Aberration of a photographic lens is the last thing a novice photographer should think about. They absolutely do not affect the artistic value of your photos, and their impact on the technical quality of images is negligible. Nevertheless, if you do not know what to take your time, reading this article will help you understand the variety of optical aberrations and methods of dealing with them, which, of course, is invaluable for real photo-erudite.
Aberrations of the optical system (in our case, the photographic lens) is the imperfection of the image, which is caused by the deviation of the rays of light from the path that they should follow in an ideal (absolute) optical system.
Light from any point source, passing through an ideal lens, should have formed an infinitely small point on the plane of the matrix or film. In fact, this naturally does not occur, and the point turns into the so-called. a spot of scattering, but the optical engineers who design the lenses try to get as close to the ideal as possible.
Monochromatic aberrations are distinguished, equally inherent in light rays of any wavelength, and chromatic aberrations, depending on the wavelength, i.e. from color.
Diffraction stands apart, which, although it can be attributed to lens aberrations, however, due to its fundamental nature and fundamental fatigue, it is usually considered separately from other aberrations.
In 1857, the German mathematician and astronomer Philip Ludwig Seidel identified and mathematically described five so-called monochromatic aberrations of the third order. Here they are:
Image field curvature
This article was written for photographers, not mathematicians, and therefore we are primarily interested not in what formulas describe each of the aberrations, but in how aberrations manifest themselves in practical photography.
Let’s consider them in order.
The peculiarity of a spherical lens is such that the rays of light passing through the lens near its edge are more refracted than the rays passing through the center. This is explained by the fact that initially parallel rays of light fall on the spherical surface of the lens at different angles. The further the path of the beam lies from the optical axis of the lens, the greater the angle of incidence, and the more it is refracted. Ultimately, this leads to the inability to focus the point except in the form of a blurred at the edges of the spot, and the whole image is blurry.
The course of light rays in an ideal lens.
Ray path with spherical aberration.
Aperture of the lens significantly reduces spherical aberration, since when the aperture is reduced, a part of the rays cut off through the edge of the lens is cut off, and the rays remaining near the optical axis form a sharper image.
When designing lenses, spherical aberrations are eliminated by combining positive and negative lenses, as well as using special aspherical elements, i.e. lenses whose refractive surface has an aspherical shape, so that, regardless of the distance of the light rays from the optical axis of the lens, they all refract equally, if possible, and converge when focusing to one point. An excessive correction of spherical aberrations, by the way, also does not lead to anything good: the spot of scattering becomes brighter at the edges than at the center, which manifests itself in the form of an annular bokeh.
Comic aberration or coma occurs when light rays pass through the lens at an angle to the optical axis. As a result, the image of point light sources acquires the shape of asymmetric spots of a drop-like (or, in severe cases, comet-like) shape at the edges of the frame.
A coma is noticeable at the edges of the frame when shooting with a wide aperture. Since diaphragm reduces the number of rays passing through the edge of the lens, it usually eliminates comic aberrations as well.
Structurally, coma is fought in much the same way as with spherical aberrations.
Astigmatism is manifested in the fact that for an inclined (not parallel to the optical axis of the lens) light beam, rays lying in the meridional plane, i.e. the plane to which the optical axis belongs is focused differently from the rays lying in the sagittal plane, which is perpendicular to the meridional plane. This ultimately leads to asymmetric stretching of the blur spot. Astigmatism is noticeable at the edges of the image, but not at its center.
By the curvature of the image field is meant a phenomenon characteristic of very many lenses, in which a sharp image of a flat object is focused by the lens not on the plane, but on a certain curved surface. For example, many wide-angle lenses exhibit a pronounced curvature of the image field, as a result of which the edges of the frame appear to be focused as if closer to the observer than the center.