The focus in photography.

The focal length is formed by the optical lens of the camera lens. A group of lenses with curvature (convex or concave) on both sides or one side are installed in the metal tube of a camera or projector to form a comprehensive lens. The light emitted from different parts of the object, after passing through the lens, is focused on a point on the negative, so that the image has a clear outline and real texture. This point is called the focus. The focal length is the distance from the point in the lens of the lens to the point where the light can be clearly focused.

When the camera lens is adjusted to infinity, it is actually a nominal focal length. In design, the distance between the main plane of the lens and the negative or imaging sensor is adjusted to the length of the focal length, and then the image far away from the lens can form a clear image on the negative or sensor. When the lens wants to shoot a close object, the actual focal length of the lens changes. The focal length is usually marked in millimeters (mm), but you can still see that some old lenses are marked in centimeters (cm) or inches. The size of the field of view depends on the ratio of lens focal length to film size. Because the most popular specification now is 35mm, the field of view of the lens is often marked according to this specification. The fields of view of standard lens (50mm), wide-angle lens (24mm) and telescope head (500mm) are different. The same is true for digital cameras, whose photoreceptors are smaller than the traditional 35mm film, so they can get the same image with a shorter focal length.

The concept of extension

1. Zoom: Adjust the focus and focal length when shooting.

2. Focus: Adjust the focal length, so that the subject is in focus and the image is clear.

3. Out of focus: the subject deviates from the focal length and the image is blurred.

4. Focus selection: in the depth of field, select a certain level to focus clearly, and other levels are blurred (out of focus).

5. Follow the focus: change the focus so that the moving person is in focus.

6. Rack focusing or zooming: The focus moves from one focus to another at a rather sudden speed.

focal distance

Lens focal length classification

Common ones are: 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 600mm, 1200mm, etc. , with a 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. For convenience and intuition, when talking about DC lens, the so-called equivalent focal length of a 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

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. 135 A typical wide-angle lens of a camera has a focal length of 28mm and a viewing angle of 75 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 shot

The telephoto lens is commonly known as the "telescope head". The telephoto lens is suitable for shooting long-distance scenes, 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.

Understanding of lens focal length

Generally speaking, the focal length we are talking about 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.

How to determine the main plane and focal length

When the incident parallel ray (or its extension line) intersects with the emergent convergent ray (or its extension line), the principal refraction surface can be determined, and the intersection of this imaginary plane and the optical axis of the lens is the principal 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.

Use case of lens focal length

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.

equivalent focal length

Digital cameras have photosensitive elements of different sizes (CCD or CMOS) (such as 1/2.5 inches,11.8 inches, etc.). ), so the lens with the same focal length has different imaging angles on digital cameras with different photosensitive element sizes (see figure for details). For example, a lens with a focal length of 50mm is used on a 135 film camera with a viewing angle of about 46 degrees, while it is used on an APS-c DSLR (such as Nikon's D90 and D300). The viewing angle of the film with diagonal length of 135 is about 30 degrees. The shooting angle of this 50mm lens on APS-c machine is similar to that of the lens with 75mm focal length on 135 film machine, both of which are about 30 degrees.

Therefore, it is impossible to compare the shooting range (imaging angle) of different cameras only by the real focal length of the lens. But for users, what really matters is the shooting range (viewing angle) of the camera. Because people always define the shooting angle by the lens focal length of 135 film camera (the photosensitive surface of 135 film camera is fixed), people are also used to converting the imaging angles of photosensitive elements with different sizes into the lens focal length corresponding to the same imaging angle of 135 camera, and this converted focal length is the equivalent focal length of 135.

focal length conversion coefficient

For example, the 50mm lens used on the DSLR of APS-c mentioned above, the equivalent focal length of its 135 camera is 75 mm ... Because 75mm=50mm* 1.5, here 1.5 is the so-called "focal length conversion factor".

135 film cameras all use the 35mm film shown in the above figure, while the CCD/CMOS size of digital cameras is often relatively small. For the same lens, the viewing angle of digital camera is smaller than that of 135 camera, and the decrease of viewing angle can be equivalent to the increase of focal length (the viewing angle is inversely proportional to the focal length).

There is such a simple method to calculate the zoom factor of a digital camera. For example, the CCD size of Nikon D80 is about 24 mm x16 mm. We divide the width of 36mm film imaging plane by the CCD width of Nikon D40 to get the zoom factor 1.5. When we add 18mm fisheye lens to Nikon D40, the equivalent focal length of this fisheye lens is 18* 1.5=27mm.