The classification of lenses is more complicated than the classification of microscopes. Lenses are divided according to the principle of calculated image quality, parametric and constructive-technological features, as well as research and contrast methods.According to the image image class, lenses can be:
- plan achromatic;
Achromatic lenses are designed for use in the spectral range 486–656 nm. Correction of any aberration (achromat) is performed for two wavelengths. These lenses eliminate spherical aberration, position chromatic aberration, coma, astigmatism, and partially spherochromatic aberration. The image of the object has a slightly bluish-reddish tint. As the focus of the specimen changes, so does the color reproduction, making it difficult to interpret the result.
Plan achromatic lenses
In plan lenses, the curvature of the image along the field has been corrected, which provides a sharp image of the object throughout the entire field of observation. In addition, the lenses also have achromatic correction, which allows them to be used for routine work. Plan achromatic lenses are commonly used in photography and video shooting in medicine and biology.
Semi-apochromats are modern lenses that have an image quality intermediate between planachromats and apochromats. Semi-apochromats have an extended spectral region and achromat is performed for three wavelengths. They are made of special glasses containing fluorite. The extended spectral range allows them to be used for fluorescence microscopy in the UV range, which is necessary for work on the FISH method, as well as in scientific research, where differential interference contrast DIC is needed.
Apochromatic lenses have an extended spectral region and achromatization is performed for three wavelengths. At the same time, in addition to position chromatism, spherical aberration, coma and astigmatism, the secondary spectrum and spherochromatic aberration are also corrected quite well due to the introduction of lenses made of crystals and special glasses (fluorite) into the scheme. Compared to achromats, these lenses typically have larger numerical apertures, produce sharper images, and accurately reproduce the color of an object. Apochromatic lenses are used in research microscopes as well as confocal systems. Since these lenses eliminate all possible image aberrations, such lenses are more expensive than plan lenses.
Routine and working laboratory-grade microscopes used in medicine are equipped with cost-effective planachromat lenses. Their color reproduction and correction are fully suitable for working with both native and stained preparations.
According to parametric features, lenses are divided as follows:
- Lenses with a finite tube length (eg 160 mm) and lenses corrected for infinity tube length (eg Olympus UIS2);
- Small lenses (up to 10x); medium (up to 50x) and large (more than 50x) magnifications, as well as lenses with extra high magnification (over 100x);
- Small lenses (up to 0.25), medium (up to 0.65) and large (more than 0.65) numerical apertures, as well as lenses with increased (compared to conventional) numerical apertures (for example, apochromatic correction lenses , as well as special objectives for luminescent (fluorescent) microscopes);
- Lenses with extended (compared to conventional) working distances, as well as with large and extra long working distances (lenses for work in inverted microscopes). The working distance is the free distance between the object (the plane of the coverslip) and the bottom edge of the frame (lens if it protrudes) of the frontal lens component;
- Lenses that provide observation within a normal linear field (20 mm); wide-field (22 mm) ultra-wide-field lenses (from 23 to 26.5 mm);
- Lenses are standard (45 mm) and non-standard in height.
Parfocal height, parfocality – the distance from the reference plane of the lens (the plane of contact of the screwed-in lens with the revolving device) to the preparation plane with a focused microscope, is a constant value and provides parfocality of a set of lenses of different magnifications of similar height, installed in the revolving device. In other words, if a sharp image of an object is obtained with a 10x lens, then when moving to subsequent magnifications (20x, 40x, 60x, 100x), the image of the object remains sharp within the depth of field of the lens.
According to the design and technological features, there is the following division:
- Lenses with and without a spring-loaded frame (starting with a numerical aperture of 0.50);
- Lenses having an iris diaphragm inside to change the numerical aperture (for example, in lenses with an increased numerical aperture, in transmitted light lenses for implementing the dark field method, in polarized reflected light lenses);
- Lenses with a corrective (control) frame, which provides the movement of optical elements inside the lens (for example, to correct the image quality of the lens when working with different thicknesses of the coverslip or with different immersion liquids; as well as to change the magnification during a smooth – pancratic – change of magnification).
By providing research methods and contrasting lenses can be divided as follows:
- Lenses working with and without cover glass;
- Lenses of transmitted and reflected light (reflexless); luminescent lenses (with a minimum of autofluorescence (autoluminescence); polarizing lenses (without glass tension in optical elements, i.e., not introducing their own depolarization); phase-contrast lenses (having a phase element – a translucent ring inside the lens); DIC lenses (DIC), working according to the method of differential interference contrast (polarizing with a prism element);
- Immersion and non-immersion lenses.