What is the complete concept of focal length?

Focal length is an important parameter of optical lens. When a beam of parallel light passes through the convex lens along its main axis, it will converge to a point on the other side of the convex lens, which is called the focal point, and the distance from the focal point to the optical center of the convex lens is called the focal length of this convex lens. Photographic lenses, like convex lenses, have standard focal lengths, long focal lengths and short focal lengths. The focal length = the diagonal length of the imaging frame is called the standard lens; What is longer than the standard lens is called a long focal length lens, and what is shorter than the standard lens is called a short focal length lens. Lenses with different focal lengths have different modeling characteristics, for example, short focal length lenses have the characteristics of wide viewing angle, large depth of field, large perspective effect and small imaging. These characteristics are used in creation, that is, modeling means. A lens with variable focal length is called a zoom lens, and the ratio of the shortest focal length to the longest focal length is called the zoom multiple of the edge focal length lens. (not called amplification).

Basic characteristics of wide-angle lens: 1. Wide viewing angle, covering a wide range of scenery. The so-called wide viewing angle range means that at the same viewing point (the distance from the subject is constant), the scene is viewed with wide-angle, standard and telephoto lenses with three different focal lengths. As a result, the former shoots more up, down, left and right scenes than the latter. When the photographer has no retreat, it is difficult to take a complete picture (such as taking a group photo) with a standard lens of 50 mm ), using a wide-angle lens can easily solve the problem. Furthermore, for example, shooting a vast Yuan Ye or a tall building in a city, perhaps only a part of the scenery can be photographed with a standard lens, which cannot show the breadth or height of the scenery. Shooting with a wide-angle lens can effectively show the open momentum of large scenes or majestic buildings. 2. Short focal length and long depth of field. Photographers usually rely on the short focal length and long depth of field of wide-angle lenses to bring the whole scene from near to far into a clear range of expression when shooting a wide scene. In addition, when shooting with a wide-angle lens, if a smaller aperture is used at the same time, the depth of field of the scene will become longer. For example, if a photographer shoots with a 28 mm wide-angle lens, the focus is on a subject about 3 meters, and the aperture is adjusted to F8, then from 1 meter to infinity, it almost enters the depth of field. It is precisely because of this long depth of field that wide-angle lenses are often used by photographers as fast-moving lenses with strong maneuverability. On some occasions, the photographer can quickly capture the picture by manipulating the wide-angle lens without focusing on the subject. 3. You can emphasize the prospect and highlight the contrast between far and near. This is another important performance of wide-angle lens. The so-called emphasis on foreground and far-near contrast means that wide-angle lens can emphasize far-near contrast more than other lenses. In other words, the photos taken with wide-angle lens are closer and farther, which makes people feel that the distance has been widened and has a strong perspective effect in the depth direction. Especially when shooting with a super wide-angle lens with short focal length, the effect of being near big and far small is particularly remarkable. 4. It can be exaggerated and deformed. Generally speaking, it is taboo to use a wide-angle lens when the subject is exaggerated and deformed. In fact, it is not necessarily undesirable for the subject to exaggerate and deform appropriately. Experienced photographers often use wide-angle lenses to deform the subject moderately and take some unusual photos of very inconspicuous scenes that people ignore. Of course, using a wide-angle lens to express exaggeration and distortion should be based on the needs of the subject matter, and the second should be few and precise. No matter whether the theme is necessary or not, it is not worthwhile to abuse the exaggerated deformation of wide-angle lens and blindly pursue the bizarre effect in form.

Basic characteristics of standard lens: Standard lens gives people a vivid visual effect picture, so it is often used in actual shooting. On the other hand, because the picture effect of standard lens is very similar to the visual effect of human eyes, the picture effect taken with standard lens is very ordinary, even very "plain", and it is difficult to obtain the dramatic effect of rendering the picture like wide-angle lens or telephoto lens. So it is not easy to take vivid photos with standard lenses. Even experienced photographers think it is not easy to use standard lenses well. However, the visual effect of the standard lens has a natural sense of closeness, and the distance between the standard lens and the subject is moderate, so it is widely used in ordinary scenes, ordinary portraits, snapshots and other photography occasions. The most common commemorative photos are taken with standard lenses. In addition, what photographers often overlook is that the standard lens is also a lens with good imaging quality, which is very effective for showing the details of the subject. The basic characteristics of telephoto lens: telephoto lens has long focal length, small viewing angle and large imaging on negative. Therefore, at the same distance, you can take larger images than the standard lens. Suitable for shooting distant objects. Because its depth of field is smaller than the standard lens, it can blur the background more effectively and highlight the focused subject, and the subject is generally far away from the camera, which makes the portrait more realistic in perspective, so people often call the telephoto lens a portrait lens. The telephoto lens is often used in professional photography because of its long lens barrel, heavy weight, relatively expensive price and small depth of field, and it is difficult to focus in practical use.

