Want to know more about the camera lens!

The focal length and aperture of the lens will be marked on the name of the lens and engraved on the lens ring. Visible focal length and aperture are important performance indexes of lens. Aperture plays an important role in controlling exposure. So we must first know the focal length, aperture and aperture of the lens.

First, the focal length

The focal length of a lens can be understood from a practical point of view as the distance from the center of the lens to the film plane. Theoretically, it is defined as the vertical distance from the optical center of the lens or lens group to the focal plane when the scene at infinity forms a clear image on the focal plane through the lens or lens group. For a fixed-focus lens, the position of its optical center is fixed; For zoom lens, the change of lens optical center brings about the change of lens focal length.

The focal length of modern 135 camera lens ranges from 6mm to 2000mm, and the common focal length range is 15mm to 600 mm For a camera with the same frame and facing the same subject, the change of imaging effect caused by the change of lens focal length can be summarized as follows:

1, the focal length of the lens is inversely proportional to the viewing angle. Long focal length and small viewing angle mean that you can shoot large scenes from a distance; Short focal length and large viewing angle mean that you can shoot a wide range of scenery at close range.

2. The focal length of the lens is inversely proportional to the depth of field. Long focal length and small depth of field mean that the clear range of foreground and background is small; Short focal length and large depth of field mean that the clear range of the scene before and after is large. Depth of field represents the image clarity of deep scenes, which is an important theoretical and practical problem in photography. We will introduce it in detail in the next few chapters.

Second, the caliber

The aperture of the lens is also called absolute aperture and effective aperture, which means the maximum incident aperture of the lens, that is, the maximum aperture of the lens. The size of the aperture is expressed by the aperture coefficient f, F= lens focal length/maximum aperture diameter, or it can be expressed by the reciprocal of the f coefficient, such as F2.8 or 1: 2.8. The smaller the f, the larger the caliber. For the zoom lens, we will see the expressions of F3.5-5.6, which are the maximum aperture of the wide-angle end and telephoto end of the lens respectively.

The larger the aperture of the lens, the greater the practical value. The advantages of large aperture lens mainly include: it is convenient for hand-held camera to shoot with field lamp in dim light; It is convenient to capture the effect of small depth of field and combine the picture with reality; It is convenient to use a higher shutter speed to cure the moving body. However, the manufacturing process of large aperture lens is complicated, so the larger the aperture, the larger the lens and the higher the price. Usually the caliber is from first gear to half gear, and the price doubles several times. For example, Canon EF 50mm F 1.8 is around 700 yuan, EF 50mm F 1.4 USM is around 3000 yuan, and EF 50mm F 1.2 LUSM is around 10000 yuan.

Third, the aperture

Aperture, also known as relative aperture, is an adjustable light entrance hole composed of several metal sheets in a lens.

1, aperture coefficient

The size of the aperture is expressed by the aperture coefficient f, and f= lens focal length/aperture diameter. Therefore, the smaller the F coefficient, the larger the aperture of the lens with the same focal length; The larger the F coefficient, the smaller the aperture.

Usually, the expression method of the F-coefficient on the lens changes one step, such as f 1, 1.4, 2, 2.8, 4, 5.6, 8,1,16, 22, 32, 45. Half gear changes, such as f2, 2.4, 2.8, 3.5, 4, 4.8, 5.6, 6.7, 8, 9.5, 1 1 3, 16, etc. There are also changes in the third gear, such as f4, 4.5, 5.6, 6.3, 7. 1, 8, 9,1,1,13, 14. Specific to a lens, its F coefficient is usually only 5 to 8 consecutive steps.

For different aperture coefficients f, we can use "2n" to calculate the amplification relation of incident light, where n is the number of different apertures. For example, f4 differs from f 1 1 by three steps, 2n=23=8, which means that the light intensity of f4 is eight times that of f 1 1. When n is 1, 2n=2 1=2, that is, two adjacent apertures, and the illumination of the large aperture is twice that of the small aperture.

2. Adjustment of aperture

The aperture of a lens can be divided into continuous aperture and discontinuous aperture. Continuous aperture can realize stepless adjustment and can be adjusted at any position between two apertures. The discontinuous aperture is adjusted according to the first, second or third gear of the aperture, which has obvious positioning feeling when adjusting. Some brands of autofocus lenses don't have an aperture adjusting ring, which can be adjusted by the aperture adjusting mechanism of the camera, such as Canon's EF series lenses and Nikon's AF G series lenses.

