Characteristics of Objective Lenses
Objective lenses are simply the lenses of any microscope that are closest to the object being observed. These lenses come in several different types, depending on what the observer needs to accomplish, and a myriad of different magnifications. The most common types of objective lenses available are achromatic, super high contrast, plan, semi-plan and phase contrast.
Achromatic Objective Lenses
Since visible light is in reality composed of various colors, microscope lenses will experience difficulties as the different colors bend at different angles when the lens gathers them, causing the lens problems in producing a common focus. This results in varied image sizes and "chromatic aberrations." An achromatic lens uses two different types of glass with dissimilar refractive indices in order to minimize the effect. These lens types are the most common sold with microscopes today.
Super High Contrast Lenses
Since the achromatic lenses still have blurring due to the color aberrations, some lenses can have a contrast-enhancing coating applied in order to supply a more-detailed, in-focus image. Some super-high-contrast lenses are simple achromatic lenses containing internal field stops to supply higher-contrast images.
Plan and Semi-plan Lenses
A typical achromatic lens provides a "flat" focus of around 60 percent of the observed image due to the manner in which light bends. The areas out of focus can be brought into better focus by adjusting the lens, but the 60 percent area will then be out of focus. If an observer is using the microscope to take pictures of subject images and needs more than 60 percent to be in focus, a semi-plan lens will flatten 80 percent of the image view, while plan lenses will flatten 100 percent.
Phase Contrast Lenses
These objective lenses are only able to be used on subjects that do not absorb light, such as cell parts in protozoans, bacteria, sperm tails and other cell types that are not stained. Early in the 20th century, Frits Zernike developed the technique of phase contrast to view microscopic detail. A phase contrast objective lens does not need specimen staining to view it. As a result, the cell cycle can be observed using a microscope that has a phase contrast objective lens. Due to his techniques advancing microscopy, Zernike won the Nobel prize for physics in 1953.
The majority of details in cells cannot be detected using other lenses with bright fields because there is not enough contrast between structures with similar transparency and natural pigmentation. This principle works by having different wavelengths between the light at the center and the edges of an object, making the light rays "out of phase."
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