Supplemented by the respondent: 2009-06-29 14:0 1

The basic characteristics of macro photography lens: macro photography lens is a kind of photography lens with the purpose of shooting tiny objects or retaking small pictures. This kind of lens has quite high resolution, minimum distortion and aberration, high contrast and good color reproducibility. Macro camera lens has good close-range resolution, which can keep the image quality unchanged in the whole focusing range. General photographic lenses are mainly used to shoot scenes within the usual focal length, and cannot be directly used for close-ups. To shoot close-ups with general photographic lens, close-up accessories such as close-up lens, close-up collar or close-up leather cavity must be added to the lens before it can be taken. However, after adding close-up accessories such as close-up lens, close-up collar or close-up leather cavity to the general photographic lens, it is in the "close-up" state and cannot be quickly restored to the normal photographic state from the "close-up" state. In other words, it is difficult to take close-ups alternately by using general photographic lenses and close-up accessories. Macro photography lens is different. Its close-up does not depend on other close-up accessories, and all close-up operations are carried out on the lens itself. It can connect the focal length from close-up to infinity, so that it can be quickly adjusted from close-up to ordinary photography, which provides convenience for photographers to alternately take close-up photography and ordinary photography. Macro photographic lenses generally have two structures. One macro photography lens adopts the structure of built-in telescopic lens barrel, and the other adopts the structure of exchanging the front and rear positions of optical lens groups in the lens. The former, in ordinary photography, the photographer can focus normally only by turning the focus ring of the lens. If he wants to take a close shot, as long as the focus ring that has been rotated to the nearest focus position continues to rotate, he can move the entire optical system of the lens forward synchronously with the built-in lens barrel, thereby increasing the image distance and achieving the purpose of close shot. The latter obtains higher image magnification by changing the front and rear positions of the optical lens group in the lens, thus achieving the purpose of close-up. Common macro photography lenses include Nicole Micro 55mm/F2.8 manual focusing macro photography lens, etc.

Wide-angle lens: A lens with a focal length between 24 mm and 38 mm is called a "wide-angle lens", which can provide a wide angle of view and bring more scenes into the shooting range, which is beneficial to shooting large scenes and shooting in narrow places. Generally, there are 35mm, 28mm and 24mm lenses in this focal length section. The 28mm lens is called "standard wide-angle lens" because of its wide viewing angle and no obvious aberration.

Ultra-wide-angle lens: A lens with a focal length less than 24mm is called an "ultra-wide-angle lens". As the name implies, its viewing angle is wider than that of a wide-angle lens, and its shooting range is also larger than that of a wide-angle lens. Because shooting a large scene, it is difficult to correct all the aberrations of the lens, so the deformation is obvious and the illumination of the image field is not very uniform. However, compared with its special shooting ability, these shortcomings are acceptable.

Fisheye lens: A lens whose focal length is less than 16mm and whose aberration has not been corrected in the manufacturing process is called "fisheye lens". The viewing angle is above 180 degrees, just like the viewing angle range of a fish eye. This kind of lens actually belongs to a special photographic lens and is used in special occasions. You should also pay great attention when using it. When shooting with a camera, you may accidentally take the photographer's own feet into the picture.

Medium focal length lens: A lens with a focal length between 70mm and 135mm is called a "medium focal length lens". The medium focal length lens has moderate focal length and excellent aberration correction, and most of them are high-speed photographic lenses, which have good perspective effect and can give people a comfortable and natural feeling. Widely used in portrait photography, landscape photography and other topics.

Long focal length lens: A lens with a focal length of 135mm or more is called a "long focal length lens". Because of the narrow viewing angle, the distant scene can be brought closer to form a larger image on the film, and the shooting effect has a strong sense of perspective compression, which is also conducive to separating the subject from the background. Mainly used for long-distance shooting, or for close-up photography.