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The lens is the most important part of the camera and is referred to as the human eye. Therefore, the lens is the eye of the camera. The clarity of the picture and the richness of the image levels are constrained by the inherent quality of the optical lens. The lenses of various cameras commonly seen in the market today are all film-added lenses. Coating is to coat a layer of colored film on the surface of the lens to reduce the chromatic dispersion between the lens and the lens. It can also reduce the glare generated during backlight shooting, protect the light from passing through the lens smoothly, and increase the light transmission of the lens. The ability to make the captured picture clearer. To
The camera lens is a device that images the shooting scene on the sensor (CCD or CMOS), and it usually consists of several lenses. From the material point of view, the lens can be divided into plastic lens (Plastic) and glass lens (Glass). Usually the lens structure is: 1P, 2P, 1G1P, 1G2P, 2G2P, 4G, etc. (P, G here represent plastic lens and glass lens respectively, for example, 1G1P means that the lens of this camera is composed of a plastic lens and a glass lens) . The more lenses, the higher the cost; glass lenses are more expensive than plastic.
The image quality, saturation, and chromatic aberration of glass lenses are better than those of plastic lenses, and they are often used in higher-end cameras. In order to reduce costs, most of the camera products on the market generally use plastic lenses or semi-plastic and semi-glass lenses (ie: 1P, 2P, 1G1P, 1G2P, etc.). Plastic lens: lower cost, smaller spherical aberration. To
Lens types: digital camera lens, wide-angle lens (focal length is generally lower than f=28), fisheye lens is an ultra-wide-angle lens with a field of view not less than 180 degrees, mobile phone lens, network lens, surveillance lens, CCTV lens.
The lens usually has two more important parameters. The other is the focal length, which is basically the distance from the center of the lens to the clear image formed on the sensor plane. The focal length of the lens determines the size of the image formed by the object shot by the lens on the sensor. Assuming that the same object is shot at the same distance, the longer the focal length of the lens, the larger the image formed by the object. One is the aperture, which is a device installed on the lens to control the amount of light passing through the lens to the sensor. In addition to controlling the amount of light, the aperture also has the function of controlling the depth of field, that is, the larger the aperture, the smaller the depth of field. To
The focal length f is the abbreviation for focal distance. Refers to the distance from the center of the lens to the focal point of the light. (The distance from the center of the lens to the imaging plane such as the lens or CCD) For example, when one of the magnifying glass faces the sun and the other faces the piece of paper, when moving up and down to a certain distance, the piece of paper will gather a very bright The light spot, and the paper can be burnt into small holes in a short while, so it is called "focus". The distance from the center of the lens to the sheet of paper is the focal length of the lens. For a camera, the focal length is equivalent to the distance from the "center" of the lens to the imaging surface of the camera tube or solid-state imaging device. A lens with a fixed focal length is a fixed-focus lens; a lens whose focal length can be adjusted and changed is a zoom lens. Optical zoom (change the focal length of the lens)/digital zoom (not change the focal length of the lens)
The focal length is one of the important data that marks the performance of an optical lens, because the size of the image captured by the lens is controlled by the focal length. When shooting the same target at the same distance, the longer the focal length of the lens, the narrower the horizontal angle of view of the lens, and the smaller the range of the scene captured; the shorter the focal length of the lens, the wider the horizontal angle of view of the lens, and the scene captured The greater the range. To
When the imaging size of the camera lens is determined, a fixed focal length lens has a relatively fixed field of view, and the field of view is often used to indicate the size of the field of view. Its law is that the shorter the focal length, the larger the angle of view and field of view. So the short focal length lens is also called the wide-angle lens.
Aperture F: In order to control the size of the luminous flux through the lens, an aperture is set at the back of the lens. f/D=F The relative aperture of the lens determines the illuminance (brightness) of the camera. The smaller the F value, the larger the aperture, and the greater the amount of light that reaches the SENSOR. Therefore, when the focal length f is the same, the smaller the F value, the better the lens resolution.
