Optimum Aperture Selection
The ability to effectively use an existing lens has a much greater effect on the sharpness of a photograph than the choice of the lens itself. The number of apertures 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 can be much more noticeable than the difference between different lenses with the same aperture.
Obviously, for the standard fast lens used in this test, sharpness is ideal at f / 5.6 aperture, but f / 4 is almost as good. f / 1.8 is somewhat soft, which is natural for maximum aperture. At f / 11, the drop in sharpness due to diffraction is already noticeable, but not fatal, but at f / 22, the picture is very blurry.
No lens is perfect. The laws of physics do not allow a ray of light to exactly follow the path that is prescribed to it by calculations performed for an ideal optical system. This leads to spherical, chromatic and other aberrations, which by no means can always be completely corrected. The center of the lens, as a rule, is impeccable, but the closer to the edge, the more light is distorted, scattered and refracted.
When the aperture is fully open, light collected from the entire surface of the lens enters the film or sensor. In this case, the aberrations of the lens are fully manifested. Covering the diaphragm, we cut off the part of the light flux passing through the edges of the lenses, allowing only the center, free from distortion, to participate in image formation.
It would seem that the smaller the size of the relative aperture, the higher the image quality should be, but there it was. At the other end of the aperture scale, an insidious enemy awaits us.
As the size of the opening of the diaphragm becomes smaller, an increasing percentage of light rays passing through the hole touches its edges. In this case, the rays deviate somewhat from their original path, as if bending around the edge of the hole – this is diffraction. As a result, each point in the scene, even being strictly in focus, is projected onto the sensor, not 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 a single photodiode, the blur becomes apparent. Further closure of the diaphragm only exacerbates diffraction.
The resolution of modern cameras is so high that a slight blurring of the image due to diffraction can be seen already at f / 11 apertures and more. Compact cameras with tiny sensors will in principle not allow you to use an aperture larger than f / 8, since the small size of the photodiodes makes diffraction especially noticeable.
Best Perception Zone
The optimal aperture value is individual for each lens, but, most often, lies in the area 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 distortion will become more noticeable, cover the aperture and the diffraction will begin to blur the image.
The better the lens, the more worthy it looks at the fully open aperture. This is especially true for the edges of the frame. For large apertures such as f / 11-f / 16, virtually all lenses behave the same.
The choice of aperture is the balance between the sharpness itself and the depth of the sharply depicted space. The artistic taste, experience and a clear understanding of the photographic tasks you are facing will render you immeasurably more significant help than any theoretical considerations. However, I still try to make your existence easier.
Optimum Aperture Selection Strategy
Find the aperture value at which your lens shows the best sharpness, and use this value whenever it is possible (most often it is f / 8 or so).
If you do not have enough light or if you want to highlight the main subject with a shallow depth of field, increase the size of the aperture, but try not to open it completely without need.
If need has arisen, boldly open the diaphragm and do not worry about this. In situations where you may need it, the aperture value is by no means the most important factor limiting the sharpness of pictures. Wiggling spoils the image much more ruthlessly than any lens aberrations.
If greater depth of field is required, cover the aperture, but no further than f / 11 for wide-angle lenses and f / 16 for telephoto lenses.
If you still do not have enough depth of field, which should not happen often, use f / 16 for wide-angle lenses and f / 22 for telephoto lenses. In no case is it worth tightening the aperture – you will pay for an increase in the depth of field as a too noticeable drop in overall sharpness.