intelligent focal length presbyopic glasses
Smart focus presbyopic glasses did not have a focal length that could be changed automatically. Instead, there were multiple focal lengths on a pair of glasses. The lenses were distributed with different focal lengths from top to bottom, so the degrees of different positions of the lenses were different. There were many degrees, such as 50 - 250 degrees auto-focusing, and 100 - 700 degrees auto-adjusting. Some smart zooming presbyopic glasses had anti-blue light functions, and the price was different. There were 2.99 yuan and 29.9 yuan for two pairs. This kind of glasses could better assist the eyes in adjusting the focal length to solve the problem of short-distance vision caused by the weakening of the elasticity of the crystalline lens due to age, so that the wearer could obtain a clearer visual effect. Read more exciting novels for free
Normal focal length of glasses
Normal glasses included short-sighted glasses and far-sighted glasses (presbyopic glasses). The focal length was related to the degree of the glasses. For short-sighted glasses, the degree and focal length were in an inverse relationship, that is, the shorter the focal length, the higher the degree. For long-sighted glasses (presbyopic glasses), the degree of the concave lens (far-sighted lens) was positive, and there was a similar relationship between the degree and focal length. To be specific, the power of the lens was equal to 100 divided by the focal length (in meters). For example, a 100-degree spectacles would have a focal length of 1 meter. However, it was not clear whether "normal glasses" were short-sighted or farsighted, so it was impossible to determine a specific focal length value. This was because different degrees of glasses had different focal lengths. From correcting mild vision problems to high vision problems, the focal length would vary greatly. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
Distance and the focal length of glasses
The relationship between the distance and the power of the glasses was: the power of the glasses = 10000/distance (in centimeters). As for glasses for short-sightedness, the focal length corresponding to the degree was related to the distance of the far point. The degree of short-sightedness could be seen as a concave lens of the same degree in the eye. The far point was the focal length of the concave lens, and the focal length of the concave lens was the inverse of the focal length, that is, focal length x focal length = 1. For example, if you have 50 degrees of nearsightedness, the focal length can be calculated to be 2 meters according to the relationship between focal power and focal length. This is also the distance of the far point when you have 50 degrees of nearsightedness. The distance between the far point of the myope and the front of the eyeball is equal to the focal length of the concave lens needed to correct the myope. For example, the far point of-2.0D myope is 50cm in front of the eye, and the far point of-4.0D is 25cm in front of the eye. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
How to calculate the focal length of glasses
The power of the glasses was multiplied by the focal power by 100 (focal power refers to the inverse of the focal length, and the unit of focal length is meters), that is, the power = 1/f × 100. From this, the focal length f = 100/degrees (meters) could be derived. For example, a pair of 400-degree glasses would have a focal power of <i>= 400/100 = 4, and a focal length of f = 1/4 = 0.25m = 25cm. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
Wearing glasses, focal length changes
Under normal circumstances, glasses were mainly worn to correct vision so that light could be focused on the retina. For nearsighted people, the concave lens they wore would diverge the light, causing the image that was originally in front of the retina to move back to the retina. For farsighted people, the concave lens would converge the light, causing the image to move from the back of the retina to the retina. When wearing glasses, the optical system formed by the eyes and glasses changed the path of light transmission, thus affecting the focal length. For example, the higher the degree of the glasses, the shorter the focal length, and the stronger the ability to disperse light, so that the eyes can focus and form images in a relatively short distance; the higher the degree of the glasses, the shorter the focal length, and the stronger the ability to converge light, helping the eyes to focus in a normal distance. If you take off your glasses with one hand, it is easy to distort the glasses. This distortion may affect the focal length of the glasses and damage your vision. When looking at nearby objects (such as mobile phones), if a nearsighted person wears glasses, the ciliary muscles need to adjust the focal length again. If he wears glasses to look at nearby objects for a long time, the overwork of the ciliary muscles may lead to an increase in nearsightedness. In addition, different degrees of glasses correspond to different focal lengths. The calculation method is that the degree is 100 times the inverse of the focal length (for concave lenses, the degree is negative, and for concave lenses, the degree is positive). For example, glasses with 400 degrees of nearsightedness (-4.00D) have a focal length of 0.25 meters. In short, the wearing of glasses was closely related to the change in focal length, and the accuracy of focal length directly affected the health of vision. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
The principle of measuring the focal length of glasses with sunlight
