Common sense of focal length

The lens of a camera is a set of lenses. When parallel light passes through a lens, it will converge to a point, which is called focus. The distance from the focal point to the center of the lens is called the focal length.

A lens with a fixed focal length is called a fixed focus lens; A lens with adjustable focal length is called a zoom lens. In the field of photography, the focal length mainly reflects the lens angle.

For the traditional135 camera, the lens with a viewing angle of about 50mm is closest to the human eye, which is called a standard lens, and generally covers the range of 40-70 mm. 18-40mm is called a wide-angle or short-focus lens, 70- 135mm is called a medium-focus lens, and135 mm is called a medium-focus lens. Above 500mm is called telescope head, and below 18mm is called fisheye or super wide-angle lens. The division of this range is only people's habit, and there is no strict definition. The CCD of a digital camera is much smaller than that of 135 film, so the focal length of the lens is much shorter at the same viewing angle.

For example, use 0. For a digital camera with 33 "CCD, when a lens of about 13mm is used, its viewing angle is equivalent to the standard lens of a 50mm 135 camera.

Because the specifications and models of CCD used by digital camera manufacturers are different, the lens coefficient should be carefully considered before adding lenses.

2. Physical knowledge: the relationship between object distance and focal length.

Imaging law of convex lens;

U & gt2f f<u & lt2f Inverted to Reduce the Real Image

F<u & lt2f u & gt2f Reverse Magnification Real Image

U<f _ _ _ vertically enlarges the virtual image

U=f does not image

U=2f becomes an inverted real image.

F represents the focal length of the lens.

U stands for the distance between the object and the lens (object distance for short).

Image distance is the distance from the image body of the imaged object to the lens (main plane).

Principle: converge light and have focus, so imaging. Rule: the object distance is more than twice the focal length, and it is inverted and reduced. The object distance is equal to twice the focal length, forming an inverted image. The object distance is less than twice the focal length, but more than twice the focal length, forming an inverted enlarged image. The object distance is equal to a focal length, and no image is formed. The object distance is less than a focal length, and the virtual image is vertically enlarged. Except the last one, the front is full of real images.

Image distance is the distance between the image formed by convex lens and the lens. Can be changed.

The focal length is the point where parallel rays converge after refraction by a convex lens, and the focal length remains unchanged.

The image distance is the distance between the image formed by the convex lens and the lens, which can be changed at will. Changing the image distance can change the image size and handstand.

Focal length is the point where parallel rays converge after refraction by a convex lens, which is the property of the lens itself and is fixed.

The focal length is fixed by a convex lens, which is the distance from the focal point of the convex lens to the convex lens. The image distance is the distance from the image to the convex lens, which varies with the object distance (the distance from the object to the convex lens). It can be said that the focal length has little to do with the image distance.

The focal length is fixed and the image distance is not fixed.

For a convex lens, the parallel light perpendicular to the mirror emitted from one side at infinity will converge on a point on the other side of the convex lens, which is called the focal point of the convex lens on this side (if the curvatures of the two convex lenses are different, the focal points of the two convex lenses are asymmetric), and the distance from the focal point to the center of the mirror is the focal length.

The light emitted by a specific luminous body is generally not parallel light, but if the luminous body is close to the main axis (the line passing through the center of the convex lens and perpendicular to the mirror surface), it can be imaged on the convex lens side (it can be a real image or a virtual image, depending on the distance between the object and the mirror and the curvature of both sides of the convex lens), and the distance between this image and the mirror surface is the image distance. Even if there is a focus on the image, its meaning is different.

What is the complete concept of focal length?

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. It is also the distance from the center of the lens to the imaging plane such as the backplane or CCD in the camera. 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.

For thick lenses (lenses whose thickness cannot be ignored) or systems with several lenses or mirrors (such as photography, lens or telescope), the focal length is usually EFL, effective focal length (EFL).

Length), 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.

4. Seek all the knowledge about physical optics concave-convex lens in junior high school.

Concave-convex lens convex lens is a kind of lens with a thick central part. Convex lenses are divided into biconvex, plano-convex and concave-convex (or positive meniscus) shapes. Thin convex lens is also called condenser because of its convergence, while thick convex lens has the function of telescope, divergence or convergence, which is related to the thickness of lens. Parallel light (such as sunlight) parallel to the axis (the line connecting the spherical center of the convex lens) is called the main optical axis of this lens. After the light is refracted twice on both sides of the lens, it is concentrated at a point on the axis, which is called the focus of the convex lens (marked F). A convex lens has a focal point on both sides of the mirror. If it is a thin lens, the distance between the two focal points and the center of the lens is approximately equal. The focal length of the convex lens refers to the distance from the focal point to the center of the lens, which is usually expressed by F. The smaller the spherical radius of the convex lens, the shorter the focal length. The convex lens can be used for magnifying glasses, glasses worn by presbyopia and hyperopia, lenses of microscopes and telescopes, etc. The larger the focal length, the more convex the convex mirror surface, and the better the condensing effect. Both sides are spherical or one side is spherical and the other side is flat, and the middle part is thin, which is called concave lens. There are three kinds of lenses: biconcave lens, plano-concave lens and convex-concave lens. The line connecting the curvature centers on both sides is called the principal axis, and the point o in the center is called light. No matter where the beam parallel to the principal axis comes from, it will refract and then diverge in all directions. The extension line opposite to its divergence direction will all be at point F on the same side as the light source, and its refracted light will just emanate from point F, which is called virtual focus. There is one on each side of the lens. The focal length of concave lens is also called divergent lens, which refers to the distance from the focal point to the center of the lens. For example, the larger the radius of curvature of a lens, the longer its focal length. Both sides have equal focal lengths. Focal length If you have seen "f =" in the English manual of the camera, then the number behind it is usually its focal length, that is, the focal length. For example, "f = 8-24mm, 38- 1 15 mm (equivalent to 35 mm)" means that the focal length of this camera is 8-24mm, and the oblique viewing angle is equivalent to the focal length of 38-15 mm of a traditional 35mm camera. Generally speaking, the focal length of the standard lens of a 35mm camera is about 28-70mm, so if the focal length is higher than 70mm, it means that telephoto effect is supported, and if it is lower than 28mm, it means that it has wide-angle shooting ability. "Focal range" is the extension of focal length, which is usually divided into general shooting distance and close-up distance. The general shooting distance of a camera is usually marked as "from a certain centimeter to infinity", and advanced products often provide macro function to make up for the inability to focus in general shooting mode. Some cameras emphasize the magical ability of supporting 1 cm close-up and are suitable for shooting fine objects. The lens of a camera is a set of lenses. When the light parallel to the main optical axis passes through the lens, it will converge to one point. The lens with adjustable focal length is a zoom lens. (When a beam of parallel light passes through the convex lens with its principal 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 the convex lens. ) optical center: the center of the convex lens can be approximately regarded as the optical center. The lens of our camera is equivalent to a convex lens. The film (or the photosensitive device of digital camera) is located near the focal point of the convex lens, or the distance between the film and the optical center of the convex lens is about equal to the focal length of the convex lens. A convex lens with focal length can be imaged. Generally, a convex lens is used as the lens of a camera, and the clearest image it forms will not fall on the focal point, or the distance from the clearest image to the optical center (image distance) is generally not equal to the focal length. But slightly larger than the focal length. The specific distance is related to the distance between the subject and the lens (object distance). The greater the object distance, the smaller the image distance (but actually always greater than the focal length). Because when we take pictures, the distance between the object and the camera (lens) is not always the same, such as taking pictures of people. Sometimes we have to shoot the whole body, we will be far away, and we will be very close when we take a bust.