4. Set system to high magnification

Zoom in and out.

Select an image size that lets you clearly see which object of interest to mark or not to mark.

A suitably sized image clearly shows cells or other anatomical features as they appear and disappear between the reference and lookup sections.
Your choice will vary, depending on your tissue and staining; some types of staining are easier to see at lower powers and some may only be clearly visible at higher powers.

Change the objective on your microscope.