The objective lens is the most important optical component of the microscope. It’s the part that sits in closest proximity to the specimen being examined, gathering light to produce optimal images for observation and analysis. This lens creates the first magnification by spreading out the light’s rays to make the object appear considerably larger by the time it meets your field of view at the other end of the eyepiece. Such a critical piece of equipment doesn’t come in a one-size-fits-all package. Below, we will discuss some of the different types of microscope objective lenses and the unique roles they play in microscopy.
Correcting for Aberration
Achromatic lenses are used to diminish chromatic and spherical aberrations which are the loss of color and focus that can happen when light wavelengths refract in direct light. These aberrations can be controlled by using an objective lens that contains both a convex and concave lens inside. Mounting these two different types of lenses to each other can bring wavelengths of red and blue light closer together, which puts them in the same focus and cancels out chromatic aberration. Another type of lens used to correct for both color and spherical aberration is the plan (or planar) lens. These produce a flatter field and can also give you a much larger working distance. However, they can be more expensive than achromatic lenses, so choosing between the two depends largely on how much power you need in your objective lens, and whether or not you need to adjust for field curvature, which only plan lenses can do. Achromatic lenses and plan lenses both come in dozens of magnifications and types, accommodating a wide variety of microscopy needs.
Infinity objective lenses did not become common until the 1980s but have since carved out a permanent spot in the microscope objective market. Previously, all microscopes had a standard tube length–the distance from the eyepiece to the objective lens was always 160 mm. Once microscope manufacturers began developing microscopes with varying tube lengths, lens manufacturers had to catch up with the changing technology. New tube lengths meant that microscopy equipment developers needed to adjust for these changes in their accessories, including objective lenses. Infinity optical systems use multiple sets of lenses within the lens house to correct a wide range of tube lengths–typically from 160-200 mm. This enables the lenses to be more versatile between microscopes of varying tube lengths.
Phase Contrast Microscopy
Obtaining high-contrast images of transparent specimens is difficult, especially when your specimen is alive and moving on a slide. Phase-contrast lenses allow you to observe microorganisms without having to fix and stain them. When your specimens are kept alive, a variety of biological functions can be examined and analyzed in real-time. Phase plates at the top of the objective lens diffract light, allowing these specialized lenses to tap into tiny changes in wavelength amplitude, which appears to the viewer as starker contrast on the slide. This makes the specimen much easier to view and observe.
Other Parts to Consider
Specialized microscopes, such as metallurgical microscopes, require their own specific metallurgical objective lenses. These devices are most often used to examine structural detail of ceramics, metals and other non-living materials. Another common microscope objective accessory is a Barlow lens. These can be added to the bottom of an objective lens to either increase or decrease its working distance, field of view or magnification. Since they can be interchanged between lenses, they are a cost-effective way to change the power and magnification of lenses you already own. Lastly, if all these lenses are starting to seem overwhelming, remember one quick trick for determining magnification at a glance: look at the band of color near the bottom of your objective lens. While the magnification number is usually written right on the lens, you can also quickly determine its strength by the color ring. Red indicates 5x magnification, while yellow means 10x, light blue means 40x and white can mean 100-250x.
There are hundreds of unique objective lenses to choose from, but once you have a greater understanding of the most common types, you can make a more informed decision regarding which lens is right for you. Whether you are a hobbyist or whether you require the use of a microscope in your day-to-day research, it’s important to gain an understanding of the strengths and weaknesses across the spectrum of objective lenses. Once you know exactly what you’re looking for, you’ll be well on your way to obtaining the best results and having an optimal viewing experience.
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