Image sharpness ~~ optical: the degree of distinctness (clarity, distinguishability) of points, contours, details of the optical image. Depends on the accuracy of focusing of the optical system, the magnitude of its aberrations, the range of wavelengths of light waves that create the image.
~~ photographic: the degree to which the boundary between two areas of a photographic image with different exposures is blurred. Depends on the properties of the photographic material, the conditions of its exposure and processing. Quantitatively, the sharpness of a photographic image (boundary unsharpness) is estimated using the so-called boundary curve, or the sharpness curve, which characterizes the distribution of the optical density of blackening in the direction perpendicular to the boundary of the selected image element.
Sharpness is also affected by: light scattering optical system, diffraction light on lens frames and other parts. In photography, when shooting spatial objects, the blur of an optical image is associated with the impossibility of reproducing with the same clarity (with the same circle of confusion) in the plane of the photolayer of points of the object at different distances from it due to the finite value of the depth of field created by the lens.
How is image sharpness achieved during focusing?
Pay attention to the picture below. This is a conditional image of how focusing occurs.
At the same time, it turns out that there is a difference where to point the camera. Which of the pictures correctly and incorrectly focused on the face? In any case, we see it like this:
The sensor responsible for focusing is small, sitting at the very bottom of the camera behind the lenses.
He sees it like this:
Sensors of old types in DSLRs sit in the same place, but they see worse:
And there are several types of focusing sensors themselves. Horizontal, vertical, cross. And the more powerful this sensor, the better it sees sharpness.