The function of the eye in refracting light is called refraction. The ability to refract light is expressed by optical power, which is called diopter. The refractive state of the eye when it is not used for adjustment is called static refraction. The optical power of the standard eye's static refraction is -58.64D. The refractive state of the human eye when using adjustment is called dynamic refraction, and its power is stronger than the power of static refraction. Due to incorrect refractive power of the eye, the image cannot be accurately formed on the retina, which is a vision defect. Generally, it is necessary to wear the eyes and supplement and correct the refractive power of the eye itself through lenses to achieve the purpose of correct retinal imaging!

Some optical instruments, such as cameras, telescopes, microscopes and other equipment, take into account the differences in vision of each eye when it is inconvenient for users to wear their own eyes, so they all have diopter adjustment devices. Therefore, the concept of "diopter" refers to glasses. For example, the diopter of 200-degree myopia glasses is -2D, and the diopter of 150-degree reading glasses is 1.5D.

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Some high-end cameras have increased diopter in order to facilitate some people with slight myopia or presbyopia to directly view the effects in the viewfinder without wearing glasses. Adjustment device. The diopter adjustment device is actually a set of lenses added to the viewfinder of the camera, and the position relationship of the lenses is adjusted through knobs to achieve continuous diopter conversion within a certain range. The range is generally between -3D and 2D. Since the diopter adjustment range is limited, users with moderate myopia who exceed this range can purchase a special eyepiece adjustment lens. Eyepiece adjustment glasses specially developed for myopia users, the diopters are -1, -2, -3, -4, etc.; eyepiece adjustment glasses specially developed for farsighted users, the diopters are +0.5, +1, +1.5, +2 respectively. , +3, etc. Users can choose a suitable eyepiece correction lens and install it in the eyepiece according to their own vision conditions, so they can shoot with naked vision. The diopter adjustment function is not only available in SLR cameras, but also in many large and full-featured small and medium-sized non-SLR cameras. However, because the structure is relatively complex, takes up a certain amount of space, and increases part of the cost, card-type digital cameras that value compactness and portability are generally not available.