المسافة فوق البؤرية .. كتاب أسس التصوير الضوئي

Hyperfocal distance

If we suppose that we use a bellows camera, and set the lens at an infinity distance, the image would thus be collected on the sensitive film or frosted glass exactly at the focal length distance of the lens. So if we want to lengthen the extension of the blower slowly. Gradually, we found at the beginning that the close objects appear sharp in detail, and at the same time the objects located at infinity remain sharp in detail as well, until the length of the blower reaches a certain limit, and it is noticed that if that limit was prolonged, the bodies located in infinity would not appear sharp in detail, and then it is said that the lens It has been set at the suprafocal distance, and this distance is symbolized by the letter H. Therefore, it is possible. The suprafocal distance is defined as the point in front of the lens and to. If you adjust the lens to it, all the details appear sharp from infinity.

We now know that the distance between . Between infinity, the details become sharp if the distance is set to H, but what is the ruling on objects located between this point and the lens? Do the details also appear sharp? This relationship is defined as follows:
A) _ If you set the lens to the hyperfocal distance H, then the objects that lie in the middle of the hyperfocal distance to infinity appear. the details . For example, if H = 12 meters, we find that all the bodies located. Between infinity and 6 meters in front of the sharp detail lens.

b) _ If you set the lens at a distance of +H, i.e. at a distance of 6 meters, according to the previous example, then all objects between H to +H appear sharp in detail. Thus, objects that are four meters away from the lens and extend to 12 meters are all sharp in detail.
h) _ If you set the lens at a distance of 1 over 2 (ie a distance of 8 meters - the previous example), we find that . Objects between H and H + are sharp in detail (i.e. objects confined at a distance of
3-6 meters). From the foregoing, you can decide that H is the one that decides the depth of field in the image. Therefore, if photography is done with a lens, it is easy, if the distance H is known, to estimate the depth of field in the image.
Mathematical equations were able to calculate the distance H for the lenses, and it was found that the only factor affecting the length or shortness of this distance is the diameter of the actual diphragm aperture, and this distance is calculated as follows:
suprafocal distance. The diameter of the actual diaphragm aperture, and thus the distance H is equal in any two lenses, of which the focal length differs for them, as long as the diameter of the actual diaphragm aperture is the same in the two lenses, and the depth of field that can be obtained becomes the same for the two lenses as well. Example: the actual diaphragm aperture lens has a name, so what is it depth of field. What can be obtained if the lens is set at a distance equal to half the distance above the lens?
The details are sharp in the area between HP and H, which is located at a distance from the lens between about ten meters and 3.35 meters. It may seem that it is difficult to measure the diameter of the actual diaphragm aperture, and in fact you find that it is easy to calculate it if we know the focal length of the lens, and the focal number of the aperture used (f No).