The application of more than twice the focal length
When the object distance is greater than twice the focal length, an inverted and reduced real image is formed. If the image distance is greater than one focal length and less than two focal lengths, it can be used as a camera. In addition, when retaking old photos, if you want to get a photo as big as the original photo, you can add a circle between the lens and the camera body to make the distance between the lens and the film equal to 2 times the focal length. Then, place the original photo at 2 times the focal length in front of the lens. This operation uses the object distance equal to 2 times the focal length imaging law, and this law is related to the object distance greater than twice the focal length imaging law. It is based on the different application of the imaging law of the concave lens. Read more exciting novels for free
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>
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>
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>
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>
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 big is the focal length of the monitor?
The focus of the monitor depended on a number of factors. First, he had to consider the scope of surveillance. If 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 camera with a 3.6mm or 4mm focal length can have a viewing angle of 75°-90 °, and the effective monitoring distance can reach more than 5 meters, covering a space of 40 - 80 square meters. If the monitoring range is large, such as a space of 10m × 10m, you need to choose a surveillance camera with a larger focal length. Secondly, pay attention to the size of the scene to be monitored, the distance between the camera and the scene, and the size of the camera's imaging device (such as 1/3", 1/2", 2/3", or 1"). The appropriate focal length can be determined by the calculation method f = vD/V or f = hD/H (where f represents the focal length, f = vD/V: v represents the vertical height of the CCD-target surface, D represents the distance from the lens to the scene, and V represents the height of the observed object;f = hD/H: h represents the horizontal width of the CCD-target surface, D represents the distance from the lens to the scene, and H represents the width of the observed object). In addition, different focal lengths had different shooting effects. The larger the focal length, the farther the view, but the smaller the field of view. The smaller the focal length, the closer the view, and the larger the field of view. At the same distance, the larger the focal length, the smaller the angle of view, but the clearer the relative details. At the same focal length, the further the distance, the wider the angle of view, but the lower the degree of recognition. If one wanted to see a human face or figure clearly, the further the distance, the greater the focal length needed. <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>
Laser engraving machine focal length
In the laser engraving machine, the focal length is the distance from the center of the lens to the focal point of the light. For example, the focal length of the 3DJQL- * 00L series laser engraving machine was 960 - 1440mm. In the laser marking equipment, after the laser was shaped by the laser, it would illuminate the focusing field lens (usually a concave lens) with a parallel beam. The concave lens would be refracted by the optical refraction to focus the parallel beam onto a focal point, forming a focal surface. The vertical distance from the center of the concave lens to the focal surface was the focal length, which was generally represented by F =. Most of the laser equipment's matching focusing field lens would have a label on it. <a href="/?from=ask_words" style="color:red" target="_blank">Read more exciting novels for free</a>
90 focal length lens group photo
When taking group photos, a 90mm focal length lens was a more suitable choice. The standard lens had many advantages when taking group photos, such as the smallest distortion and the closest to the human eye's visual perception. When using a 120 medium format 6×7 camera to take a group photo of more than 500 people, using a 90mm lens could achieve better results. However, when using this focal length lens to take a group photo, one also had to consider other factors such as shooting distance, aperture, shooting time, queue position, etc., in order to take the ideal group photo. For example, it is recommended to choose soft natural light when shooting, such as 9 - 10 a.m. or 4 - 5 p.m.(It needs to be adjusted according to the season and regional differences). It is best to use a cloudy morning, and the direction of the light is best to be in the light. In terms of queue position, when there are more people, they can be arranged in a square array (for example, 50 - 100 people can be arranged in 4 - 5 rows). The height of the front and back rows should be reasonably arranged by chairs, stools, steps, etc. to avoid blocking. It is recommended to use a small aperture such as F11 or F16 (different cameras may have different aperture ranges). <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>