With this in mind, the depth of field can be calculated by using this formula: So, a high numerical aperture results in a low depth of field, and vice versa. The general rule is that depth of field is inversely proportional to the numerical aperture, which is the size of the opening of an optical component where light passes through- in this case, the objective lens. Having said that, since the depth of field concerns the objective lens, there are a few other factors that must also be taken into account. In this sense, the microscope’s depth of field and depth of focus are somewhat similar, since these both generally increase as the numerical aperture is decreased. The numerical aperture of the objective lens is the main factor that determines the depth of field. Knowing the depth of field of the microscope at any given setting is important since it affects how much you have to move the specimen slide up, down, left, or right to image certain areas of the specimen, especially since it determines the required stability of the focusing axis. What factors affect the depth of field? source The depth of focus is determined by both the numerical aperture or sensor size and the magnification of the objective lens, and is also, in a way, related to the resolution. It’s also affected by aberrations and diffraction figures extending above and below the image plane. It’s a somewhat more advanced and complex microscopy concept, since it takes into account the tilt and tip of the space between the image plane and the objective lens sensor plane. While the depth of field refers to the object space, or the quality of the image coming from a stationary lens as the specimen is being repositioned, depth of focus talks about the image space, or the ability of the sensor to retain the focus of the image as the sensor changes positions. Out of all natural and artificial optical instruments, the human eye has one of the best depth of field. This number is largely determined by the numerical aperture of the objective lens, and is considerably small that it’s typically measured in microns. It is the axial or longitudinal resolving power of the objective lens, measured parallel to the optical axis. In microscopy, the depth of field is how far above and below the sample plane the objective lens and the specimen can be while remaining in perfect focus. The depth of field is defined as the distance between the nearest and farthest object planes that are both in focus at any given moment. Average figures What is depth of field in a microscope? Source
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