To no surprise, a microscope objective is the most important imaging component of an optical microscope. A microscope objective is a multi-lens assembly that focuses light from an object and forms an intermediate image that is projected onto a sensor or magnified by an eyepiece. There are two types of microscope objectives: finite and infinite conjugate. Finite conjugate objectives work like any other lens where an image is formed when an object is located at a finite distance beyond the front focal point. On the other hand, infinite conjugate objectives require additional optics, often referred to as a tube lens, to focus the ray bundles to form an image. This objective design allows for the introduction of other optics such as filters, polarizers, and beamsplitters, making it a popular choice for high magnification imaging systems.
Besides deciding on the type of objective, imaging system designers often consider the magnification, numerical aperture (NA), and cost options available to make their decision. Numerical aperture often has the biggest influence in a decision between two objectives with the same magnification as NA is believed to directly equal to performance. However, NA does not tell the whole story about a microscope objective’s imaging performance. This article will go over what defines a microscope objective’s performance and how this can be applied when developing high-magnification imaging systems.