Magnifying glasses enlarge images by small degrees, blurring the image around the edges as lens magnification increases. Microscopes combine two lenses, requiring adjustments in depth of field and magnification to observe small objects clearly.
Depth of Field Microscope Description
Microscopes contain two lenses, positioned at opposite ends of a cylinder. The cylinder is connected to an eyepiece, a base and a lighted slide shelf. Placing slides beyond the focal distance of the first lens produces an inverted image approximately 16 cm behind the lens, which is re-magnified by the second lens. Magnification increases as focal length for each lens is shortened.
Depth of Field Vs. Magnification
Depth of field describes a layer that is in focus. The layer grows thinner with increased magnification. For example, the hairs on a mite's legs can be viewed at 1,000x magnification, but this shrinks the depth of field, making the mite blur in or out of focus depending on its distance from the first lens.
Adjusting Plane of Focus
This setup produces a balance problem between the depth of field and the magnification. Decreasing the lens's focal length and diameter keeps the magnification / depth-of-field ratio relatively even. At higher magnifications, fine adjustments focus the slide by moving the layers of interest in and out of focus.