Depth of field

The depht of field describes how many optical plans you can focus on simultaneously with an optical instrument.

Example 1

Fig1. Let's observe the wedge-shaped object showed in (a: perspective view) and (b: top view) under the compound microscope with various objectives. The focus will be set at the level of the line seperating the red and yellow rows.

Fig2. 4X objective. Left: side view of the object. The blue area shows the region (made up of several optical planes) that appears in focus in these conditions. Right: Schematic representation of the object as viewed under the microscope with the 4x objective.

Fig3. 10x Objective. Left: the blue area is narrower with this objective. Right: The magnification factor is greater (2.5x more than on figure 1 but only about one third on the object now appears in focus.

Fig4. 40x Objective. The region where the object appears in focus is even more shallow (in blue, left) as the magnification increases (right).

Example 2

Fig5. Cimex sp. (bed bug) leg, observed under the 4x objective. Note that the whole appendage appears in focus, including several layers of bristles (hair-like structures).

Fig6. Same sample under the 10x objective. Only part of the appendage and a few bristles are now in focus, the rest of the picture is blurry.

Fig7. Sample under the 40x objective. The picture can only be focused on one row of bristles, which can now been observed with great detail.