A field microscope is a small compound light microscope with a long focal length objective. Its purpose is to do microscopy in a variety of locations, so portability, small size, and light weight are primary considerations for a field or portable microscope. Many are small enough to be hand-held.
A compound microscope has two lenses: the eyepiece lens and the objective lens. The objective lens is the most important part of a microscope, as it is the lens closest to the object. The focal length is the distance between the objective and the sample, and field microscopes have a long focal length in order to accommodate larger samples like a whole leaf or rock, and for ease of use in switching between slides.
All field microscopes are light microscopes, and can be configured for brightfield, dark field, and phase contrast microscopy. Inexpensive ones use ambient light, but this light is not sufficient for good resolution in some situations, so some field microscopes have a battery-powered light source. High-quality models use light-emitting diode (LED) light sources, while others use incandescent light bulbs.
A field microscope can be monocular or binocular. They often use low magnification 10X or 20X fixed objective lenses, but higher magnification objectives are available in more expensive models. Field microscopes usually do not have revolving nosepieces, so when switching between objectives, the objective must be removed and replaced. Some field microscopes have camera attachments so images can be acquired on site, and others can be attached to a digital camera through a Universal Serial Bus (USB) connector.
A field microscope can be anything from a simple ambient light compound microscope used to explore the world in the backyard, to a serious piece of equipment used by professionals. Forensic scientists may use field microscopes at a crime scene. Doctors on medical missions in remote areas with no electricity depend on field microscopes for lab analysis, and biologists may require a field microscope to examine specimens in their research outdoors or in remote locations. Field microscopes vary in price, depending upon features such as the light source, the quality of the objectives, and the number of objectives.