Key Optical Parameters of Microscopes: Magnification and Effective Magnification
Since the image undergoes two successive magnifications by the objective lens and the eyepiece, the total magnification Γ of the microscope is the product of the objective magnification β and the eyepiece magnification Γ₁:\(\boldsymbol{\Gamma=\beta\Gamma_1}\)
Obviously, compared with a magnifying glass, a microscope can achieve a much higher magnification. The total magnification can be conveniently adjusted by changing objective lenses and eyepieces of different magnifying powers.
Magnification is also an important parameter of a microscope, but it is inadvisable to simply assume that higher magnification is always better. The upper limit of microscope magnification is the effective magnification.
Resolution and magnification are two distinct yet interrelated concepts, which satisfy the relation:\(\boldsymbol{500NA<\Gamma<1000NA}\)
When the numerical aperture of the selected objective lens is insufficient, meaning the resolution is inadequate, the microscope cannot distinguish the fine microstructure of the specimen. In this case, excessive magnification only produces an image with a large outline but blurred details, which is known as empty magnification. Conversely, if the resolution meets the requirement but the magnification is too low, the image formed by the microscope, though resolvable, will be too small to be clearly distinguished by human eyes. Therefore, to fully exploit the resolving power of a microscope, the numerical aperture and the total magnification should be properly matched.
Depth of Focus
Depth of focus is the abbreviation for focal depth. When focusing on an object plane with a microscope, not only points on this plane can be seen clearly, but also features within a certain thickness range above and below this plane remain sharp. This clear thickness range is the depth of focus. A large depth of focus allows observation of the full layer of the specimen, while a small depth of focus only enables observation of a thin slice of the specimen. The depth of focus has the following relationships with other technical parameters:(1) Depth of focus is inversely proportional to the total magnification and the numerical aperture of the objective lens.(2) A greater depth of focus corresponds to lower resolution.
Low-power objectives feature a relatively large depth of focus, which brings difficulties for low-magnification photomicrography. This will be described in detail in the section on photomicrography.
