预防近视英文小报内容（近视防控-眼光学）

​The basic optic function of the eye is to form an image of whatever object being looked at (fixated on) on the retina. This image formation is done by the combination of the cornea and the lens which behaves like a convex lens. The focus length of this lens is adjustable by the muscles attached to it according to the distance of the object so that its image is always formed on the retina in normal situation.

眼睛的基本视觉功能是在视网膜上形成所看物体的图像，这种图像的形成是由角膜和晶状体共同完成的。晶状体的焦距可根据物体的距离由附着在其上的肌肉来调节，使其在正常情况下始终在视网膜上形成图像。

The simple relationship between the focal length f of the lens and the distances of the object and its image is

晶状体的焦距f与物距d_o、像距d_i之间的简单关系为

where d_o and d_i are the distances from the lens to the object and its image, respectively. For a given f, distant object (large d_o) images on the retina ，with the focus length equal to the distance of the image.

其中d_o和d_i分别是晶状体到物体及其图像的距离。对于给定的f，远处的物体(d_o无限大)成像于视网膜，焦距则等于像距。

In the human eye, as the image always needs to be formed on the retina, di is fixed to be the distance between the lens and the retina, which is about 0.017 meters (17 mm), and the focal length of the lens need to be adjuected as a function of the distance of the object:

在视网膜上成像，d_i是晶状体和视网膜之间的距离，是固定的，即约0.017米(17毫米), 则需要调节晶状体的焦距f，它与物距的函数关系是:

If, for various reasons, the eye fails to focus the images on the retina (either in front or behind the retina), eye glasses are needed to change the focal length so that the image can still be formed on the retina.

如果由于各种的原因导致眼睛不能将图像聚焦在视网膜上(不管是在视网膜的前还是后)，就需要眼镜来改变焦距，从而在视网膜上成像。

屈光度=1/焦距

The power of a lens to bend the incoming light is measured by diopter, defined as the reciprocal of the focal length in meter. To see objects of different distances, the power of the lens needs to be changed accordingly.

晶状体屈光能力是通过屈光度来测量的，其定义为焦距的倒数(以米为单位)。要看到不同距离的物体，需要相应改变晶状体的屈光能力。

When the object is far away (d_o-∞), its image is formed at the distance equal to the focal length ( di=f=0.017 mm); however, when the object is close, its image will be formed farther away from the lens (d_i>f).

For the image to still be formed on the retina, the lens needs to change its shape to increase its power (shorten f). As one can see in the plot below (showing f and 1/f as functions of object distance d_o, given d_i=17 mm), major adjustment of the lens is needed only when viewing objects within the range of about half a meter. For example, the focal length needs to be at 0.0145 meters for an object at 0.1 meters, or 0.0163 meters for an object at 0.4 meters. To view any object farther away than 0.5 meters, the lens needs to be pretty relaxed so that f is very close to 0.017 meters.

当物距较大(d_o-∞)时，在像距等于焦距(di=f=0.017 mm)处形成物像；然而，当物体靠近时，图像会成在离晶状体较远处(d_i>f)，如果仍在视网膜上成像，晶状体则需要改变形状，增大其屈光力(缩短f)。如下图((用f和1/f来表示与物距d_o的函数关系, d_i= 17mm))，只有在看半米范围内的物体时，才需要对晶状体进行大的调节。例如，看0.1米的物体，焦距为0.0145米，看0.4米的物体，焦距为0.0163米。要想观察到比0.5米更远的物体，晶状体需要非常放松，使f非常接近0.017米。

Figure: Focal length and power of eye as functions of object distance

焦距和眼睛屈光度与物距的函数关系