When we look at the world, the eye is constantly moving, it grabs the small pieces, and the brain combines it all into the overall picture. But if we take a photograph in which all objects are cut, then it would seem to us painted, it happens when we remove the "dark" optics or a compact camera, why? Because the depth of field depends on: 1) focal length lens, and 2) the distance to the subject; 3) The amount of solar cell, matrix it or film (behold why in compact cameras DOF always big); 4) luminosity or on much now open diapliragm; Focal distance and distance obtained binds than more focal distance the less DOF but if we want storyline in one scale on different focal lengths, we need "zoom legs" ie derogate or approached and if we shoot on telephoto lens and waste, increasing distance and increasing the depth of field. It turns out we can administer the flu only by the size of solar cell and open or close the aperture, as well as the matrix in our a camera, only the diaphragm. And now a little bit of high-aperture optics, if the clamp, close the aperture, we can always, then open it, the more value the luminosity of the lens, we did not succeed. That is, if we have a lens with aperture 2.8 aperture is close to 22 when we will need a large depth of field, or will be a lot of light, we can, but to open up to 2.8 on the lens which has a value of 4.5 – no! That is obtained by the smaller number of the more aperture, since luminosity is 1/na aperture. Remember this formula, we will not get confused as to open or close the aperture to a darkened room we had was barely visible contours on a black background, and in the solar day of overexposed persons and to reduce the depth of field, make sharp eyes only, and everything else, including debris in the street, gone in a blur.