Objectives - The world under the microscope

In order to view the specimen with different objectives, they are mounted in a objective-revolver. Most microscopes have a revolver that can hold between three and six objectives. The Leitz Orthoplan has a revolver with five objectives. By twisting the revolver it is possible to look at the specimen with another objective. The objectives are centred in the revolver in such a way that the same part of the specimen is viewed each time. In most objectives, the lenses are fixed in the lens. Only the front lens of the objective is visible and is therefore the most sensitive to damage and dirt. In particular, objectives that have a higher magnification factor come close to very close to the specimen cover glass (e.g. a Leitz 63x Planapochromate has a working distance of 0.15mm[1]). To prevent damage to the front lens, these objectives have a spring-loaded mechanism that allows the front part to slide telescopically inwards when contact is actually made with the cover glass. Objectives can be divided into different categories due to their constructive way. So there are: Achromates, Apochromates, Fluorites, PlanAchromates and PlanFluorites[3]. Achromatic objectives have been corrected for two colors and do not give a sharp image all the way to the edge. These objectives are quite simple in construction and therefore relatively inexpensive. The planAchromatic has the same color correction, however, with an extra lens the entire field of view is sharp. This is particularly important for micro photography. The Apochromatic objective is corrected for three colors and has a high Numerical Aperture due to many lenses of high quality glass types which creates a high dissolving power. PlanApochromatic objectives are also corrected for planity and therefore very suitable for microphotography. Of course these objectives are expensive to very expensive. The Fluoriten and PlanFluoriten are regularly called half Apo's in the microscopy world. The quality is between the Achromates and Apochromates[3]. Click on the schematic image below for an enlargement.

As already described in Microscopes, the exposure, the objective, the brightness and the resolving power are the four most important criteria that determine the quality of the image. The resolving power of a microscope (the resolution) is defined as the smallest distance at which two points are just separated. The resolving power depends mainly on the objective, to a lesser extent on the condenser, while the eyepiece does not contribute much. Image quality is determined by the color reproduction, transparency, contrast and resolution of the objective, while specimen properties are also important. Good image information is obtained when magnification and resolution are balanced. When higher magnification is not accompanied by higher resolving power, this results in meaningless magnification. A very important specification of the objective lens is the numerical aperture (NA), which is mainly determined by the top angle of the cone of light that can be captured by a lens. In the formula used to calculate the NA, the half aperture angle of the light cone (µ) is multiplied by the refractive index (N) of the medium between lens and specimen[2]. Theoretically, the NA of a dry lens (a lens that does not use immersion oil) can never be higher than 1.0. Since the refractive index of air is 1, the aperture angle of the lens should be 180° and the front lens is then infinitely large. Practically, these lenses reach a maximum NA value of 0.95. With oil immersion lenses, however, a higher NA can be achieved. The refractive index of this oil is 1.515. The maximum achievable NA is now 1.4.

The Leitz Orthoplan is designed for objectives with a tube length of 170mm. In 1976 Leitz switched to the then new standard of 160mm (see memo in downloads). According to Leitz this new standard can also be used normally in the Orthoplan and on several types of Leitz microscopes. Practice has proven that the 160mm tube length is indeed fine to use. Even later a new standard was introduced: the infinite objectives. These can not be used on the Leitz Orthoplan.

The thread with which Leitz objectives are screwed into the revolver is quite standard.

Objectives with a tube length of 160 and 170mm all have DIN-RMS W0.8" x 1/36".

The infinite objectives have a thread size of M19 x 0.75mm.

However, most manufacturers use this size, some have different sizes[6].  

Most Leitz objectives have a standardized optical correction for a cover glass thickness of 0.17mm. From a numerical aperture of 0.4 a wrong thickness of the cover glass is noticeable. In the fifties and sixties, lenses were also manufactured where the thickness of the cover glass could be adjusted on the objective by means of a rotating ring. The thickness could be set between 0.12 and 0.22mm. This was necessary because the different manufacturers did not produce a constant 0.17mm. Even now, a 0.17mm is not always guaranteed. This should be taken into account when purchasing[5].

On all objectives there are indications such as: magnification factor, tube length, type of lens etcetera.

The enumeration below gives an explanation for the most commonly used engravings[6].