When adjusting the aperture, the aperture is always at the maximum aperture. Only when the shutter button is pressed to release the shutter, the aperture will automatically shrink to the set f coefficient, and the aperture will return to the maximum aperture after exposure.

3, the role of the aperture

The function of aperture is mainly manifested in the following three aspects:

(1) Adjust the lighting. This is the basic function of the aperture. When the aperture increases, the light intensity increases; When the aperture decreases, the light intensity decreases. It solves the exposure problem together with the shutter speed.

(2) Adjust and control the depth of field. This is the important role of aperture. Large aperture and small depth of field; Small aperture and large depth of field.

(3) affect the imaging quality. This is the function that the aperture is easily overlooked. Any lens has an aperture with the best imaging quality, that is, it is least affected by various aberrations. This aperture is commonly known as the best aperture. The exact position of the optimal aperture can be measured by using special instruments. Generally speaking, the best aperture is located at the position where the maximum aperture of the lens is reduced by about 2 ~ 3 steps. If it is greater than or smaller than the optimal aperture, lens aberrations such as spherical aberration, longitudinal chromatic aberration, coma, field curvature and astigmatism will increase, so try to use the optimal aperture when shooting to improve the imaging quality. The shape of the aperture will also affect the imaging quality. The closer the polygon is to the circle, the better the imaging quality.

Black head: low-end mass lens, including the head of low-end SLR camera.

The division of shots can be divided into many categories. For example, it can be divided by focal length, fixed focus and zoom, nearest focal length, brand and bayonet, or grade. It sounds complicated, but it is not difficult to explain.

Let's start with the focal length of the lens. When we understand or choose to buy a lens, the first factor to consider is the focal length of the lens. This parameter directly affects our shooting perspective and shooting theme. The focal length of a lens is generally expressed in millimeters, such as 35mm, 85mm, 105mm and so on. The shorter the focal length (the smaller the number), the wider the viewing angle; The longer the focal length (the larger the number), the narrower the viewing angle. In addition to the width of the viewing angle, the change of focal length also tells us another intuitive feeling: the shorter the focal length, the farther away the subject is from us; The longer the focal length, the closer the object is to us. So you should understand that the zoom function on consumer digital cameras can give us the feeling of "zooming in and out", which is actually achieved by changing the focal length of the lens.

For 135 digital SLR, we usually refer to the lens with a focal length of less than 24mm as super wide-angle lens, the lens with a focal length of 24 mm ~ 35 mm as wide-angle lens, the lens with a focal length of 50 mm ~ 85 mm as standard lens, the lens with a focal length of 85 mm ~ 135 mm as medium telephoto lens, and the lens with a focal length of more than 200mm as telephoto lens.

How to use the lens of each focal segment? Wide-angle lens has a wide angle of view, which is suitable for shooting scenery, especially the ultra-wide-angle lens, which has a wide field of vision and the scenery shot is particularly imposing. The angle of view of the standard lens is close to that of the human eye, hence the name. Standard lenses are widely used, both in landscape and portrait, but they are also quite difficult to control. Ordinary people may take the same photos with the standard lens, but experienced photographers can take many photos with a sense of scene by using the similar perspective of the standard lens and the human eye. 85 mm ~ 135 mm medium telephoto lens is generally used to shoot busts. It's easy to take photos with blurred background with this lens, which looks more artistic. A telephoto lens over 200mm is usually used to photograph insects, ecology, stage and sports. Because of the long focal length, we can bring distant objects closer to our eyes, so we can take clear photos without getting close to the subject. We obviously need this ability to shoot insects, ecology, stage and sports.

Of course, the use of lenses with various focal lengths is not static. Sometimes, we can also take some portraits with a wide-angle lens to achieve an exaggerated effect. You can also shoot some landscape works with telephoto lens. This requires us to gain experience through constant practice. We already know that different focal lengths correspond to different shooting occasions and themes. However, it is also a troublesome thing to change lenses with different focal lengths for different shooting themes. Is there a shot that can take care of multiple occasions?

Of course there is! The zoom lens can meet our requirements. The focal length of the zoom lens can be changed, and the freedom of shooting is naturally expanded. For example, a zoom lens of 24 ~ 70 mm can not only shoot landscapes, but also be suitable for portraits. A fixed-focus lens with a fixed focal length can't do this.

