Optical Theory

Optical lenses are generally made of glass or acrylic. The optical glass is manufactured from various ingredients such as silica, oxidized lanthanum and boric acid etc. that are dissolved at the temperature of 1200 to 1400 degrees. Natural crystals like fluorite are used in some cases.

Lenses are formed into a lot of shapes that provide various optical effects. The figures indicated above show respectively the shapes of convex, plano-convex, meniscus-convex, concave, plano-concave, meniscus-concave and aspherical-convex. A surface of spherical lens is, as well known, a part of sphere, but prolongation of aspherical surface forms into a parabola instead of sphere.

A refractive index varies by the wave length of ray. Each ray has a different focal point on the axis after passing through a spherical convex lens. That is why it causes aberrations in image. The convex and concave optical elements made of different types of glass are combined to compose efficient corrected lenses that provide quite clear images without aberrations. The above figures are respectively a combination of plano-convex and the other two, a doublet and a triplet called achromatic lenses.

Magnification power of optical lens is indicated as D(diopter) or X(multiple). A X is equivalent to 4Ds. However, for lower magnification, one more X is added on the converted figure, i.e. in case of 8 diopters, 8/4=2 2+1=3X, but no addition on higher power. It is calculated by distance of focal point of lens on the basis that 100cm length is converted to a diopter, for example a lens with 25cm focal point has 4D magnification and it is calculated as 100cm/25cm=4D 4/4=1 1+1=2X.

It restrains reflection and helps light ray to penetrate through a lens. Approx. 4 percent of light is lost by reflection when passing through a face of lens. Even a single lens, it has two surfaces and light is reduced by a little less than 8 percent in total. Therefore images become dark and indistinct while observing with a complex optical system. The coating with fluorinated magnesium helps passage of more light to improve images for much more distinct observation.

When light ray goes from a medium to another that differs in optical characteristics, it turns with some angle in its incident course. Refractive indexes of various mediums are as follows. 1.00 for atmosphere at the temperature of 20 degrees, 1.50 to 1.80 for optical glass, 1.33 for water and 2.40 for diamond. The refractive index varies in temperature and wave length. Higher index provides more magnification.

Natural light, namely sun ray, is called white light that is decomposed into spectrum of 7 colors. White light is able to be made out from equal mixture of 3 primary color rays, i.e., red (abt.700nm long), green abt.540nm and blue abt.430nm. Any other color is obtained by proportional mixture of those three.

According to the theory a spherical convex lens must converge parallel rays on a point of the axis, but it generally has some expansion. Sometimes an image observed by lens fades away and is distorted or partially indistinct. Those are typical occurrences of aberrations. There are spherical aberration, curvature of field, distortion, astigmatism, coma aberration and chromatic aberration etc. The latest optical systems and technology have been developing to completely correct those aberrations, but it has not been realised yet. An aspherical system corrects both of spherical and coma aberration and must be ideal for convergence of rays on a focal point. But on the other hand, it leaves some objections for observing image as the curvature of field and chromatic aberration still remain uncorrected.

It corrects chromatic aberration and offers more distinct images, consisting of convex and concave optical elements that respectively have different characteristics.(See the fig. of doublet lens in the Corrected Lens section.) It is effective only for correcting two colors, red and blue, but the other one, green, is left. Then more multiple optical systems and less dispersion glass are utilized to settle the problem. Such efficient components are called as apochromatic and ED lenses etc.

A range of distance on the optical axis where objects are clearly observed throughout back and forth. Lower magnification provides more depth of focus that enables to look at uneven things in different height at a time. On the contrary, it has to be very careful to set the focus in case of higher magnification due to less depth of focus.

It is most important to choose proper magnification according to the applications. You are likely to prefer higher magnification than what it is actually needed. If you complain about your magnifier and want to change it, try lower power first and the problem would be settled as it improves brightness, viewing field and aberrations of image. But if still unsatisfied, then try higher ones in order. Generally the power of 1.8x to 3x must be effective both for reading and complicated precision works, 3x to 8x for photography, 10x to 12x for jewel appraisal and inspection of electronic micro chips and 15x or more for observing printing dots, ticks, pollen and surface of metal etc. It has to be minded that continuous use for long time with excessive high power will definitely bring more inconvenience caused by fatigue that may degrade your health and quality of works. But you should not be too careful when selecting a lower power magnifier for reading or precision works as it seldom has serious aberrations. Therefore do not think much of famous brands or plausible descriptions in this case, but only have to make sure that it provides an distinct image without distortion. On the other hand, for higher power to observe microscopic objects, it always annoys with considerable aberrations that cannot completely be deleted by all means. At least you had better to be careful about the following faults to refrain inconvenience.