Selecting the optimal aperture
The ability to effectively use an existing lens has a much greater impact on the sharpness of the photo than the choice of the lens itself. The aperture number is the most important of the shooting parameters that affect the technical quality of the image. The difference between different aperture values of the same lens may be much more noticeable than the difference between different lenses at the same aperture.
Aperture f / 1.8 f/1.8
Aperture f/2.8 f/2.8
Aperture f/4 f/4
Aperture f/5.6 f/5.6
Aperture f/8 f/8
Aperture of f/11 to f/11
Aperture f/16 f/16
Aperture of f/22 to f/22
Obviously, for the standard high-speed lens used in this test, the sharpness is ideal at an aperture of f/5.6, but f / 4 is almost as good. f / 1.8 is somewhat soft, which is natural for the maximum aperture. At f / 11, the sharpness drop due to diffraction is already noticeable, but not fatal, but at f / 22, the image is blurred very significantly.
Aberration of the lens
No lens is perfect. The laws of physics do not allow a ray of light to follow exactly the path prescribed for it by calculations made for an ideal optical system. This leads to spherical, chromatic, and other aberrations that can not always be completely corrected. The center of the lens is usually perfect, but the closer to the edge, the more light is distorted, scattering and refracting.
When the aperture is fully open, the film or sensor receives light collected from the entire surface of the lens. In this case, the lens aberrations are fully manifested. By covering the aperture, we cut off part of the light stream that passes through the edges of the lenses, allowing only the center, free from distortion, to participate in the image formation.
It would seem that the smaller the size of the relative hole, the higher the image quality should be, but this was not the case. At the other end of the aperture scale, we have an insidious enemy waiting for us.
As the size of the aperture becomes smaller, an increasing percentage of light rays passing through the aperture touch its edges. In this case, the rays deviate somewhat from their original path, as if skirting the edge of the hole – this is diffraction. As a result, each point of the scene, even when strictly in focus, is projected onto the sensor no longer as a point, but as a small blurry spot called the airy disk. Its size is larger the smaller the aperture. When the diameter of the airy disk begins to exceed the size of the individual photodiode of the matrix, the blurring becomes obvious. Further closing of the diaphragm only aggravates the diffraction.
The resolution of modern cameras is so high that a slight blurring of the image due to diffraction can be noticed already at apertures from f / 11 and more. Compact cameras with tiny sensors will not allow you to use an aperture greater than f/8 in principle, since the small size of the photodiodes makes diffraction particularly noticeable.
The optimal aperture value is individual for each lens, but most often lies in the region of two steps from the minimum, i.e. f/5,6-f/11, depending on the specific model. Open the aperture wider, and the optical distortions will become more noticeable, cover the aperture, and diffraction will begin to blur the image.
The better the lens, the more worthy it looks at a fully open aperture. This is especially true for the edges of the frame. At large aperture values, such as f / 11-f / 16, almost all lenses behave the same way.
The choice of aperture is a balance between the actual sharpness and the depth of the sharply displayed space. Artistic taste, experience, and a clear understanding of the photographic challenges you face will help you immeasurably more than any theoretical reasoning. However, I will try to make life easier for you.