ABC knowledge talk about lens and lens focal length

I recently saw some discussions about focal lengths in the forum, so I want to talk about the focal lengths. I hope it will be good for you. If any, please note.

The focal length of the camera lens is a very important indicator of the lens. The length of the lens focal length determines the size of the object to be imaged on the imaging medium (film or CCD, etc.), which is equivalent to the scale of the object and the image. When shooting a shot of the same distant target, the camera focal length is large, and the camera focal length is short. Depending on the purpose, the focal length of the camera lens varies greatly, ranging from a few millimeters to a dozen millimeters. Common examples include 8mm, 15, 24, 28, 35, 50, 85, 105, 135, 200, 400, 600, 1200mm and so on, as well as up to 2500mm super telephoto lenses.

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

Standard LensThe angle of view is about 50 degrees. This is what a person can see when his/her head and eyes do not rotate, the feeling we observe from the standard lens is basically the same as what we usually see. The focal length of the 35mm standard lens is 40mm, 50mm mm, or 55mm. The focal length of the standard lens of the 120 camera is generally 80mm or 75mm. The larger the camera width, the larger the focal length of the standard lens. Because of the large and small imaging medium (CCD or CMOS) of digital cameras, the focal lengths of standard lenses are also inconsistent. To facilitate direct sensing, the DC lens often uses the so-called equivalent focal length equivalent to 35mm cameras, which is equivalent to the angle of view. The following is only about the lens of the 35mm camera, the camera with other frames and the digital camera, and so on.

Wide angle lensAs the name suggests, it has a wide range of photography perspectives and is suitable for shooting close-to-distance and wide-range scenes. It is sometimes used to deliberately exaggerate the foreground performance, strong sense of distance, and perspective. The typical wide-angle lens of the 35mm camera is a focal length of 28mm and the angle of view is 72 degrees. The commonly used 35mm is a little longer than 28mm, and the so-called small Wide Angle of 38mm (mostly seen in dummies ). Larger than the general wide-angle lens, the ultra-wide angle lens (for example, the focal length is 24mm, and the angle of view is 84 degrees) and the so-called fish eye lens, the focal length is 8mm, and the angle of view can reach 180 degrees.

Long focal length lensIt is suitable for shooting distant scenes with a small depth of field, so it is easy to blur the background and expose the subject. A 35mm-inch long focal length lens is generally divided into three levels, with a medium focal length of less than 135mm, such as 85mm, 28 degrees angle, 105mm, 23 degrees angle and 135mm, and 18 degrees angle. A middle focal length lens is often used to take portraits. It is also called a portrait lens. 200-400 long focal length, such as mm, 12 degrees of angle of view, mm, 6 degrees of angle of view. More than 500mm of the images are referred to as super-long focal lengths, and the angle of view is smaller than 5 degrees. It is suitable for shooting distant scenes. For example, close-up on the stadium and wildlife shooting, because they cannot be close to the subject, the ultra-long focal length lens will be useful.

What is the definition of lens focal length? Generally, the focal length is the distance from the center to the focus of the lens. However, this is only a single-chip thin lens. Because the camera lens is composed of many lenses, the situation is far from that simple.

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

Indicates how to determine the primary plane and focal length: the incident parallel light (or its extension line) and the outbound convergence light (or its extension line) intersection, you can determine the refraction of the primary plane, the imaginary plane and the optical axis of the lens are the main point. The distance between the focal plane (Focus) formed by the primary point of the image and the infinite remote line 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 the object.

The position of the main face may appear outside the lens according to the design. This is important in many cases.

For example, for a 8mm fisheye lens, the image's Main Plane should be located at 8mm in front of the focal plane, but within 8mm, it cannot accommodate the thickness of the mirror, the exposure window, and the focal plane shutter. Therefore, the 8mm fisheye lens actually uses the design of the front plus negative group optical system (also called the inverted far structure), so that the lens can be installed on the fuselage with a much larger Face Positioning distance than 8mm.

