Basic knowledge of photography: lens principle

Basic knowledge of photography: lens principle

After pressing the shutter for several years and taking so many photos, looking back, I still have to ask myself, have you really mastered the principle of photography? Do you really know the lens in your hand? Has it really become a good helper for your creation? You should know that the theoretical basis determines the superstructure, and a thorough understanding of the basic knowledge of photography is helpful to give full play to the value of the equipment in your hand. Below, I will give you a systematic introduction to the principle of lens. After reading it patiently, you will certainly gain something.

From pinhole to lens

The reflection and divergence of light are sometimes like light and sometimes like waves. Light emitted or reflected from all points in any scene is scattered in all directions. The formation of an image does not need all the light reflected by a point or an object, but needs to choose from it.

When the light reflected by an object meets an obstacle with a small pinhole, as shown in the figure below, most of the light at each point will be deflected when it meets an obstacle. A small amount of light reflected from an object passes through a pinhole and propagates in a straight line. When they reach a plane (like the film on the back of a camera), an image will be formed. The image will be rotated upside down. Everything at the top of the subject appears at the bottom of the image, and everything at the bottom appears at the top of the image. Similarly, what was originally on the left appeared on the right, and what was originally on the right appeared on the left.

In order to achieve this picture effect, the lens of the camera is replaced by a thin metal disk pierced by tiny pinholes, which is equivalent to the aperture of f/ 182. The light comes from the main body, and only a small amount of light passes through the tiny opening at each point, forming a soft and relatively clear photo. Because the pinhole size is small, the exposure time reaches 6 seconds.

The disadvantage of using pinhole as lens is that the opening is very small. It only allows a very small amount of light to pass through, so it takes a very long exposure time to form an image on the film or sensor. If the pinhole is enlarged, the exposure time will be shorter, but at the same time the image will become unclear.

When the pinhole size increases to f/65, the result is only 1/5 seconds of exposure, but the image will be extremely out of focus. Larger holes allow more light from points on the body to pass through. These rays will spread widely before reaching the photosensitive surface, so that the apertures overlap each other and form a very unclear image.

Although the pinhole is small, it can still let small clusters of light pass through. These rays are generated from slightly different angles and continue to propagate in slightly different directions through small holes. Because they are divergent, when they touch a surface (such as film), the light from a certain point of the subject will form a tiny, fuzzy circle instead of a point. With the increase of hole size, a larger beam passes through the film or sensor, forming a larger and more blurred circle. These circles overlap each other, making each circle bigger and the picture more blurred.

The lens will form a clear image through a relatively short exposure. In order to get a clear picture, the image of a point should also be a point. But the pinhole on the camera can't achieve this. Even the smallest opening allows a small amount of light to pass through, and a long exposure will not form a very clear image. In order to let more light pass through and form a clear picture, we need a method different from making the smallest pinhole. This is what the lens provides us.

This time, with a simple convex lens with an aperture of f/ 16, the picture is clearer than with a smaller pinhole, and the exposure time is much shorter, only1100 second. When the lens is opened, it is much larger than the pinhole, allowing more light to pass through. It can accurately focus the light emitted by each point of the subject, thus making the photo look very clear.

A compound lens usually consists of six or eight individual lenses. Each lens is added to correct some aberrations (focusing defects). The six element of that lens are divide into four groups. Each element is a simple lens with two curved sides or one curved side and one smooth side. Each group can be composed of a single element or two or more elements bonded together.

The lens aberration can be predicted mathematically, and some obvious defects can be removed digitally after the photo is formed, but using a well-designed lens is always a good start.

Most photographic lenses use convex lenses. The convex lens is thick in the middle and thin in the edge. It collects a lot of light from any point on the object in front of the lens, and after refraction or bending, it makes them close to each other, and the final gathering is in the corresponding single-point picture behind the lens. Each point (object point) corresponds to a point (image point) behind the lens. The film in the camera is flattened (or has a flat digital sensor) on the image plane (sometimes called the film plane). All points on the plane of the corresponding object in the scene will converge on the surface, so the image taken on the plane will become clear.

The schematic diagram of light passing through four glass blocks explains the working principle of refraction well. The light enters from the left, passes through the first glass block (1) and travels in a straight line. The next glass block (2) is placed at an angle so that the light rays are bent, but when they are emitted from the glass block, they are bent again to return to the previous direction. The concave surface of the third block (3) disperses the light, but the last glass (4) is a convex lens, similar to the basic condenser lens in a camera, which gathers the light together again and makes them converge on the focus.

How does a lens form an image by refracting (bending) light? When light enters different transparent media, such as water or glass, it will be refracted or bent. A common example of refraction is observed when observing a spoon that is half immersed in a glass of water: the light reflected by the spoon will be bent by water and glass, so the part of the spoon that is immersed in water will be partially displaced.

In order to be refracted, light must be injected into a new medium at a certain angle. If light is incident perpendicular to the surface, it will pass straight when entering and leaving the medium. But if they enter or leave at an oblique angle, the light will bend at a predictable angle. The greater the incident angle, the greater the bending degree.

When light shines on a transparent medium with a curved surface, such as a lens. The light will be emitted according to the incident angle on the lens surface.

