What is Objective Lens?
An objective lens is a lens located closest to the specimen and receives the rays coming from the specimen and forms the image. It is one of the most important optical units of a microscope since it determines the basic performance and function of a microscope.
The objective lens collects as much light as possible from the specimen and combines this light to produce the real image that is seen on the eyepiece lens. Since an objective lens Forms the image in the focal plane of the eyepiece, it is important to have good quality eyepiece on your microscope to observe the details of the images.
In a compound Microscope, The objective lens is connected to the eyepiece lens through a tube also called the eyepiece tube. The eyepiece enhances the magnification power of the respective objective lens.
The Objective lens is held by a rotating nosepiece or turret just above the specimen. Nosepiece holds around two or more objective lenses, and you can easily switch between them to change magnification power.
Generally, objective lenses come in various magnification power, among those, the most common one is 4x, 10x, 40x, and 100x this lens is also known as a scanning lens, low power, high power, and oil immersion lens respectively.
Different Types of Objective lens and its magnification
The majority of compound microscopes come with interchangeable objective lenses, Essentially, objective lenses can be categorized into four main categories based on their magnification power. These include.
- Scanning Objective Lens (4x)
- Low Power Objective (10x)
- High Power Objective Lens (40x)
- Oil Immersion Objective Lens (100x)
1. Scanning Objective Lens (4x)
Combined with the eyepiece lens, this lens will provide the lowest magnification power. For example, a 10x eyepiece lens, multiplied by the 4x objective lens gives a total magnification of 40x.
This objective is often referred to as the scanning objective lens since the low power provides enough magnification to give the observer a good overview of the entire slide and sample.
2. Low Power Objective (10x)
This objective lens is the next lowest powered and is often the most helpful when it comes to analyzing glass slide samples. The total magnification for this lens is equal to 100x magnification (10x eyepiece lens x the 10x objective equals 100).
Learn more: How to calculate total magnification?
Since it still provides a good amount of magnification at a good distance from the slide, there is a limited risk of it breaking the glass and potentially ruining the sample. Hence, why it is often preferred before going for a high-powered lens.
3. High Power Objective Lens (40x)
This is referred to as the high-powered objective lens since it is ideal for observing the small details within a specimen sample. The total magnification for this lens is equal to 400x magnification (10x eyepiece lens x the 40x objective equals 400).
4. Oil Immersion Objective (100x)
This objective lens will achieve the greatest magnification and has a total magnification of 1000x (10x eyepiece lens x the 100x objective equals 1000).
However, since the refractive index of air and the glass slide are slightly different, a special oil must be used to help fill the gap between the two. Without a drop of oil, the objective lens will not work properly and you will not achieve the desired magnification and resolution.
How to classify Microscope objective lens?
A wide variety of objective lenses are available according to the purpose. Objective lenses are roughly classified basically according to the intended purpose, microscopy method, magnification, and performance (aberration correction).
Classification according to the concept of aberration correction among those items is a characteristic way of classification of microscope objectives.
1. Classification According to Purpose
Using this classification method, objective lenses are classified into
- Biological-use objectives:
- Industrial-use objectives
Since a biological-use objective lens is used for observation through this cover glass, the optical design is performed in consideration of the cover glass thickness (commonly 0.17mm).
Meanwhile, in industrial use, a specimen such as a metallography specimen, semiconductor wafer, and an electronic component is usually observed with nothing covered on it.
An industrial-use objective lens is optically designed so as to be optimal for observation without any cover glass between the lens end and a specimen.
2. Classification According to Microscopy Method
The dedicated objective lenses for each microscopy method have been developed and are classified according to such a method.
For example:
- Reflected darkfield objective: A circular-zone light path is applied to the periphery of an inner lens.
- Differential Interference Contrast (DIC) objective: The combination of optical properties with a DIC prism is optimized by reducing lens distortions.
- Fluorescence objective: The transmittance in the near-ultraviolet region is improved.
- Polarization objective: lens distortions are drastically reduced and
- Phase difference objective: A phase plate is built in available.
3. Classification According to Magnification
As we discussed above, objective lenses can be categorized into four main categories based on their magnification power. These include: Scanning Objective Lens (4x), Low Power Objective (10x), High Power Objective Lens (40x), Oil Immersion Objective Lens (100x)
Apart from the differences in their magnifications, objective lenses are also different in how they are used. For instance, with a high magnification lens (100x) immersion oil is often used to obtain high resolving power. This is not the case with lower magnification objectives.
4. Classification According to Aberration Correction
Essentially, with regard to chromatic aberration correction, there are two main levels of correction. These include the achromatic ad apochromatic.
Achromatic objectives are the simplest, least expensive, and most common objectives used. These objectives are designed to correct for chromatic aberration in both the red and blue wavelengths. They are also corrected for spherical aberration in the green wavelength.
The main weakness of this type of objective is that there is limited correction when it comes to chromatic aberration as well as the lack of a flat field of view. These issues reduce the objective performance of these objective lenses.
These lenses are particularly well-suited for monochromatic applications. With apochromatic objectives, there is higher precision. These objectives are chromatically corrected for red, blue, and yellow.
With apochromatic objectives, there is also spherical aberration correction for two and three wavelengths in addition to a higher numerical aperture and long working distance. Because of their better design, apochromatic objectives are ideal for white light applications.
Understanding Labeling and Specification of Objective lens
Specifications of any objectives are listed in the body of the objective. It is important to understand what labeling means if one is to select the right objectives for their intended purpose.
Specifications include:
1. Objective standard
Such objective standards as DIN or JIS will be listed on the body of the objective depending on the type of standard. This shows the required specification present in the system.
For instance, the DIN, which is the most common standard, has a 160mm distance from the objective fringe to the fringe of the eyepiece while JIS has a 170mm distance.
2. Magnification
On the objective, this is usually denoted by an X next to a numeric value (100X, 10X, etc). On the other hand, objectives will also have a colored band around the circumference of the objective that indicates the magnification of the objective.
For instance, a yellow band around the objectives (lower part of the objective) indicates that it is a 10x objective.
3. Numerical aperture (NA)
Numerical aperture refers to the function of focal length and entrance pupil diameter. This is usually labeled next to the magnification of the objective (1, 1.30, etc).
A large numerical aperture (more than 1) means that immersion oil may have to be used given that the highest NA that can be achieved without immersion oils (in the air) is NA of 1.
This labeling is therefore important in that it directs the user on how to use the objective for better quality images.
4. Cover slip thickness
Denoted by a number (such as 0.17mm) the coverslip thickness is labeled on the objective to note the type of coverslip that should be used.
A coverslip changes the way light is refracted from the specimen. Therefore, it is important to ensure that the right coverslip is used in order to produce a good-quality image.
5. Quality correction
Quality corrections such as achromatic, apochromatic, plan, and semi-plan are often denoted on the objective in order to show the design of the objective. Plan and semi-plan objectives (also referred to as a micro plan, planar or semi-planar) correct for field curvature.
Field curvature often results in blurred images and correction for this helps produce good-quality images. Whereas plan objectives correct better, allowing for better display (over 90 percent) of field flat, semi-plain objectives produce about 80 percent.
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