In light microscopy, lower magnification objective lenses are further from the specimen and survey a larger area, meaning more light enters the microscope, explains How Stuff Works. More magnification means the lens is closer to the stage, which also means less light enters the microscope, making the field of view dimmer.
A light microscope works by shining a light up through objects, such as cells, on a slide. The light passes through the objects up into a small, spherical objective lens, up through the body of the microscope, through a second lens and finally into the eye of the microscope user. By this time, the image on the slide has been magnified many times.
Most light microscopes have a variety of lenses that allow a user to see objects under different magnifications. Each lens is a different magnification and a different distance from the stage. Lower magnification lenses are further from the stage, meaning they observe a larger area and allow in more light. Higher magnification lenses are much closer to the stage. In fact, at the highest resolutions, the lens is separated from the specimen by just a thin layer of oil. Since the lens is so close, the user sees a much smaller area, and less light enters the microscope and the eye. The differing amounts of light reaching the eye make the field of view brighter or dimmer.