LENS ABERRATION YOU SHOULD KNOW

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• Friday 25 February 2022, 11:00-12:00
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An image created by an ideal photographic lens must have the following characteristics:

1) a dot must be formed as a dot; 2) a plane (such as a wall) perpendicular to the optical axis must be formed as a plane; 3) the image formed by the lens must have the same shape as the object itself. In addition, in terms of image expression, the lens must show the true color of the reproduced object. Almost perfect lens performance is only possible if only light rays entering the lens near the optical axis are used, and if the light is monochromatic (light of only one specific wavelength). However, in the case of a conventional lens, where a large aperture is used to obtain sufficient brightness and the lens must bring together rays passing not only near the optical axis, but from all parts of the image, it is extremely difficult to create the above ideal conditions due to the existence of the following interference:

1) Because most lenses are built only from lenses with spherical surfaces, the rays of light from one point of the object are not displayed as an ideal point in the image. (A problem that cannot be avoided with spherical surfaces.) 2) Different types of light (i.e., different wavelengths) have different focal point positions. 3) There are many requirements associated with changes in the angle of view (especially in zoom lenses and telephoto lenses). The general term used to describe the difference between an ideal and a real image under the influence of the above factors is “aberration”. Thus, in order to design a high-quality lens, the aberration must be very small, and the highest goal should be to obtain an image as close to ideal as possible. In general, aberrations can be divided into two broad categories: chromatic aberrations, which occur due to differences in wavelengths, and monochromatic aberrations, which occur even at a single wavelength.

Lens aberrations

Aberrations visible in the continuous spectrum Chromatic aberrations

- Longitudinal aberration (longitudinal chromatic aberration) - Cross-sectional chromatic aberration (transverse chromatic aberration)

Aberrations visible at specific wavelengths

Five aberrations of Seidl

Spherical aberration Coma Astigmatism Field curvature Distortion

Chromatic aberration When white light (light consisting of many colors mixed evenly so that the eye does not distinguish any specific color and thus perceives the light as white), such as sunlight, passes through a prism, a rainbow spectrum can be observed. . This phenomenon occurs because the prism’s refractive index (and dispersion intensity) varies with wavelength (short waves are refracted more intensely than long ones). Although it is most visible in a prism, this phenomenon also occurs in photographic lenses, and since it occurs at different wavelengths, it is called chromatic aberration.

There are two types of chromatic aberration: “longitudinal chromatic aberration”, in which the position of the focal point on the optical axis varies with wavelength, and “chromatic magnification difference”, in which the image magnification in peripheral areas varies with wavelength. In real photographs, longitudinal chromatic aberration appears as color blur or flare, and chromatic magnification difference appears as color fringing (when color is visible at the edges).

Chromatic aberration in a photographic lens is corrected by combining different types of optical glass with different refractive and dispersion characteristics. Since the effect of chromatic aberration increases at longer focal lengths, precise correction of chromatic aberration is especially important in super-telephoto lenses in order to obtain good image sharpness. Although there is a limit to the amount of correction allowed by optical glass, results can be greatly improved with an artificial crystal such as fluorite or UD glass.

Chromatic magnification difference can be called “transverse chromatic aberration” (because it occurs across the optical axis).

Note: While chromatic aberration is most noticeable when using color film, it also affects black and white images, manifesting itself as a decrease in sharpness.

Apochromat (apochromatic lens) Apochromat, also called APO means lens that corrects chromatic aberration for three wavelengths of light, the aberration being reduced to a large extent, especially in the secondary spectrum. An electronically focused supertelephoto lens is an example of an apochromatic lens.

Achromat (achromatic lens) A lens that corrects chromatic aberration for two wavelengths of light. When it comes to photographic lenses, these two correctable wavelengths are in the blue-violet and yellow ranges.

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