If you want to shoot distant and near scenes, you must adjust the focal length so that the scenes can be clearly projected on the negative. In a real camera, the mechanism is to adjust the distance between the lens and the film. In the eyes, this step is controlled by the ciliary muscle. Ciliary muscles are a group of involuntary muscles around the lens. When looking at close objects, this group of muscles makes the curvature of the lens more curved, the thickness increases and the diopter increases, so that the image is clearly projected on the retina. On the contrary, when looking at the distant view, the ciliary muscle reduces the curvature of the lens, makes the front and back surfaces flatter, and reduces the refractive power accordingly. Finally, the image is still clearly projected on the retina.
The function of ciliary muscle to control the diopter of lens is called accommodation. When the eye looks at an object at infinity, it can clearly project the image onto the retina without adjustment. This refractive state is called "emmetropia". On the other hand, if the scene at infinity cannot be clearly projected onto the retina without adjustment, it is called "ametropia" or "ametropia", which is what we usually call myopia, hyperopia or astigmatism.
In fact, even if there is a clear retinal image, it does not mean that we can "see" clearly, but whether there is a problem in the process of visual information from the optic nerve to the visual cortex of the brain. In other words, the eyeball, optic nerve, visual area and visual cortex of the brain must work normally, so that we can see the external images clearly and accurately.
When light enters a single spherical refractor composed of another medium from the air, its refraction in the material depends on the radius of curvature r of the interface between the material and the air and the refractive index N2 of the material. If the refractive index of air is n 1, the relationship is:
The empty side focal length is the front main focal length or 1 focal length. F2, called the back principal focal length or the second focal length, refers to the distance from the refractive surface to the back principal focal length, which can represent the refractive power of this refractive body; Or put it another way, that is, the main focal length is expressed in m (meters), and then the reciprocal of this value is called diopter; If the main focal length of a lens is 10cm, which is equivalent to 0. 1m, then the refractive power of this lens is 10 power (10D). Generally speaking, it is stipulated that the power of convex lens is positive, and the refractive system that is concave through human eyes is a complex optical system. Light entering the eye can be imaged on the retina through four media with different refractive indexes, namely cornea, aqueous humor, lens and vitreous body, and through four refractive surfaces with different diopters (front and rear surfaces of cornea and front and rear interfaces of lens), in which the most important refraction of incident light occurs on the front surface of cornea. According to the principle of geometrical optics, the calculation results show that the posterior main focus of normal adult eyes is exactly where the retina is when they are quiet and unadjusted. This anatomical relationship is very important for understanding the refractive imaging ability of normal eyes. It shows that all the objects located 6m away from the eyes until infinity, when they reach the refractive system of the eyes, the light they emit or reflect is almost parallel, so they can form a clear image on the retina, just like a negative placed in the main focus of a camera, and can shoot a clear vision. Of course, the human eye will not unconditionally see objects at any distance. For example, the human eye can clearly see the Chu Moon (or other distant stars) and its larger shadows, but it can't clearly see the smaller objects or features on the surface of the Moon. The reason is that if the light from an object is too weak, or the light is scattered or absorbed when it propagates in space or eyes, it is weak enough to excite photoreceptor cells when it reaches the retina, so it cannot be perceived; In addition, if the objects are too small or too far away from the eyes, their images on the retina will be less than the limit of retina resolution, so they cannot be perceived.
The refraction of light through the refractive system of the eye is called refraction, and the total refractive power of the eye can be expressed by diopter (D). The diopter value is equal to the reciprocal of the main focal length (in m) of the refractor. The total refractive power of the human eye in the unadjusted state is about 59 days.
The focusing ability of the mirror is negative.
The main focal length is the most important optical parameter of the refractor, and the position of the refracted image formed by the object at any position can be calculated through it. Taking a thin lens as an example, if the object distance a is known, the image distance b can be calculated by the following formula:
It can be seen from Formula (2) that when the object distance A tends to infinity, 1/a tends to zero, so 1/b tends to 1/F2, that is, the image distance b is almost equal to f2; That is to say, when the object is infinitely far away from the convex lens, its imaging position will be at the position of the back main focus. Similarly, it is not difficult to see that the image distance b of an object whose object distance is less than infinity is always greater than F2, that is, it will be imaged further than the main focus. The above conclusions are very important for understanding the refractive imaging ability of eyes.
In addition, according to the optical principle, the position of the main focus is the position where the parallel rays are refracted and focused into a point, which is consistent with the first conclusion mentioned above. The surface of every object can be considered to be composed of countless luminous points or reflective points, and the light emitted by each point is divergent; Only when the distance between these points and the corresponding refraction surface tends to infinity, the light rays reaching the refraction surface from these points can be nearly parallel, so they are refracted on the surface where the main focus is located to form a point, and then the object image is composed of these points. Of course, infinity is an impossible position. In fact, for human eyes and general optical systems, the light emitted by each spot of an object beyond 6m can be regarded as almost parallel, so an object image can be formed on the plane where the back main focus is located.