DEPTH OF FOCUS
strictly speaking, the lens draws sharply only in one plane, in that, has which he has been set (theoretically, it even has a bowl surface, corresponding to the curvature of the lens system). All, what is outside the focus plane, is more or less blurry. ,
The human eye is not such a precise tool, so that it can notice too small deviations from the maximum sharpness, and yet apart from reproductions of flat objects (np. prints, illustrations etc.) we usually photograph objects extending into the depths and we want to, to both closer, and the others were sharp enough, therefore the lens must have a certain depth of field.
Depth of field the ability of the lens to render sharp images of objects on the film is called, located at different distances from the lens, and therefore not in the plane of adjustment.
When focused on the focusing screen of a distant object image, i.e. after setting the focusing screen at a focal distance from the lens, can be noticed, that images of closer objects, on the focusing screen are less sharp. Make them sharp, move the focusing screen farther away from the lens, causing the sharpness of the image of distant objects to deteriorate again. However, if you skillfully reduce the aperture, you can set the screen like this, so that the images of near and far objects are sharp. The smaller the aperture diameter will be, the greater the difference in the distance of different objects can be, whose images on the focusing screen are still sharp enough. So when you change the aperture, the sharpness in depth increases, and because photography takes pictures of three-dimensional objects (except photos, drawings, map itd.), at different distances from the lens, therefore the use of diaphragms is often necessary.
Drawing. Depth of field: a - ray coming from a far point, b - ray coming from a close point, 1 i 2 — planes of emerging images.
If on the focusing screen set behind the lens (drawing) in one place we will get a bright spot (as an image of a distant shining point projected by a lens), this is after moving the focusing screen to a different position, instead of a point we will see a bright circle on it. The size of this disc depends on the diameter of the aperture of the lens (the smaller the aperture will be, the smaller the diameter of the disc will be). Be sharp in depth, is the depth of field of the lens, so it does not depend on its construction. It decreases with the increase of the relative aperture or focal length. The longer the focal length or the larger the maximum aperture, the greater the difference in the sharpness of images of unequally distant objects.