When focusing on a slide, ALWAYS start with either the 4X or 10X objective. Once you have the object in focus, then switch to the next higher power objective.

Why do you start with the objective lens?

When using a light microscope it's important to start with the low power objective lens as the field of view will be wider, increasing the number of cells you are able to see. This makes it easier to find what you're looking for.

Is the objective the first lens on a microscope?

The first lens is called the objective lens, and has typical magnification values from 5× to 100×. In standard microscopes, the objectives are mounted such that when you switch between objectives, the sample remains in focus. Objectives arranged in this way are described as parfocal.