The difference is that it is bent at many different angles. They will disperse and diffuse through the concave surface and converge and approach through the convex surface. Light comes from a point on an object, bends to the other side through a convex lens (the simplest form of camera lens), and finally focuses on the image point.

focal distance

The biggest difference between lenses is the focal length. Because you are most likely to use a camera with interchangeable lenses, you can also choose to buy or use one or several lenses. A lens is usually described by its focal length (50 mm lens and 0.3 m lens) or its relative focal length (normal, long or short). Technically, the focal length is the distance from the rear node of the lens to the focal plane when the lens is focused at infinity. Theoretically, infinity is an unmeasurable distance that goes beyond the edge of the universe. In photography, infinity is the distance that parallel light enters the lens. The lens designer calls the image point on the back of the lens where the light converges as the focus.

The focal length controls the magnification and size of the image formed by the lens. A longer lens will enlarge the size of the object projected in the film or sensor image, and a shorter lens will bend the light greatly. The light is focused at a close distance behind the lens to form a small image of the object.

A long focal length lens has less light bending than a short focal length lens. The longer the focal length, the smaller the curvature of the light, and the farther away from the lens, the greater the imaging. The size of the image is proportional to the size of the focal length. If the distance between the subject and the lens remains the same, the image formed by a 50 mm lens will be twice that of a 25 mm lens.

The focal length also controls the viewing angle and the number of pictures displayed on a sensor or film of a given size. Compared with short focal length lens, long focal length lens images larger. Therefore, on a sensor or film of a given size, a lens with a longer focal length takes fewer photos. If you draw a circle with your thumb and forefinger, close to your eyes, you will see most of the pictures in front of you, similar to short shots. If you take away the circle formed by your hands, the effect will be similar to that of a telephoto lens, and there will be fewer pictures in the circle, and the viewing angle from the circle formed by your fingers will become smaller. Similarly, the longer the focal length, the smaller or narrower the angle of view you see from the lens.

Increasing the focal length while keeping the distance from the lens to the subject unchanged has the effects of enlarging the image, increasing the magnification and reducing the viewing angle. Because the photographer's position remains the same, the size ratio of the subject in the scene remains the same. The picture below shows that some of them can be used in 35mm or? Quan Huafu? Viewing angles of lenses with different focal lengths in digital cameras. Is the focal length of this lens what other beginners of digital cameras often say? 35mm equivalent? .

Lens focal length? Normal focal length

Normal focal length lens is also called standard focal length lens, which is roughly the same as the viewing angle of human eyes. One of the greatest photographers in modern times: Henri Cartier Bresson described the camera as? The expansion of my eyes? He often uses ordinary focal length lenses. His photos contain images as if you had observed them on the spot. His works see through nature, and the distance is in harmony with the size of the nearby objects.

A normal focal length lens on one camera can be used as a long focal length lens or a short focal length lens of another camera. The size of the sensor or film determines the normal focal length of the camera. The larger the size, the longer the normal focal length of the specification; It roughly conforms to the diagonal length of the sensor surface or each frame of film.

Cameras using 35mm film use a 50mm lens as the normal focal length lens, which is also 24? Normal focal length lens of 36mm (Quan Huafu) digital sensor. Because it used 4? For a 5-inch film camera, the focal length of the 150mm lens is normal. Most digital cameras have sensors smaller than 35mm film frames, so their normal lens focal length is less than 50mm. Of course, these rules are flexible. For example, a 40 mm to 58 mm lens can be used as a normal focal length lens for a 35mm camera.

Lenses with normal focal length have some advantages over lenses with longer or shorter focal length. Most normal focal length lenses are faster, that is, they can open the aperture wider at the same time, so they can shoot at a faster shutter speed or in dim light. They tend to be cheaper, more compact and lighter.

The choice of focal length is a personal preference. Some photographers habitually use short focal length lenses because most of the time they want a wide angle of view; Others prefer a lens with a longer focal length, which highlights the main object in the scene by narrowing the viewing angle. If you are not sure about your preference, start with a normal focal length lens.

A lens with a given focal length may be ordinary, short or long, depending on the size of the film you are using or the sensor of your digital camera. If the focal length of a lens is equal to the diagonal length of the light-sensitive surface (shown by the dotted line in the figure), we can think that this is a? Normal? The focus lens is about 50? Collect light from a perspective.

Is a 4 used on the right? 5-inch camera, 150mm lens. 4? The diagonal length of 5-inch film is about 150mm, so the lens of 150mm is the normal focal length lens of this size film.

However, 150mm is longer than the diagonal of a 35mm film frame or a sensor of the same size (called Quan Huafu). 150mm lens is a long focal length lens for this kind of camera, because most digital sensors are smaller than 35mm film frames, while 150mm is very long for most digital single-lens reflex cameras.

Using a lens with normal focal length is a way for photographers to express their subjective feelings. Wide-angle lens and telephoto lens emphasize the photographer's technique, which involves how to choose the perspective relationship and depth of field.

Although Cartier Bresson is opposed to discussing technology, it is well known that he can perfectly use a normal focal length (50 mm) lens on his 35mm Leica camera. We are attracted by the pictures that photographers want us to pay attention to, and we are shocked by the poetry of time itself captured in an instant. Cartier Bresson called it? A decisive moment? .

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