The lens can be divided into two types: variable aperture and fixed aperture. The lens currently used by our company is a fixed aperture lens.
Field of view: the range that the lens can cover, (objects beyond this angle will not be collected in the lens), how large a camera lens can cover the scene, usually expressed in angle, this angle is called the angle of view of the lens FOV . The area covered by the visible image formed by the subject through the lens on the focal plane is the field of view of the lens.
The concept of depth of field: when an object is in focus, all the scenes from a certain distance in front of the object to a certain distance behind it should also be clear. The distance from front to back where the focus is quite clear is called the depth of field. The depth of field is divided into the foreground depth and the back depth of field, the back depth of field is greater than the foreground depth. The deeper the depth of field, the clearer the scene far away from the focus, while the shallower the depth of field, the blurred scene far away from the focus. To
Depth of field
When the lens is focused on a certain point of the photographed object, the object at this point can be clearly imaged on the TV screen. Scenes within a certain range before and after this point can also be recorded more clearly. That is to say, there is a certain limit to the clear range of the scene captured by the lens. This kind of "clearer" depth of the subject that can be recorded before and after the camera tube is focused on imaging is the depth of field. When the lens is aimed at the subject, the clear range in front of the subject is called foreground depth, and the clear range behind is called back depth of field. The foreground depth and the back depth of field are added together, that is, the depth of the entire TV picture from the nearest clear point to the farthest clear point, called panoramic depth. Generally speaking, the depth of field refers to the panoramic depth. To
On some pictures, the front of the subject is clear and the back is blurred, some pictures on the picture are clear at the back and the front is blurred, and on the other pictures only the subject is clear but the front and back are blurred. These phenomena are caused by the lens. The depth of field characteristics. It can be said that the principle of depth of field plays an extremely important role in photography. The correct understanding and use of depth of field will help to take a satisfactory picture. The main factors that determine the depth of field are as follows:
Aperture When the focal length of the lens is the same and the shooting distance is the same, the smaller the aperture, the larger the range of depth of field; the larger the aperture, the smaller the range of depth of field. This is because the smaller the aperture, the thinner the light beam entering the lens, the more obvious the paraxial effect, and the smaller the angle at which the light converges. So in front of and behind the imaging. The condensed light will leave a smaller spot on the imaging surface, making the original unclear scenes closer and farther away from the lens have acceptable clarity. To
Focal length When the aperture factor and shooting distance are the same, the shorter the focal length of the lens, the greater the depth of field range; the longer the lens focal length, the smaller the depth of field range. This is because a lens with a short focal length is much narrower than a lens with a long focal length, and the focal zone (depth of focus) formed by the light from the scene at different distances from the front and rear is much narrower, so there will be more light spots entering the acceptable definition area. To
Object distance When the focal length of the lens and the aperture coefficient are equal, the farther the object distance, the larger the depth of field range; the closer the object distance, the smaller the depth of field range. This is because the scene far away from the lens only needs to make a few adjustments to get a clear focus, and the front and back scenes are focused very tightly. This will cause more light spots to enter the acceptable sharpness area, so the depth of field is increased. On the contrary, focusing on the scene close to the lens, because the distance between the front and rear focal points is enlarged, that is, the focal depth range is enlarged, so that the light spot entering the acceptable sharpness area is reduced, and the depth of field becomes smaller. For this reason, the foreground depth of the lens is always smaller than the back depth of field. To
It is related to the focal length of the lens, a lens with a long focal length has a small depth of field, and a lens with a short focal length has a large depth of field. Secondly, the depth of field is related to the aperture. The smaller the aperture (the larger the value, for example, the aperture of f16 is smaller than the aperture of f11), the greater the depth of field; the larger the aperture (the smaller the value, for example, the aperture of f2.8 is greater than f5.6) The smaller the depth of field. Secondly, the foreground depth is smaller than the rear depth of field, that is, after precise focusing, only a short distance in front of the focus point can be clearly imaged, while the scene behind the focus point for a long distance is clear.
