In microscopes of any type or design, the lens closest to the object or specimen being viewed is known as the objective lens. This is the instrument’s most important lens. It gathers light from the object being viewed and produces a magnified image. This image is sent up the microscope’s tube to the ocular, more commonly known as the eyepiece; here the final changes to the image are made so that it can be viewed or photographed. The design and quality of the objective lens is also the primary determining factor in degree of magnification, and in resolution – the clarity and detail in the final image. Once again, while a microscope’s ocular and other elements are important and affect final image quality, by far the most vital element is the objective lens.
There are many different objective lenses available, designed for different types of viewing and manufactured with varying degrees of quality. As with most any other type of instrument or equipment, the finer the quality of the final result (in this case, the image seen by the viewer or captured by the camera), the higher the cost.
There are four major types of microscope objective lenses.
Achromatic Objective
The simplest type of objective lens is known as achromatic or achromat objective. The name refers to the fact that the image the lens produces has been corrected to prevent chromatic aberration – which, simply put, is a fuzzy, multi-colored blur appearing around the edges of an image produced by the lens. This blurring results from the effects of the lens glass on light passing through it. Correction for chromatic aberration is also sometimes referred to as colour correction.
The image produced by an achromatic objective has significantly reduced chromatic aberration. Because of another type of optical effect known as spherical aberration, only objects in the centre 60 to 65% of the lens’s field of view appear in focus – outside that area they go increasingly out of focus.
Semi-plan Objective
The semi-plan or semi-planar objective (sometimes also called micro-plan) is the next step upward in objective lens quality. This type of lens also corrects chromatic aberration, but provides an in-focus image over 80 to 85% of its field of view. Semi-plan lenses are more complex in design and construction than achromatic lenses, and so their cost is generally higher.
Plan Objective
Plan or planar objectives are yet another grade higher in terms of image quality. A plan objective produces images which are in focus over 95 to 100% of the field of view, and with a finer degree of colour correction than the two types described previously. As you might expect, the plan objective lens’s increased image quality is the result of more numerous and higher-quality elements, more difficult to assemble precisely – and therefore more expensive than achromatic or semi-plan objectives.
Infinite Plan Objective
The infinite plan or infinity-corrected objective is the most advanced type of objective lens. Because of its superior resolution and very high degree of correction for both chromatic and spherical aberration, the infinite plan objective has become the first choice for research-grade, medical and industrial microscopes.
The infinite plan objective is actually part of an overall optical system, different from the system used in instruments with the objective types described above. That is, infinite plan objectives can only be used in microscopes specifically designed for objectives of this type.
The special optical properties of the infinite plan objective and system not only produce superior image quality, they also allow the addition of auxiliary components along the optical path, without compromising image quality (as would occur in other types of microscopes). This factor makes infinite plan objectives and microscopes even more versatile, and increases their importance as serious scientific and industrial instruments. Though their cost is higher, when there is no room for compromise with quality and clarity, infinite plan objectives are the obvious choice.