العين وآلة التصوير .. كتاب آلة التصوير

Chapter VIII

The eye and the camera

Similarities between the functions of the eye and the camera:

The eye and the camera are similar in the function they perform, as they are the only two optical devices that (1) record optical images, and in each of them there are parts to receive and collect light rays, and another to control the amount of light needed to form the image, and there is in both of them a means for recording images (Fig. 115). Below we discuss each of these functions:

First - receiving and collecting light rays:

(a) In the camera: The camera has a lens whose function is to receive light rays and collect them on a flat surface, which is the film on its focal plane. In order for the optical image to be sharp, the distance between the lens and the image must be adjusted according to the distance between the object and the lens. So the process of adjusting the distance requires a mechanical means of adjusting the relationship. between these two dimensions.

(b) In the eye: the eye also has a lens that receives light rays and collects them in their focal plane on a curved surface that is the retina. But the eye lens differs from the photographic lens in that the first one changes its focal length always by means of involuntary muscles that contract when the object is near, so the lens becomes rounded, so its focal length decreases, and it relaxes when the object is far away, so the lens flattens, so its focal length increases. The processes of rounding and flattening are the two means that the eye follows in order for the images of objects to fall into the focus of its lens (on the retina), because the distance between the lens and the retina is fixed and does not change.

Accordingly, we summarize the difference between the two means of adjusting the distance in the two cases in that the lens of the eye changes its focal length according to the distance of the object from it, while the lens of the camera has a fixed bore dimension, and therefore the distance between the lens and the focal plane (or film) must be adjusted according to the distance of the object from it.

Second – controlling the amount of light:

(a) In the camera: The camera controls the amount of light needed to form the image by two ways:

1- The intensity of the light passing through the lens by means of an iris diaphragm, which the photographer adjusts to be narrow or wide.

2- The duration of exposure to light by the shutter. The two factors cooperate together in estimating the amount of light needed to affect the sensitive film according to the intensity of the illumination of the objects being photographed and according to the speed of sensitivity of these films.

(b) In the eye: the human eye controls the intensity of the light that is allowed to pass through it to form an image on the retina. If the amount of light increases, other muscles contract involuntarily, so its diameter narrows, and vice versa, its diameter widens if the amount of light decreases. On the other hand, we see the eyelid performing a function almost similar to (1) what the shutter does in a camera.

Third - Image registration and color:

(a) In the camera: the lens transmits inverted light images to the objects in front of it and records them on sensitive films as latent images that require chemical treatment to convert them into negative images or direct positive images according to the method that was used. The color of the resulting image depends on several factors, including the spectral sensitivity of these films, whether it is Photography is “black and white or color, as it depends on the exposure time, the color of each of the objects, the rays of the light sources, etc.

(b) In the eye: Inverted light images are also formed for visible objects (Fig. 116) and are recorded on a sensitive layer inside the eye, the retina, which contains two types of cells: rods and cones. The cone cells are concentrated in a hidden point on the retina, which is known as the yellow spot or Fovea, located opposite the human eye from the other side (Fig. 117). It is the center of clear and sharp vision. These cone cells and rod cells mix outside the yellow spot. . The percentage of rod cells is very few around the yellow spot and their percentage increases with distance from it, and on the contrary, the percentage of cone cells increases around the yellow spot and decreases with distance from it.

And when the sight focuses on a small object close to the eye in order to examine it and see it clearly, the axis of the eyes adapts so that the light rays emanating from the object gather in this yellow spot. Given the color sensitivity of cone cells, we finally see that the correctness of distinguishing between different colors depends on factors similar to what was previously mentioned, such as the color of objects, the color of the rays of light sources, etc.