The central piece of an optical microscope is its objective lens. In a strict sense it should be called 'objective lens array' because it is made out of many lenses, but no-one does that. Microscopy objective lenses look like small metallic cylinders of some centimetres height and about one centimetre in diameter, this cylinder is ofter termed 'objective barrel'. On top and on the bottom of the barrel are the lenses. On the top side there's also a screw with which one can install the objective lens onto the microscope (normally on a revolving objective drum); the other side will come close to the object to be inspected. There are various designations engraved on the side of the cylinder giving some information about the type, the magnification power and the resolution of the objective lens. This entry will describe the particular aspects of objective lenses concerning microscopy. Obviously, since objective lenses are very generally defined as the first lens of any optical apparatus from the perspective of the object1, their use is not restricted to microscopy.
The Objective Lens
In microscopy there are different types of objectives (their magnification can vary, some special objectives require extra handling and adjustment, some are more simple), however their general appearance is more or less the same. Below a schematic cut through a generic microscope objective lens (array). In the depicted case the objective lens is a compound of two lenses which work together as if they were just one stronger lens. Many of the microscope objective lenses are compound of many 'weaker' lenses to form one strong lens.
TO THE TUBUS
II II ————— adapting screw
II _____——————_____ II
++———_____ _____———++ ————— additional lens(es)
|| —————— ||
|| || ————— metal cylinder
\\ ___————___ //
\\———__________———// ——————— first objective lens
) ( —————————— air or immersion oil
|________________| ———————— coverslip
| —— object holder (glass plate)
Magnification, Numeric Aperture and Resolution
Strangely, the magnification factor is the least important property of an objective. It's merely an equivalent to the strength of the lens array. The bigger the magnification factor the more the user will be able to 'zoom-in'. But this capability alone is of little help if the resolution is low. On the other hand, if the resolution is high it makes little sense if one would not be able to 'zoom-in'.
The resolution is defined by the smallest distance between two dots so that they can still be recognized as two distinct points. The problem with microscopy is that these dots have a minimum size. And this size is in its turn is given by the sharpness of the dots. So, the crucial question is: How sharp can images get? Or, the other way round: How blurry do dots get? The reason for the blurriness lies in the fact that not all light stemming from one point of the sample can be collected. The objective lense has a finite width, and it is placed at a finite distance from the observed dot. Hence, only a cone of light can be collected by the objective lens. And it's not the loss of intensity, but the loss of directional information that is going to account for the trouble. The problem lies in the missing interference pattern at the edge of the projected dot - it gets blurry.
As if that was not enough, the missing interference also depends on the colour of the light. So, in the end the blurriness and thus the resolution depending on the wavelength of the used light and the width of the collection cone. The distance between the two dots decreases with the cone-size (numeric aperture) and increases with the wave-length. The rough formula is the wavelength of light (hundreds of nanometers) divided by (roughly) twice the numeric aperture (which is a number around 1). So, even more roughly the resolution is about half the wavelength.
Increasing numeric aperture
There is one neat little effect in optics called total internal reflection, which is why the surface of a lake or a glass plate looks like a mirror at shallow angles. The angle where light starts to get reflected (the critical angle) is proportional to the quotioent of the refractive indices of the media in question. If this quotioent is high (e.g. going from water to air) then the angles get less shallow, if the quotient is low (e.g. going from glycerine to glass) then the angles get wider. Now, how does this affect numeric aperture?
How can the detection cone get wider? The answer is: When the refractive indices of the medium between the object and the first lens come close. On the other hand, because of this, the magnification power of this lens will diminish. There is one optimal compromise between losing lens power and increasing numeric aperture. For that reason some objectives work in special immersion media, like special immersion oils or water. These objective lenses have a very complex design to optimise the effect of the immersion medium.
Types of objectives
Some types of objectives were already presented in the preceeding section, while explaining some special working mechanisms. Namely: Immersion and the so-called 'air' (= non-immersion) objectives which also vary in magnification factors and numeric apertures.
There are more variations which come from the effort to correct certain optical flaws, or aberrations. There are two types of aberration, which can be corrected: First, the chromatic abberation happens because blue light is bent stronger than red light by a material. To circumvent this the polychromatic light is sent through a convex lens with a lower refractive index but with the same curvature after each lens. The magnification effect will remain, but the colours will be shifted the other way round compensating for the aberration. Objectives corrected with regard to this abberation are called achromatic. Second, the spheric aberration is due to the fact that most of the lens curvatures are derived from the surface of a sphere2, for that reason the peripherial part of the lens will focus a bundle of light to a position different from the focal spot for light going through the central part of the lens. This aberration is weak for lenses with small curvatures and more pronounced for stronger lenses. A combination of lenses arranged at very special positions compensates for this aberration. Lenses with that corrective arrangement are called aspheric. However, aspheric lenses are very sensitive to changes in the medium between the object and the lens, like coverslips with different thickness or different optical quality. This problem can be corrected by re-adjusting the lens positions.
Summing up there are two 'main' types of objectives: Immersion and air objectives which can both be achromatic and/or aspheric. One further exotic type is the mirror objective, which works in the same way as a mirror telescope, but is a lot more difficult to handle. It has the advantage that it doesn't lose as much light-intensity as lens objectives. All of these objectives are available in further more specialized variations, which allow a finer tuning of optical possibilities.