When observing an object with a lens, the object is most clearly viewed when it is at the focal point of the lens. If the distance between the lens and the object is slightly varied, then there is a range or tolerance in which the object can still be clearly observed. This range is known as the depth-of-field (DoF). When using a lens with a large depth-of-field, it is possible to precisely and quickly observe a target, even when that target has large variations in its surface. A large DoF is especially useful, if not critical, when viewing from an angle.

Shallow Depth-of -Field : Optical microscope LargeDepth-of-Field : Digital microscope