What is the focal length in photography?

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of the lens to the focal point of light convergence when parallel light is incident. So what do you know about focal length? The following is what I have compiled about focal length, I hope you like it!

Focal length introduces the distance from the lens center to the imaging plane (such as back plate or CCD). An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the center of the mirror.

The focal length of an optical system is usually expressed by the effective focal length (EFL) of a thick lens (the thickness of which cannot be ignored) or a system with several lenses or mirrors (such as a camera, lens or telescope), which is different from the commonly used parameters:

The front focal length (FFD) or the front focal length (FFL) is the distance from the focal point in front of the system to the vertex of the first optical surface.

The back focal length (BFD) or back focal length (BFL) is the distance from the vertex of the last optical surface of the system to the back focal point.

In an optical system in air, the effective focal length is the distance from the front and rear main planes to the corresponding focal points. If the surrounding environment is not air, the distance should be multiplied by the refractive index of the substance. Some authors call this distance the front (back) focal length to distinguish it from the front (back) focal length defined above.

Generally, focal length or effective focal length is a value that describes the ability of an optical system to collect light, and is usually used to calculate magnification. Other parameters are used to calculate where the image of a specific object will be formed.

For a lens with thickness d, radius of curvature R 1 and R2 in air, the effective focal length is:

1/f =(n- 1)[ 1/r 1- 1/R2+(n- 1)d/NR 1r 2]

Where n is the refractive index of the lens material, the value 1/f is the optical magnification of the lens, and f is the focal length. It can be seen that the smaller the refractive index n of the lens material, the larger the focal length of the lens.

The focal length is marked by the most common convention. If the lens on the first side is convex, the value of R 1 is positive, and if it is concave, it is negative. R2 is positive if the second surface is a concave lens, and negative if it is a convex lens. It should be noted that even so, different authors may have different annotation habits.

For a mirror with spherical curvature, the focal length is equal to half the radius of curvature of the mirror. The focal length of convex mirror is positive and the focal length of concave mirror is negative.

The distance from the object to the center of the lens u The size of the positive and negative images of the image v The distance between the virtual image and the real image to the center of the lens applies the relationship between the object distance and the image distance.

The concept of focal length 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.