According to optical principles, for a concave lens, parallel rays (such as sunlight can be regarded as parallel rays) will converge at a point after passing through the concave lens. The distance from this point to the center of the lens is the focal length. Mythic spectacles were concave lenses, and the focal length of concave lenses could be obtained indirectly by using the relationship between concave lenses and concave lenses. When a beam of parallel sunlight passes through the concave lens, it will converge into a focal point. Put the glasses (concave lens) in front of the concave lens. Because the concave lens has a dispersing effect on the light, it will make the light that was originally converged by the concave lens diverge again. At this time, the light screen needs to move back a certain distance to get a clear image point again. By measuring the distance change before and after this movement and the known focal length of the concave lens, etc. The focal length of the concave lens (spectacles) could be calculated using optical imaging formulas. However, this process involved more complicated optical imaging principles and formula calculations. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
100 to 600 degree presbyopic glasses
presbyopic glasses were optical products used to correct presbyopic, usually with a concave lens. According to the degree range, there were 100 to 600 degrees to choose from. Different degrees of presbyopic glasses were suitable for people with different degrees of presbyopic symptoms. If you want to choose the right degree of presbyopic glasses, it is recommended not to buy the finished presbyopic glasses on the street stall, because the optical center distance of the finished presbyopic glasses is fixed, the lens has the same degree and no scattering adjustment function, the brightness accuracy is poor, and the light transmission is low, which may lead to visual fatigue, blurred vision and other problems. It was best to go to a regular optometrist to ensure that they could get presbyopic glasses that suited their eyes. In addition, there were also some multi-functional presbyopic glasses, such as blue light protection, radiation protection, and other functions. There were also reading glasses ranging from 100 to 600 degrees.
objective focal length
The focal length of an objective lens is an important factor in an optical system. There were many methods to measure the focal length of an objective lens. The fixed-focal length collimator method had a large measurement range and high accuracy. The relative error was generally less than 1%. It was a commonly used measurement method. In a telescope, the multiplying power was equal to the focal length of the objective lens divided by the focal length of the eyepieces. In terms of aperture, the F value was the ratio of the focal length of the objective lens (f) to the perimeter of the entrance pupil (D). Different types of telescopes or instruments had different focal length values of the objective lens. For example, the focal length of an astronomical telescope was 400mm, and the focal length of the objective lens of the ZWJ - 851 Collimator was 200mm. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
On the application of focal length
In the field of photography, focal length was widely used. When shooting portraits, different focal lengths would produce different effects. For example, a focal length of 85mm was a better choice for close-up portraits, which could show a more natural facial features ratio. When shooting close-up portraits with a focal length of 17mm (a wide-angle focal length), the parts close to the lens such as the mouth, nose, and arms would be magnified, easily deformed, and the ratio of the character to the background would be exaggerated. As the focal length increased, the ratio of the character to the background would become more natural, the distance between the five facial features would be shortened, and the face would become flatter. The hairstyle would look thicker, and the face would not be so sharp. For the lens label, if the focal length label on the lens had two numbers (such as 17 - 40mm), it meant that it was a zoom lens, covering the focal length from 17mm to 40mm; if there was only one number (such as 50mm), it was a fixed-focus lens. The smaller the number, the shorter the focal length, the wider the angle of view, and the wider the viewing range. The picture could accommodate more elements but the proportion of each element was small. The larger the number, the longer the focal length, the narrower the angle of view, and the narrower the viewing range. The picture could accommodate fewer elements but the proportion of each element was larger. In some simple self-made microscopes (such as the glue stick microscope with an adjusted focal length), the focal length needed to be adjusted to observe the object (such as garlic film). <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
Staircase camera focal length
For a relatively narrow environment such as stairs, a 2.8mm focal length camera could generally be used. Its field of view was larger and the range that could be seen was the largest. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
The focal length of the surveillance camera
In an optical system, the focal length of the surveillance camera was the distance from the optical center point of the lens to the camera's CCD-based/CMOS-based sensor. It can also be understood as the distance from the center of the lens to the focal point of the light when parallel light is incident. The size of the focal length determined the image size, field of view, and depth of field of the surveillance image. Generally speaking, the larger the focal length, the farther the view, and the smaller the field of view. The smaller the focal length, the closer the view, and the larger the field of view. The focal length of mainstream surveillance cameras was generally 2.8mm, 3.6mm/4mm, 6mm, 8mm, 12mm, 16mm, etc. Some even had a zooming function. Cameras with different focal lengths are suitable for different monitoring ranges. For example, for situations where the monitoring range is small and the viewing angle is large, such as a family living room, small shops, etc., you can choose a surveillance camera with a smaller focal length.(For example, a 3.6mm or 4mm focal length camera can have a viewing angle of 75°-90 °, an effective monitoring distance of more than 5 meters, and can cover 40 - 80 degrees of space); for a larger monitoring range (such as a space of 10m * 10m), you need to choose a surveillance camera with a larger focal length. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>