One of the most important parts of a microscope, like a telescope, is the lens. The selection and purchase of lenses for a microscope must be approached very carefully. The quality of the image given by the microscope depends on this, and how fine details you can see in it. On the market you can find a large number of lenses for microscopes from different manufacturers. According to the nature of the optical correction of aberrations, lenses are divided into achromats, apochromats, planachromats, planapochromats. There are specialized lenses, but we will not consider them, because. they are needed for special studies and are very expensive for home use.
Let’s take a closer look at each type of lens.
Achromat lenses have color correction for the main and two additional wavelengths of the visible spectrum. The chromatic difference in magnification is not corrected, but it can be compensated for by the so-called. compensation eyepiece. The curvature of the field is not corrected and in lenses, especially with low magnification, the image is blurry at the edges of the field of view. The marking on the lens barrel usually does not indicate the optical correction code.
Apochromats are lenses in which chromatic aberration has been completely corrected, but the chromatic magnification difference and curvature of the field of view have not been corrected. The lens barrel is marked APO,
Planachromats are lenses that have corrected field curvature, chromatic aberration, and chromatic magnification difference. A very useful lens for low magnifications, giving a sharp image across the entire field. Marked with the code PLAN, PL, Plan.
A planapochromat is a fully chromatic corrected lens with a flat field and corrected chromatic magnification difference. This is the most advanced and expensive lens for a microscope. The lens is marked with the code PLAN-APO, Plan-apo.
In the West, they produce the so-called. semi-plans (Semi-Plan). These lenses are between achromats and planachromats, and they have reduced (not completely corrected) field curvature. These lenses are marked with the SP code.
Consider in detail the marking of lenses for microscopes.
The magnification of the lens is indicated on the lens barrel, for example 4x, 40x, 100x. To calculate the magnification of a microscope, multiply the magnification of the objective by the magnification of the eyepiece.
After the value of the magnification of the microscope lens, the so-called. numerical aperture (denoted by the symbol NA in calculations). Numerical aperture indicates what maximum useful magnification can be achieved with this lens and what resolution the lens has. The maximum useful magnification of a microscope with a given objective is calculated by multiplying the numerical aperture by 1000. For example, a microscope objective with a numerical aperture of 0.65 has a useful magnification of 650x. It makes no sense to put a significantly larger increase, because. this will not add details, but only worsen the contrast and brightness of the image. You can also calculate the lens resolution. To do this, you need to divide the wavelength in microns at which we observe by twice the numerical aperture. High-quality immersion lenses with a numerical aperture of 1.40 give a resolution of the order of 0.12 microns.
Under magnification and numerical aperture, other parameters are sometimes indicated on the microscope lens. For example, the length of the microscope tube, with which the lens can work with standard magnification. For example, the usual length of the tube is 160mm. The thickness of the cover glass with which the lens will work normally is also indicated, usually 0.17 mm.
How do you choose a lens for a microscope? If you want to observe insects or other more or less large objects, then you should strive to buy a lens with a low magnification, such as 4x. But the most important thing is that the lens has a PLAN, PL or Plan correction code. These lenses will give a sharp image across the entire field of view. If you want to take pictures through a microscope, it is advisable to buy a lens not only with field correction, but also with full correction for chromatic aberration and chromatic magnification difference. But these lenses are very expensive. To observe bacteria, one should strive to buy an immersion objective with the highest possible numerical aperture. This will allow high magnifications to be applied.