Objective Lenses – Types Based On Classification And Specifications

Arguably the most important part of a microscope is the objective lens. The objective lens is used in microscopy to gather light from the specimen being examined and focus the light to help produce the actual image that is seen through the ocular lens.

This is the element of the microscope that is closest to the specimen and therefore is the part of the microscope that has the most transformative role when it comes to changing the view of the specimen.

Objective lenses have a lot of different elements within their design often carrying out many different functions. This makes it one of the most, if not the most important, element of the microscope and is what affects the function the most.

Because of this having as much knowledge of the objective lenses you are using as possible will lead to a much easier time within microscopy. 

When studying and researching objective lenses, make sure you are specifying microscopy as objective lenses are present in many other fields working with lenses like telescopes.

While some terminology is similar it is important to not get information mixed up.

There are many different factors that change the use and purpose of an objective lens within microscopy, and these are the best way to identify and classify the use of an objective lens.

These factors include; the intended purpose of the objective lens, the microscopy method the objective lens is used with, the performance of the objective lens, the level of magnification the objective lens provides, and finally the aberration correction the lens provides.

To put these factors into perspective, the variables that the objective lens is responsible for are; primary image formation, determining the quality of the produced image, the magnification of the final image provided, as well as the resolution of the final produced image.

Now you know all of these factors let’s look at the different classifications used in more specific detail.

How The Microscopy Method Affects Objective Lens Classification

As previously mentioned, the microscopy method largely impacts the classification of an objective lens and will drastically change what type of objective lens is used.

Some of the different microscopy methods which are most commonly used include:

Reflected Dark Field Objectives

These objective lenses are specifically designed to have a 360-degree empty chamber to surround the lens element.

This makes getting a more specific image easier and helps control the lack of outside impact and controlling factors to get a more accurate final image.

Differential Interference Contrast (Known As DIC Objectives)

These objective lenses use stain-free optical elements and rely strongly on the use of Nomarski prisms (also known as Wollaston prisms).

These prisms will influence the optical path of the light collected between sheared light beams located at the rear focal plane.

Fluorescence Objectives

These objective lenses are specifically designed with quartz and a special glass that has an especially high transmission from the ultraviolet to the infrared regions.

Phase Contrast Objectives

These objective lenses can be split into several different categories that are dependent on the construction and the neutral density of the internal phase ring. These specifications include:

• Dark low objectives (DL)
• Dark low low objectives (DLL)
• Apodized dark low objectives (ADL)
• Dark medium objectives (DM)
• Bright medium objectives (BM)

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Now you know all of the main classifications that are affected by the microscopy method of the objective lens, we can move into more specific classifications.

How Objective Lenses Are Classified Based Upon Magnification

On top of more specific classifications that objective lenses are put into based upon magnification, there are 3 main categories that these classifications can fit into.

These categories are; low magnification objectives, intermediate magnification objectives, and finally high magnification objectives.

Low magnification objectives cover objectives that magnify by around 5x or 10x. Intermediate magnification objective lenses are lenses that magnify by around 20x or 50x magnification.

Finally, high magnification objective lenses can magnify up to 100x.

As well as, of course, affecting how magnified the final produced image is, the categorization of magnification lens will drastically impact how this style of the objective lens is used.

The most common example of this is that high magnification lenses like 100x, immersion oil is often necessary. This oil is used to give a better clarity to the final image by providing a higher resolving power.

Due to the lower level of magnification, this oil is not needed for lower levels of magnification lenses.

How Objective Lenses Are Classified Based Upon Aberration Correction

When it comes to chromatic aberration correction, there are two main levels of correction that are utilized with objective lenses. These two levels of correction are; achromatic and apochromatic. 

Achromatic

Achromatic objective lenses are usually the simpler, the less expensive, and more commonly used objective lenses. These lenses are made to correct both red and blue wavelengths when it comes to chromatic aberration.

These lenses are also corrected for the spherical aberration that appears in the green wavelength.

However, this is not without fault. The most glaring weakness with this type of lens is that there is a quite limited correction when it comes to chromatic aberration as well as the complete absence of a flat field of view.

Because of these problems, this style of the objective lens has reduced objective performance.

In spite of this, these objective lenses are good for being used for monochromatic applications. They lack the precision of an apochromatic objective lens as these lenses are chromatically corrected for both red, blue, and yellow.

Apochromatic

Apochromatic objective lenses, as previously mentioned are much more precise than achromatic objective lenses and are chromatically corrected for red, blue, and yellow.