Image vignetting phenomenon: the sensor package is off-core, the lens imaging surface and the sensor are not the same size
Image color cast phenomenon: the selection of lens material, the design of the scheme system, and the IR cut film. Red orange yellow green blue indigo purple
Image resolution: focus, aperture, sensor dead pixels. The lens is dirty, the lens is wet and foggy. The lens itself does not match, the lens is loose to change the air gap, and the AR coating is coated.
Glare: The lens is called "flary" (meaning shining bright light, which is what we usually call the lens reflective white flower) due to internal reflection problems. However, usually glare means that internal reflections are still visible, not that it leads to poor image quality. Typically, glare will cause the highlights to overflow, so there is usually a kind of brilliance around the bright part. Sometimes the lens will produce a main image and an attached “ghost image”, such as fluorescent tubes or street lights. Modern coating technology makes it no longer a big problem, but it still happens occasionally, especially with cheap zoom lenses and ultra-large aperture lenses. Removing filters can alleviate similar problems.
A common type of internal reflection is when light enters the lens directly, creating an image in the shape of an aperture. For internal reflections, a reasonably designed darker hood will effectively reduce glare and ghosting.
Spherical aberration (curvature of field) Few lenses can project a flat scene onto a flat surface. Instead, they form a curved or dish-shaped image: the center is farthest from the lens and the edges are closer. In order to cope with this, some early cameras made the film plane into a curved surface. However, in general, designers try to make the image field of the lens flatter so that the image on a flat film can be accepted.
Field curvature is one of the reasons for the gradual deterioration of the lens imaging from the center to the periphery, which is more serious in the super-aperture lens and the variable lens. For SLR cameras, the focus is usually at the center of the screen, so the edges may become soft, but if there is no subject on the edges, this is not a problem.
The analysis of bad lenses is actually an important issue for optical units. In addition to professional knowledge in optics, pragmatic experience is also required.
1. The resolution is the so-called spatial frequency. When the resolution cannot meet the specifications, it is judged to be bad. Even if it is a defective product, different bad phenomena can be seen from the resolution.
★ Unfavorable phenomenon: the whole chart is not clear.
Cause analysis: It may be that the lens is installed reversely or wrongly.
Solution: Disassemble the lens and reassemble it.
★ Unfavorable phenomena: The half-sided chart resolution is not clear, and there is a phenomenon of mutual pull. For example, when the left side is clear, the right side is blurred; adjust the focus, when the right side is clear, the left side is blurred, which is an obvious tilt phenomenon.
Cause analysis: lenses, parts, assembly methods, projection tables, projection resolution meters, etc. can all be caused.
Solution: first check the problem of the projection instrument, and then check the lens, parts and assembly operations.
★ Unfavorable phenomenon: the center and Zhoubian (0.7~~0.9F) resolution mutually pull. For example, the center is clear and the periphery is blurred; after adjusting the focus, the periphery is clear and the center is blurred, which is due to the presence of field curvature aberration.
Cause analysis: the influence of air space and lens thickness.
Solution: Solve it from the angle of field curvature aberration correction.
2. Image quality:
Difference and resolution, imaging quality is also another important inspection standard of the lens.
★ Undesirable phenomenon: distortion aberration.
Measurement method: When using projection to test the resolution, measure the length of the top, bottom, left, and right four lines, and compare with the cross line in the center.
Cause analysis: the characteristics of spherical lenses.
Solution: Optimize from design, or use aspherical surface.
★ Bad phenomenon: chromatic aberration
Measurement method: a. When using projection to test the resolution, use visual inspection, you can see that there are blue and red lines on the white line of the pattern, but this method can only see the dispersion of the image quality, and cannot be measured. .
b. The MTF machine can measure not only the resolution, but also the color difference measurement method.
Cause analysis: Wavelength is a function of refractive index, and it is also a characteristic of composite light.
Solution: From the choice of material and design optimization.