So, how do we measure the use of zoom lenses? Like a fixed-focus lens, we look at it through the focal length. For zoom lenses only, there are two focal lengths. One is the focal length of the wide-angle end and the other is the focal length of the telephoto end. The focal length of the zoom lens can be continuously changed within these two values.

It is worth noting that for digital SLR lenses, we usually don't mention the concept of "multiple zoom". Give a very simple example, I believe everyone will understand. Zoom lenses with a focal length of 24 ~ 70 mm and zoom lenses with a focal length of 70 ~ 200 mm can both be called triple optical zoom (multiple zoom is calculated by dividing the focal length of the telephoto end by the focal length of the wide-angle end, and the number obtained is multiple zoom. 70/24 and 200/70 are both slightly equal to 3, so we can all regard them as 3x optical zoom. But obviously, the scope of application of the two is completely different. 24 ~ 70 mm lens is suitable for shooting landscapes and portraits, and 70 ~ 200 mm lens is more suitable for shooting works with ecological, sports, stage and other themes. Therefore, for the lens of digital SLR, it is of little significance to talk about multiple zoom. What we value more is its focal length.

However, on the other hand, it must be admitted that the greater the zoom factor of the lens, the wider the scope of application of this lens. For example, 18 ~ 200mm lens, the focal length ranges from wide angle to telephoto, which is suitable for almost any shooting subject. With such a lens, you can "walk the world in one mirror" and it is very convenient to use.

So is the bigger the magnification, the better? Of course not. You can't have your cake and eat it. Although the zoom lens is easy to use, it is not as good as the fixed focus lens in optical quality. Moreover, the fixed-focus lens far exceeds the zoom lens in some technical indicators. For example, the aperture of F2.8 is great for zoom lenses, but for fixed-focus lenses, the aperture can be F 1.4 or even F 1.2. Therefore, the fixed-focus lens is still the best choice for applications that require extremely high image quality. In addition, if you are particularly interested in large aperture, then fixed focus lens is also the first choice. Similarly, for those zoom lenses with good optical quality, the zoom factor is generally controlled within 3 times, and excessive zoom factor is still a negative factor for excellent optical quality.

Therefore, choosing a fixed-focus lens or a zoom lens should be based on your own requirements. Generally speaking, however, zoom lenses are enough to meet the needs of ordinary consumers. The focal length of a camera lens is a very important index of the lens. The focal length of the lens determines the imaging size of the object on the imaging medium (film or CCD, etc.). ), which is equivalent to the proportion of objects and images. When shooting the same subject at the same distance, the image with long lens focal length is large, and the image with short lens focal length is small. According to different uses, the focal length of camera lens varies greatly, from a few millimeters to a dozen millimeters to several meters. Common ones are 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 1200mm, etc. And it has a super long focal length of 2500 mm.

According to the length of its focal length, that is, the viewing angle when shooting, the lens can be divided into standard lens, wide-angle lens and long focal length lens.

The viewing angle of a standard lens is about 50 degrees, which is the viewing angle that a person can see with one eye without turning his head and eyes. The feeling observed from the standard lens is basically the same as the scenery we usually see. The standard lens focal length of a 35mm camera is mostly 40mm, 50mm or 55mm. The focal length of the standard lens of 120 camera is generally 80mm or 75 mm. The larger the camera frame, the greater the focal length of the standard lens. However, due to the size of imaging medium (CCD or CMOS) of digital camera, the focal length of standard lens is also inconsistent. In order to facilitate direct perception, when talking about DC lens, the so-called equivalent focal length of 35mm camera is often used, and this equivalence refers to the equivalence in viewing angle. Only the lens of 35mm camera can be compared with other cameras and digital cameras.

Wide-angle lens, as its name implies, has a wide shooting angle and is suitable for shooting scenes with short distance and large range. Sometimes it is used to deliberately exaggerate the foreground performance, with a strong sense of distance and perspective. A typical wide-angle lens of a 35mm camera has a focal length of 28mm and a viewing angle of 72 degrees. Commonly used are the so-called small wide angles of 35mm and 38mm, which are slightly longer than 28mm (more common in fool machines). Compared with the so-called fisheye lens, the super wide-angle lens (for example, the focal length is 24mm and the viewing angle is 84 degrees) has a focal length of 8mm and the viewing angle can reach 180 degrees.

