How does photography project?

The projection formed by light scattering from a point is called central projection. The projection lines of the central projection intersect at a point. A point light source illuminates a figure on a plane. The shadow of this figure is its central projection on this plane, and this plane is the projection plane. Every ray is a projection line. After the center projection, the space figure becomes a straight line, but parallel lines may become intersecting straight lines. Although the figure after central projection changes more than the original figure, it is intuitive and looks like the original object. Therefore, this method is often used in painting, but the central projection principle is rarely used in solid geometry. If the plane of a plane figure is parallel to the projection plane, the figure obtained after central projection is also parallel to the original figure, and the figure obtained after central projection is similar to the original figure. A unique photographic method of infrared photographic film is to use infrared as light source, and the photographic film used is sensitized by absorbing cyanine dye with infrared wavelength. The infrared spectrum for photography is in the range of 700 ~ 1300 nm. When shooting, a dark red to infrared black filter is placed in front of the lens to filter out the ultraviolet and visible light in the sun, or an infrared light source is used to illuminate the subject, and the photosensitive film is only exposed to infrared light and a small amount of red light, and then a positive film is obtained through general development processing. Because infrared rays can penetrate smoke and mist, clear scenery images can be obtained in distant view and high altitude photography, and TV is mostly used to simulate night scenes under the sun. According to the test, the infrared ray in natural light radiation can reach more than 40%. In black-and-white photography, special filters can be used to block visible light from red to dark red, so that infrared images can be sensitized on film. For example, the "Leiden 87C" filter can block almost all visible light and produce pure infrared photography effect. This filter is almost opaque to the naked eye, so you should adjust the focal length before adding the filter, and you need to shoot to get normal exposure. Generally speaking, "Leiden 25" red mirror can be used for infrared photography and can also produce obvious infrared effect. So what is the reason for infrared photography? The brightest objects seen by human eyes, such as blue water and sky, can't reflect more infrared light, so that although imaging in ordinary black and white film is normal, the color of infrared film is dark. The reason why trees and grasslands turn pale is that chlorophyll can reflect a lot of infrared rays, thus achieving an unrealistic artistic conception. Infrared ray not only provides a special creative technique for photography, but also can take clear photos in fog and smoke due to its high transmittance. It is often used in exploration and military reconnaissance in scientific research. Using the characteristics of infrared rays penetrating the surface of the picture and pigment, we can also judge the authenticity of the famous paintings of masters. Infrared ray and camera autofocus (AF) In the AF function of the camera, infrared ray is used in the design of active focusing (another automatic ranging commonly used in SLR cameras is called passive focusing). The principle of infrared ranging is that when we press the shutter half-way, the infrared emitted by the camera is projected on the subject and then reflected back. Different infrared receiving points on the camera generate different voltages according to the distance of the target, and then the AF receiver converts the voltages into electrical signals to drive the lens to adjust the focal length. The "R" mark on the lens is engraved with the red "R" or red dot on most camera lenses, which is the infrared photography with the infrared focus mark. After the wavelength of infrared light passes through the camera lens, the refractive index is smaller than that of visible light, which makes the convergence point deviate from the normal point. How to bring it back to the normal focus? The operation method is to adjust the focus according to the normal situation, and then turn the focus line forward slightly to aim at "R". If the subject is far away, it can be ignored. Of course, manual focusing camera is more suitable for infrared photography. Edit the basic principles of digital infrared photography in this section. The basic principle of infrared photography is actually the same as general photography. They all use light to shine on objects and reflect through the lens to form images in the camera. The only difference is between visible light and infrared light. Sunlight contains rich and varied light, and of course it also contains infrared light. When sunlight is reflected from an object to our eyes and cameras, we have actually received infrared light, but our optic nerve does not feel it. The principle of digital infrared photography is that the camera actually receives infrared light, which is not sensed by ordinary negative films. Traditional infrared photography relies on special negative film for infrared photography, while CCD or CMOS used in digital cameras can sense infrared light itself, but in general, infrared effect can't be seen because the amount of visible light is much larger than infrared light (another reason is that many DCs are equipped with IR cut filters). The function of infrared filter is to block visible light and let infrared light pass through smoothly, except infrared cut-off filter, which is used to block infrared light. After the infrared filter is installed in front of the lens, the negative or CCD can only see infrared light, which is the simple principle of infrared photography. Advantages of digital infrared photography Compared with traditional infrared photography, the advantages of digital infrared photography are that traditional infrared films are expensive, troublesome to store (need to be refrigerated), and have to be developed in specific places. Digital camera can see the results on the spot, which is helpful to correct the parameters and can meet the interactive application of general infrared photography without two cameras! Generally speaking, the CCD of most digital cameras is sensitive to infrared light, but some manufacturers have added infrared cut-off filters in front of the infrared CCD in cameras to block infrared light, which makes these cameras less sensitive to infrared light. There is a simple way to test whether the camera is suitable for infrared photography. Hold the infrared remote control of general household appliances and press the button of the remote control for the camera lens. At the same time, see if you can see the bright spot from the LCD or EVF of the camera. If the spot of the infrared light on the remote control is clearly visible, that is good news. (The result of shooting the remote controller with Olympus C-2 100UZ, taken by Steve Tang) Choosing an infrared filter The next step is to choose the appropriate infrared filter. The wavelength range of visible light is about 400nm-700nm, and the near-infrared light that CCD can induce is more than 700nm and less than 1200nm. Hoya R72 or B+W 092 is just qualified at 720nm for 50% cutoff of infrared photography, but R72 can still let some visible light pass. Better choices such as Tifeen 87 and B+W 093 are purer but more expensive. Experience shows that the luminosity decreases a lot after adding infrared filter. My experience is to set the sensitivity of Olympus C-2 100UZ to 100. Hoya R72 is generally used in sunny days. Even if the aperture is opened to the maximum (f2.8), the shutter is often 1/30 seconds or slower. (This situation seems to indicate that C-2 100UZ also uses an IR cut filter, which blocks some infrared rays). If the camera doesn't have hand shock prevention function, it often needs a tripod! At this time, the shortcomings of R72 become advantages, because a little visible light can make up for the lack of light. With B+W 093 with opaque visible light, the situation of insufficient light is even more serious! Infrared photography is a great boon for night owls, because the most obvious opportunity of infrared radiation energy is in the afternoon, because the whole environment absorbs a lot of heat energy after a morning of sun exposure, so you can shoot beautiful light without getting up early in the morning. Friends who want to shoot infrared rays can rest assured to sleep in the sun and bask in the ass! Because the more sunlight, the stronger the infrared radiation energy, the more obvious the effect! Editing this paragraph about perspective infrared photography often reminds people of perspective. The so-called perspective is due to the effect that infrared rays can pass through some infrared photographic substances that are not very transparent to visible light. The premise is that there is enough infrared light, the outer layer can penetrate, and the infrared filter filters out all visible light interference. For example, the best example is the dark protective film on the infrared remote control, with a filter in the sun to see the transparency. This is the truth of some well-known 0 Lux NightShot Camcoder perspective photography. Perspective photography not only requires very high filters, but also has the effect of magnifying weak light by camcoder itself. Digital cameras do not have the above-mentioned brightness enhancement function, and the perspective effect is very limited.