The microscope allows observation of objects too small to be seen with the unaided eye. To effectively use this instrument, an observer must understand the concept of the Field of View (FOV). This term describes the area of the specimen visible through the eyepiece at any given time. Understanding the dimensions of this viewing circle provides the necessary context for interpreting the size and scale of the microscopic world.
Defining the Field of View
The Field of View (FOV) is the diameter of the circular area visible through the ocular lens. This measurement represents the actual distance across the specimen being viewed. Since the objects are small, the FOV is typically expressed in metric units, such as millimeters (mm) for lower magnification and micrometers ($\mu$m) for higher magnification.
Many eyepieces are marked with a “Field Number” (FN), which is the diameter of the image formed inside the microscope’s tube, measured in millimeters. The actual FOV is calculated by dividing the Field Number by the magnification of the objective lens being used. This calculation confirms the real-world size of the area being studied.
The Inverse Relationship with Magnification
The FOV and the total magnification of the microscope have an inverse relationship. As the total magnification increases, the diameter of the visible field decreases proportionally. This occurs because higher-power objective lenses focus on a much smaller, concentrated area of the specimen. Moving to a high-power setting is a trade-off: greater detail comes at the cost of a reduced viewing area.
For example, if a low-power combination (4x objective, 10x eyepiece) yields 40x total magnification and a 4.0 mm FOV, switching to a medium-power objective (10x objective for 100x total magnification) reduces the FOV to 1.6 mm. Changing the objective to a high-power setting (40x objective for 400x total magnification) decreases the FOV significantly to 0.4 mm. This proportional decrease means that a tenfold increase in magnification results in a tenfold decrease in the diameter of the visible field.
Practical Methods for Measurement
To determine the Field of View, a measurement must be taken for each objective lens. For low-power objectives, the most straightforward method uses a transparent metric ruler placed on the stage. The observer focuses on the ruler’s millimeter scale and counts how many millimeter lines fit across the diameter of the visible field. This count provides the FOV diameter directly in millimeters.
However, standard ruler marks are too large to accurately measure the FOV at higher magnifications, such as 400x. For these objective lenses, the observer must use a specialized tool called a stage micrometer. This is a slide with a fine, precisely ruled scale that allows for accurate measurement in micrometers. Once the FOV diameter is measured for one objective, the values for the others can be calculated using the inverse relationship principle.
Using Field of View for Specimen Sizing
Determining the FOV diameter allows observers to estimate the actual size of the specimen. Since most microscopic organisms or cells are too small to measure with an external ruler, the known FOV diameter acts as an internal reference scale. Once the FOV is established in micrometers, the observer can visually estimate the specimen’s length or width as a fraction of that diameter.
If the FOV diameter is 400 $\mu$m, for instance, and a single cell spans approximately one-quarter of that field, the specimen’s estimated length is 100 $\mu$m. This estimation technique is routine in microscopy, especially when magnification is high. Using the FOV as a calibrated measuring stick, researchers can quickly approximate the physical dimensions of the structures they observe.