Microscopes can be tricky to use because of the strength of their lens magnifications, typically 4x, 10x and 40x. The difficulty lies in trying to align the specimen in a precise position so the microscope lens can capture it and allow the user to see the specimen close up. Generally, the lowest magnification is used because it is not too zoomed in so aligning the slide and lens is easier.
Using the lowest magnification means that the specimen is far enough away from the lens in comparison to the higher magnification lenses, offering the widest field of visible range. By starting with the lowest magnification, the specimen is easier to locate, center, and focus in on. Once the specimen is in view under this magnification, it is also easier to focus on the object using the fine focus knob on the side of the microscope.
Also, by using the lowest magnification and thus shorter lens, the stage is able to be raised higher to bring the sample up closer to the eye piece. Once the specimen is in sight, a gradual increase in magnification can provide clearer details and a more close-up look at the specimen. When magnification is increased, the specimen must be recentered and refocused carefully. Since the field of vision gets smaller and smaller with each increase, the difficulty of using these lenses is having to "chase" or reposition the specimen in the right line of sight.