معيار حدة تفاصيل الصورة .. كتاب أسس التصوير الضوئي

Standard of Sharpness

We have often expressed that a certain image is sharp, so what is meant by this change? Or another meaning, what are the limits of sharpness of details? And when do we consider the image sharp details? Or is there a dividing line that requires us to consider the image sharp or not sharp?

There is no doubt that the image is considered sharp if each point of light emanating from the object is represented by a point in the image, but if we take that ideal rule, we find that we will not get a sharp image, so we can tolerate the expression. An image is considered sharp if every point of light emanates from the object. It is represented by a point of light on the image or a very small optical disk, so the optical circle. Small (or the small optical disc as some express it (it is difficult to distinguish between it and the point. If we look at the previous expression, we find that it is also necessary to determine the diameter of this light circle, which is expressed as small).
Here we have some considerations, with a specific diameter, such as: a ladder may be considered small in a tangential image of 18 x 24 cm, and it is large in relation to an image of its scale, so look at this circle, it is estimated relative to the area of ​​an image - just as it is a cm that is considered relative to the distance of sight, so the circle may appear clear if seen The image is at a close distance, and does not appear large if the image is seen from a distance. 9 × 7 - From the foregoing, we can decide the factors by which the sharpness of details is known, which is the viewing distance and the image area
It is agreed that the sighting distance is 25 cm, which is the normal sighting distance of the human eye. As will come later in the chapter on perspective, for the perspective of the image to be normal, the viewing distance must be equal to the focal length of the lens. Accordingly, the focal length of the lens must be 25 cm, i.e. equal to the viewing distance. If the focal length of the lens is less than that, the image must be enlarged, and its magnification ratio is according to the following law:
The required zoom ratio = 25 cm: the focal length of the lens in centimeters: for example, if the focal length of the photographic lens is 12.5 cm, then the image must be enlarged.

B _ the diameter of the circle or optical disk:
It was agreed that the diameter of the optical disk should not exceed an inch or a millimeter. The image that was photographed with a gland with a focal length of 25 cm, or the focal length was less, and the image was enlarged according to the previous law referred to in A.. This ratio was estimated to be 4 millimeters on the basis that it is difficult for the human eye to distinguish between two points that are a millimeter apart from each other, so if The two points are seen at a distance of 25 cm. Rather, the eye sees them as more like a single point.

Now we can define a fixed measure to express the sharpness of details in the image, by linking each of the following:

(a) The focal length of the lens.
(b) Optical disc diameter.

Thus, the sharpness of the details in the image can be defined by the following definition: The details are considered sharp in the image if the diameter of the optical lens in the image does not exceed the focal length of the lens used for imaging. It is clear that we have reached the ratio 1: 1000 as follows:
The focal length of the lens according to the previous explanation = 250 mm, the diameter of the optical disk mm
The ratio between the diameter of the optical lens: the focal length of the lens = 1: 1000