Question

With reference to a lens’s Numerical Aperture (N.A.) discuss the trade-offs between the diameter of the front lens of an objective, the objective’s working distance, its optical resolution limit and the complexity of internal lenses.

Answer 1

a) Diameter of the front lens of an objective

The diameter of the front lens of an objective considered as a measure of the amount of light that can enter a microscope. It is measured in millimeters (mm). If the objective lens tends to be larger, more light can enter the microscope.

Numerical Aperture is defined by the equation

(NA)=n×sin(θ)

where θ is the angular aperture and  n is the refractive index of the medium between the objective front lens and the specimen. The numerical aperture (NA) of a microscope objective is a measure of its ability to gather light. It then resolves the specimen details at a fixed object distance.

b) Objective's working distance

The Objective's working distance of a microscope objective is defined as the distance from the surface of the sample to the front lens element of the objective when it is in focus.

the working distance is measured at the actual surface of the specimen. This condition applies when the specimen is designed to be imaged without a cover glass. As the numerical aperture and magnification increases, the working distance decreases when considering a series of matched objectives.

(c) Optical resolution limit

The Optical resolution limit (resolving power) is a measure of the ability of the objective lens to separate the image adjacent details of the image sent in the object.It is represented as the object between two points that are resolved in the image. Numerical aperture determines the resolving power of an objective. We obtain a better resolution of a specimen with the lens if the numerical aperture of the system is higher.

d) Internal lenses

A internal focus lens (IF lens) is a photographic lens. Its focus is shifted by moving inner lens without a rotation or shift of front lens element.

The physical size of an IF lens does not change during focus. Also it does not change the front of the rotated lens.This is particularly useful for large lenses. Its size more compact. For an internal focusing lens, the true focal length of the lens is reduced when it is not focussed at infinity. The focus is accomplished through the movement of internal lens elements, without affecting the overall outside length of the lens.