Similarly, if the lens is a 500mm hyper-focal lens and does not use a telescope structure, the lens will be longer than 500mm, which is undoubtedly inconvenient to use. The design of the telescope structure can make the main face much earlier than the lens, greatly reducing the length of the lens.

When it comes to focal lengths, you cannot talk about zoom lenses.

A 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 within a certain range, which also changes the shooting angle.

The zoom lens rate refers to the ratio of the maximum focal length at the focal point to the minimum focal length at the wide-angle end.

Due to the variable angle of view, the zoom lens brings great convenience to shooting and is favored by photography enthusiasts. However, due to the complex structure of the zoom lens, there are many optical components in relative motion, and the imaging quality, resolution, distortion, and other parameter consistency of each focal length cannot be ensured during optical design. Therefore, the overall imaging quality of the zoom lens is difficult to reach a high level. In particular, zoom lenses with high zoom ratios are more difficult to obtain high image quality.Therefore, the imaging quality of a zoom lens is lower than that of a fixed focal lens and that of a zoom lens is lower than that of a zoom lens.

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

The zoom ratio and focal length are two different concepts. The top is a 70-210mm zoom lens with a zoom rate of 3X. The bottom is a 28-105mm zoom lens with a zoom rate of 4x. However, the zoom capability is obviously 3x zoom (210) it is better than 4x zoom (105 ). In this forum, the "10 x zoom to bully the 5 x zoom" statement is incorrect.

Let's talk about digital zoom. The so-called digital zoom is actually a digital image processing method that captures a part of the image to change the angle of view. For example, if a x image is taken out of the center of a x image, it can be seen as a 2x digital zoom. When the storage medium capacity is relatively large, you can use a large size to take a picture and then cut it to achieve the same or better effect. Therefore, I believe that digital zoom in DC is meaningless. Only in DV, digital zoom makes sense because the shooting size is determined by the standard.

The focal length of a lens can also be changed by adding some accessories to meet the shooting requirements, such as a multiplier or near-camera. You can double the focal length of a lens. The multiplier is actually a negative lens group (as shown in the figure), and is connected between the fuselage and the lens in the FC. After a 28-105mm lens is attached to the multiplier, the zoom range becomes 56-210mm (shown below ). Close up (close up) is connected to the front of the lens. In fact, it is a positive lens that can increase the horizontal magnification (that is, the ratio of the image to the object size) during shooting ). Currently, most DC (except DSLR) lens headers cannot be replaced. Therefore, in DC, these accessories are added to the front-end of the lens, such as doubling the additional mirror and Wide Angle additional mirror.

Because the actual focal length of the DC lens is relatively small, the macro shooting function of the DC lens becomes more prominent. In this forum, there are many works taken by Macro. In optics, what macro photography does not change is not the focal length of the lens, but the image distance when shooting. According to the imaging formula, the reciprocal of the object distance plus the reciprocal of the image distance is equal to the reciprocal of the focal length. When the image distance is increased to 2 times the focal length, the object distance and the image distance are equal to 2 times the focal length. At this time, the image size is equal to the object size, and the horizontal magnification is 1 ). When the DC lens is set to the macro, the focal length remains unchanged and the image distance is changed. That is, the relative position of the fixed Lens Group and the variable Lens Group in the lens is locked and the entire lens group is moved. Because the image distance is equal to the focal length when the distance is infinite, it can be seen that the distance from the focus is infinite to is about the focal length. DC is relatively easy to achieve because of its small focal length.

The FC lens also has the macro function. However, because the focal length of the FC lens is relatively large, it is difficult to achieve a macro. As shown in the figure above, the camera macro of 28-105mm is only. In addition to the macro function of the lens, the SLR camera can also be connected to a near-camera image (under the image) or a dedicated near-camera leather cavity (on the image) between the body and the lens to change the image distance, to achieve a horizontal magnification greater than 1. However, if the transverse magnification is large (from to), it will belong to the scope of micro photography.

In addition to the accessories above, macro photography can also be achieved through a dedicated macro lens or lens inversion and other methods. However, the principle is the same. Even if the image distance increases to achieve the increase of horizontal magnification, I will not elaborate on it.