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Camera lens

Camera lens (also known as camera lens or objective lens) is an optical lens or lens component used in combination with the camera body and mechanism, which is used to electronically image an object on a photographic film or other medium capable of chemically storing the image.

The lenses used in static cameras, cameras, telescopes, microscopes or other devices are not very different in principle, but the detailed design and construction are different. The lens can be permanently fixed to the camera or interchanged with lenses of different focal lengths, apertures and other characteristics.

In principle, a simple convex lens is enough, but in fact, a composite lens composed of multiple optical lens elements is needed to correct (as much as possible) many optical aberrations. There are some aberrations in any lens system. The task of lens designers is to balance these and make designs that are suitable for photographic purposes and may be mass-produced.

A typical linear lens can be considered as an "improved" pinhole "lens. As shown in the figure, the pinhole "lens" is only a small aperture, which can block most of the light. Ideally, a light on the object will be selected for each point on the image sensor. Pinhole lenses have some serious limitations:

Pinhole cameras with large apertures are blurry because each pixel is essentially a shadow of an aperture diaphragm, so its size is not less than the aperture size (third image). The pixel here is the area where the detector is exposed to light at a point on the object.

Reducing pinholes improves resolution (to the limit), but reduces the amount of light captured.

At some point, due to the diffraction limit, reducing the hole will not improve the resolution. Beyond this limit, making the hole smaller will blur and darken the image.

Practical lenses can be regarded as the answer to the following question: "how to modify pinhole lenses to absorb more light and provide a smaller spot size?". The first step is to place a simple convex lens at the pinhole with a focal length equal to the distance from the film plane (assuming the camera will take a picture of a distant object). This causes the pinhole to open significantly (fourth image), because a thin convex lens bends the light proportionally to the distance from the lens axis so that the light hits the center of the lens. Its geometry is almost the same as a simple pinhole lens, but it is not illuminated by a single light, but by a focused "pencil" to illuminate each image point.

A camera lens can be made up of many elements: from one element in a box Brownie lunate lens to more than 20 elements in a more complex zoom. These elements themselves may include a set of lenses bonded together.

Front elements are critical to the performance of the entire component. In all modern lenses, the surface is coated to reduce wear, glare and surface reflectivity, and to adjust the color balance. In order to minimize the aberration, the curvature is usually set to make the angle of incidence and the angle of refraction equal. In a fixed focus lens, this is easy, but in a zoom lens, there is always a compromise.

Generally, the lens is focused by adjusting the distance from the lens assembly to the image plane, or by moving the elements of the lens assembly. To improve performance, some lenses have cam systems that adjust the distance between groups when focusing the lens. Manufacturers call it something different: Nikon calls it CRC; Canon calls it floating system. Hassu and Mamiya call it FLE.

Because of its good optical properties and scratch resistance, glass is the most commonly used material for lenses. Other materials can also be used, such as quartz glass, fluorite, acrylic acid and other plastics (plexiglass), and even germanium and meteorite glass. Plastics allow the manufacture of strong aspheric lens elements that are difficult or impossible to manufacture in glass, and simplify or improve the manufacture and performance of lenses. Plastic is not the cheapest lens, not the outermost element of all lenses, because they scratch easily. Molded plastic lenses have been used for many years as the cheapest disposable camera and have gained a bad reputation: manufacturers of high-quality optics tend to use euphemisms such as "optical resin.". However, many modern, high-performance (and expensive) lenses from popular manufacturers contain molded or hybrid aspheric elements, so not all lenses with plastic elements have low photographic quality.

The resolution test chart of the U.S. Air Force in 1951 is a method to measure the resolution of the lens. The quality of materials, coatings, and construction can affect resolution. The resolution of lens is limited by diffraction, which is rarely achieved by camera lens. Those known as "diffraction limits" are often very expensive.

Today, most lenses are multilayer coated to minimize glare and other undesirable effects. Some lenses have UV coating to prevent UV light that may contaminate the color. Most modern optical binders used to bond glass elements also block UV light, eliminating the need for UV filters. UV photographers must do their best to find lenses without cement or coating.

The lens usually has a diaphragm adjusting mechanism, usually an iris diaphragm, to adjust the amount of light passing through. In early camera models, spinners or sliders with holes of different sizes were used. These Waterhouse stops can still be found on modern professional lenses. In order to adjust the time that light can pass, the shutter can be installed in the lens component (for better image quality), in the camera, or even rarely in front of the lens. Some cameras with blade shutters in their lenses will omit the aperture and the shutters will do double work.