that only mildly affect light patterns (i.e., very thin, generally translucent objects). Furthermore, the use of phase contrasting significantly reduces the available resolution.