Objective Lens of Microscope

Welcome to this week's exciting POSES! This week, we will be sharing the objective lens of microscope.

Objective lens is the most essential part of microscope. It is the optical component located close to sample. Principally, the objective lens is a positive lens with very high magnification and relatively short focal length. Then, what is so special about this lens?

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Lenses inside various optical devices, such as telescope or binoculars, work based on “converging” lens (also known as “positive” or “convex” lens) and “diverging” lens (also known as “negative” or “concave” lens). The term “converging” is related to the behaviour

of light’s rays to direct onto a point (namely “focal point”) after passing through the lens. The fact that the focal point of convex lens is “in front of” the lens leads to its other name as “positive” lens. The term “convex” reflects the typical shape of the lens, which its centre is thicker than it edge. Likewise, the opposite understandings of converging, positive, and convex are assigned to “diverging”, “negative”, and “concave”, respectively.

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Image formation from a lens is basically the “transfer” of an object to “other side” of the lens. This transfer is facilitated by the ray lights emerging from every single point on the object. Following is the basic formula for the image formation and magnification:

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By rearranging equation (1), one can calculate the object distance as:

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Some typical image formation conditions of a lens are (only discussing “convex lens”) :

1.?Object on infinite distance

Infinite distance is defined when all the rays from object are almost parallel (practically about 200 times of lens focal length). The image will form at focal point with extremely small size or even just simply a “dot”

2.?Object on “far” distance/behind “centre of curvature”

Centre of curvature is defined as twice of focal length. Hence, object is practically at distance between 200 times to twice of lens’ focal length. The formed image will be smaller than the object at the shorter distance.

3. Object on “centre of curvature”

The image formed will have an exact size and distance as the object.

4.?Object between “centre of curvature” and “focal point”

The formed image will be larger than the object at the further distance.

5.?Object on the “focal point”

The formed image will be at infinite distance or simply there is no image can be found as all the refracted rays are in parallel path. One way to catch the image is using another convex lens that following the principle of object on infinite distance (see #1).

6.?Object in front of “focal point”

The formed image will be relatively much larger than the object, however, it will present as “virtual” image “behind” the lens. One way to catch the image is using another convex lens that is following principle object behind is focal point up to its centre of curvature (see #3-4).??????????????

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To achieve its purpose as very high magnifier for the tiny object, objective lens shall work following condition #6 above, where the object shall be at distance within the lens focal length. The virtual image formed with this setting will have relatively larger size than the object, where the shorter the focal length the larger magnification will be. It can be explained by following equation (3). Once can notice here that the value of image distance s’ will always be negative, representing that it is a virtual image formed behind the lens.

In conventional setup, a relay lens, knows as “eyepiece” is placed between objective lens and observer eye. Both work hand-in-hand to allow image projection of the object onto observer “retina”, where all the light sensory nerves of human eyes are located. The eyepiece provides additional magnification to the image from the objective lens, which value is ranging from 1x to 30x (the most common magnification used is 10x). Hence, the total magnification of the microscope is the multiplication of objective lens magnification and eyepiece magnification.

In modern microscopy system, the role of human eyes is replaced by the camera. Slightly different from the “eyepiece”, the relay lens used with the camera has function to project the image to the camera sensors.?

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Though the basic of objective lens is a magnifier, its real design is very complex involving combination of multiple lenses which are optimized to suppress the aberration for such optical component with very short focal length.?

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The objective lens is primarily characterized by its “magnification” (ranging from 2x to 100x) and “numerical aperture/NA”. NA is the entrance angle of light into the lens, which closely related to the lens’ focal length. NA is an important factor that defines the “field of view/FOV” of the microscope system.?

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Other characteristics which may be pasted on the objective lens body are its intended use, immersion medium, special property, lens image distance, cover slip correction, working distance, etc.

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The traditional objective lens is designed with a “finite conjugate”, which means the incident light from the object is focused to a particular point. Consequently, the design of the microscope body become challenging and is strictly limited to particular length. Furthermore, it will be more complicated when its application required additional optical component, such as filter or beam splitter, to be added within the optical path of the microscope.

Recently, most objective lens is designed with an “infinite conjugate” to allow more flexibility for microscope application. The design provides an infinity corrected rays’s pathways of the incident light from the object, which works similarly to imaging condition #5. To capture the image for the eyepiece or camera’s relay lens, it utilizes a “tube lens” placed between the eyepiece or camera’s relay lens and objective lens. The additional optical component, then, can be inserted in the infinite optical path between objective lens and tube lens without affecting the overall optical path of the microscope. With tube lens having typical focal length of 200 mm, the magnification printed on the objective lens body is derived from the following formula:

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The mechanical design of objective lens is rigorously controlled by international standards to allow its interchangeability during the application without the need to redesign the whole system. Two important criteria to be followed are the “parfocal length” and the “mounting thread”.

The parfocal length is the length measured from the object position in front of the objective lens to its back flange, which simply equal to the sum of its working distance and its body length.?

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There are 3 types of standard mounting thread applied to the objective lens screw: RMS (Royal Microscopical Society) type, which are more universally compatible; as well as M25 (metric 25 mm) and M32 (metric 32 mm) types.