معيار حدة الصورة .. كتاب آلة التصوير

Standard of Sharpne8s:

An image is considered sharp if every point of light emanating from the body is represented by a point in the image. Then we can be lenient in the expression and we decide that the image is considered sharp if every point of light emanating from the body is represented by a point of light on the image or by a very small circle of light, because it is difficult to distinguish between the small light circle or the small light dise as it is sometimes expressed. And between the point, and if we look at the Sabbath expression, we find that the diameter of this light circle, which we express as small, should also be specified. Here the reader may think that we will define a specific diameter in a millimeter, for example, in order to make it a dividing line between what we call the large optical circle and the small one. However, it is wrong for us to do that, as another test is made from us, as the diameter of an eye, such as a millimeter, may be considered large if the image is seen at a close distance, and small if the image is seen from a distance, as the distance of sight, i.e. the distance of the image from the eye, is considered to be in the diameter of the mixing circle. allowed .

Experiments have proven that it is difficult for the human eye to distinguish between two points, one of which is 1/4 millimeter away from the other, if the two points are seen at a distance of 25 cm, which is the normal vision distance for the human eye. Rather, the eye sees it as if it were one point. Likewise, it is also unable to distinguish between a point and a circle with a diameter of one millimeter if they are seen at the same distance of 25 cm. Based on the foregoing, it was agreed that the diameter of the mixing circle, i.e. the diameter of the cosmic disk, should not exceed 1/4 millimeter (1/100 of an inch), if the image is seen at a distance of 25 cm (i.e. 10 inches).

And since the image that the eye sees at a distance of 25 cm is the same as the image recorded by the lens whose focal length is 25 cm as well, because if we imagine that this image that we judge by the sharpness of its details was a light image on the back frosted glass in the camera (or it was an image Negative) and we wanted the judgment on it to be similar in its circumstances to that of the positive image, so we found that the focal length of the photographic lens must be made 25 cm as well (i.e. exactly equal to the distance of vision) so that the judgment in both cases is under similar fair conditions. Therefore, it can be said that in order for the image to be considered sharp, the diameter of the permissible circle of confusion should not exceed 1/1000 of the focal length of the lens. .
It is clear that we have arrived at this ratio of 1/1000 by means of the ratio between the diameter of the optical disc over the focal length of the lens
That is, the focal length of 250 millimeters (allowed from the focal length (i.e. we see from the previous definition that the diameter of the mixing circle) or the optical disc) allowed must be very small if the focal length of the lens is small, and vice versa, meaning that the mixing circle is large Relatively if the focal length of the lens is long. If the lens is of medium focal length
(50 riyals, for example) and is intended for photography with a 35 riyal Miniature Camera. According to the previous rule, the optical circle whose diameter is: (i.e. 1/20 mm) becomes an acceptable diameter.

But if we use a telephoto lens with a focal length of 200 millimeters, for example, then the permissible diameter of the optical disk becomes only 1/5 of a millimeter, making the ratio of the focal length of the lens to the diameter of the optical disk the only factor in estimating the standard of image sharpness, as despite the stability of the image area (24× 36 millimeters) It was allowed once that the diameter of the mixing circle not be more than 1/20 of a millimeter when the focal length was 50 millimeters, then we made it again only 1/5 of a millimeter when the focal length of the lens was 200. millimeters, and for this reason they saw the inclusion of the image area in the camera as a factor in defining the image sharpness criterion, so the definition became as follows:

The image is considered sharp if the diameter of the optical disc (ie the mixing circle)

is less than 1/1000th of the hypotenuse joining opposite angles of the negative space. And since this chord is equal to the average focal length (Fig. 67 p. 101), it has become agreed that the diameter of the permissible mixing circle is 1/1000 of the focal length of the lens if it is of medium focal length, and this is the basis on which we will rely in this book ( 1) When calculating the depth of field.