To locate a specimen on a prepared slide with a compound microscope, a student should begin with the low-power objective rather than the high-power objective because the

The field of vision is larger under low power objective than under high power.

The high power objective is more zoomed in, so you see less of the slide but more detail in that region. The low- power objective is more zoomed out and because of that allows us to see more of the slide. After you  look at the big picture, you can zoom in what you need to see in detail.