These objective lenses also have spherical aberration correction for two and three wavelengths as well as having a higher numerical aperture and a longer working distance too.

Because of this superior design, apochromatic objective lenses are great for being used with white light applications.

However, as mentioned in the achromatic section, the use of these lenses tends to be more complex and more expensive than the use of achromatic objective lenses.

How Objective Lenses Are Classified Based Upon Refractive Or Reflective Lenses

Refractive

Refractive objective lenses are more commonly used that the reflective alternative lenses. By having refractive lenses, light is bent by the optical element used.

This lens is designed specifically to reduce the back reflections and therefore will improve the quality of the light passing through this specific type of objective lens. 

Refractive lenses are most commonly used in applications that need a highly detailed resolution.

These lenses can have a very varied design sometimes just having 2 elements in the simple achromatic style to sometimes having up to 15 elements in the apochromatic objective lenses.

Reflective

Generally speaking, reflective style objective lenses are much less common than the refractive alternative, however, they do have a number of specific problems they can solve that refractive designs can not.

When it comes to reflective objective lenses, these lenses usually use a reflective mirror style design.

A good example of something a reflective objective lens can do that refractive can not is that the reflective lens can magnify as well as relay the image with its primary and secondary mirror system.

Through this double mirror system, the reflective objectives do not have the same aberration that refractive objective lenses can give because the light is reflected on metallic surfaces. 

Also, with reflective objective lenses, no further design is needed to deal with aberration. Reflective lenses also produce a higher light efficiency and as well as this, are better at resolving power which is great if you want a fine detailed image.

This is because the system is reliant on the mirror coating as opposed to the typical glass substrate.

Finally, reflective objective lenses are better at refractive objective lenses at working more into either the ultraviolet or the infrared spectral regions because of the use of mirrors.

How Objective Lenses Are Specified And How To Understand The Labelling Used

Like most lenses, the specifications that they have are visible on their body (generally speaking) but these are usually shortened and need some kind of translation to understand.

So this section of the guide will go over these different specifications, so you know what you are looking for.

Objective Standard

The most common style of objective standards include DIN and JIS, and something like this will be listed on the body of the lens depending on the standard type.

This listing shows the specification required to have a preset to use the lens on the system.

A good way to exemplify this is that a DIN (the most common standard) has a 160mm distance from the objective range to the range of the eyepiece which differs from a JIS standard objective lens that will have a 170mm range distance.

Magnification

On the body of the objective lens, there will be a notation of the magnification of the lens usually using an ‘x’ multiplication symbol. Sometimes on top of this, there will be a colored band around the circumference of the lens.

These different colored bands will reflect the magnification of the lens. The standard for this is:

• Black 1x magnification
• Brown 2x magnification
• Red 4x magnification
• Yellow 10x magnification
• Green 20x magnification
• Light blue 40x magnification
• Dark blue 63x magnification
• White 100x magnification

Numerical Aperture (NA)

The numerical aperture of an objective lens is a reference to the function of the focal length and the entrance pupil diameter. This is usually labeled near the magnification of the objective lens and is labeled as something like 1 or 1.3.

If the numerical aperture of an objective lens is above 1 this means that the use of immersion oil may be needed to get an accurate final image.

That makes this labeling quite important as if it is ignored, you might have completely ignored the use of immersion oil and be getting poor quality images because of this.

Cover Slip Thickness

The cover slip thickness of an objective lens is usually marked with a number (usually around the 0.17mm range for an example) and quite clearly labels the type of cover slip that will be needed to use the lens on a microscope.

The cover slip is actually quite important as it changes how the light is reflected from the specimen so knowing and accurately reading the necessary cover slip is important information to now overlook.

Quality Correction

Quality correction of an objective lens is a reference to one of the previously mentioned classifications like; achromatic, apochromatic, or plan and semi-plan objective lenses.

This quality correction of the lens is usually noted clearly on the objective to make sure their variable is taken into account when the objective lens is used.

Plan and semi-plan refer to the correct for field curvature and can also be referred to as microplan, planar or semi-planar. This field curvature, if not corrected, can lead to blurry images and if not corrected can lead to bad quality final images.

Plan objective lenses correct over 90 percent of field flat, semi-plan objective lenses produce around 80 percent instead.

Conclusion

So now you know everything that can affect an objective lens that can change its use drastically.

As mentioned in the introduction, objective lenses are massively important in shifting the use of a microscope, so now hopefully the range of this has fully been put into perspective.

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