Long-focus lens is suitable for shooting distant view, with small depth of field, easy to blur the background and highlight the subject. The long focal length lens of a 35mm camera is usually divided into three levels. The lens below 135mm is called medium focal length, such as 85mm, 28-degree viewing angle, 105mm viewing angle, 23-degree viewing angle and 135mm viewing angle. Medium focal length lens is often used to shoot portraits, sometimes called portrait lens. 135-500 is called long focal length, such as 200mm, viewing angle 12 degrees, 400mm, and viewing angle of 6 degrees. More than 500mm is called ultra-long focal length, and the viewing angle is less than 5 degrees, which is suitable for shooting distant view. For example, the close-up on the court and the shooting of wild animals, because it is impossible to get close to the subject, the ultra-long focal length lens is of great use. What is the definition of lens focal length? Generally speaking, we say: the focal length is the distance from the center of the lens to the focus. But this is only the case of a single thin lens. Because the lens of a camera is composed of many lenses, the situation is far from simple.

The focal length of a lens is divided into image focal length and object focal length. The focal length of an image is the distance from the main surface of the image to the focal point of the image, and the focal length of an object is the distance from the main surface of the object to the focal point of the object. It must be noted that the focal length of the object and the focal length of the image are not necessarily equal, because the telescope structure is widely used in camera lens design, especially zoom lens. What we usually call the focal length of the camera lens refers to the focal length of the image.

The following figure illustrates how to determine the main plane and focal length: when the incident parallel light (or its extension line) intersects with the emergent convergent light (or its extension line), the main plane can be determined, and the intersection of this imaginary plane and the optical axis of the lens is the main point. The distance between the main point of the image and the focal plane (focal point) formed by infinite light is called the focal length of the compound lens (strictly speaking, the effective focal length). The same principle can also be used to determine the main surface and focal length of an object.

Depending on the design, the position of the main surface may appear outside the lens. This is very important on many occasions.

For example, for an 8mm fisheye lens, the main plane of the image should be 8mm in front of the focal plane, but the thickness of the reflector, exposure window and focal plane shutter can't be accommodated within 8 mm, so the 8 mm fisheye lens actually adopts the design of adding a negative optical system in front (also called inverted telescopic structure), so that the lens can be installed on the fuselage with the positioning distance of the image plane much greater than 8 mm.

Similarly, if it is a 500mm super telephoto lens, if the telescope structure is not used, the length of the lens will exceed 500mm, which is undoubtedly very inconvenient to use. The design of the telescope structure can make the main surface far in front of the lens and greatly shorten the lens length.

When it comes to focal length, you can't help but talk about the zoom lens.

The zoom lens consists of a fixed lens group and a variable lens group. By moving the variable lens group and changing the imaging optical path, the focal length of the lens can be changed in a certain range, thus changing the shooting angle.

The magnification of zoom lens refers to the ratio of the maximum focal length of telephoto end to the minimum focal length of wide-angle end.

Zoom lens is very convenient for shooting and framing because of its variable viewing angle, which is deeply loved by ordinary photographers. However, due to the complex structure of zoom lens and many relative moving optical elements, the consistency of imaging quality, resolution and distortion of each focal length segment can not be guaranteed in optical design. Therefore, the overall imaging quality of the zoom lens is difficult to reach a higher level. Especially the zoom lens with large zoom ratio is more difficult to obtain higher image quality. Therefore, at the same technical level, the imaging quality of zoom lens is lower than that of fixed focus lens, and the imaging quality of large magnification zoom lens is lower than that of small magnification zoom lens.

In the case of FC (for example, a 35mm camera), a fixed-focus lens is often used for high-quality photography, and even if a zoom lens is used, the zoom ratio is not large. However, in the case of DC, because the imaging area of CCD is relatively small, the geometric dimensions (focal length and lens diameter) of its zoom lens are also small, so it is easier to realize large zoom magnification in optical design.

Therefore, zoom magnification and focal length are two different concepts. The top of the picture below is a 70-2 10mm zoom lens with magnification of 3 times, and the bottom is a 28- 105mm zoom lens with magnification of 4 times. However, the telephoto ability of triple zoom (2 108 mm) is obviously better than that of quadruple zoom (105mm). Sometimes in this forum, the statements of "bullying others with 5X zoom10x" and "shooting with10x zoom" are incorrect.