Magnification is typically not the problem since sperm can be adequately visualized even at a total magnification of only 25 X. Usually, difficulties observing sperm are often associated with improper or insufficient illumination.
Bottom (transmitted) illumination is preferred. If adjustments are possible, try increasing or decreasing the intensity of the light source. Also, if a mirror is present, try different positions so that the highly refractive sperm appear in a pseudo-dark field context. If a separate light source is available, light from the side can sometimes help. Even a penlight can be useful. In some student-level scopes, often there is little or no adjustment concerning intensity. Thus, if the light source is too bright the largely transparent sperm can be difficult to visualize. If there is no adjustment, a simple device can be made from the top lid of a Petri dish. With a black fine tip marker (i.e. Sharpie), make a series of straight lines in a grid-like pattern covering approximately one square inch of the Petri dish lid. Spaces between the lines should be much smaller than the thickness of the lines. By placing this lid underneath the specimen (i.e., gametophytes on a slide in water or a culture-dish) and moving it around, enough contrast can be created to enhance sperm visualizations.