Poco photography tribe

Aperture: Aperture is a device used to control the amount of light entering the photosensitive surface of the fuselage through the lens, usually inside the lens. We use the f value to represent the aperture size. For the manufactured lens, we can't change the diameter of the lens at will, but we can control the luminous flux of the lens by adding a polygon or circular aperture grating with variable area inside the lens. This device is called an aperture.

Aperture f value = lens focal length/lens aperture diameter

As can be seen from the above formula, to achieve the same aperture f value, the aperture of a long focal length lens is larger than that of a short focal length lens. The complete series of aperture values is as follows:

f 1，f 1.4，f2，f2.8，f4，f5.6，f8，f 1 1，f 16，f22，f32，f44，f64

It is worth mentioning here that the smaller the f value of the aperture, the more light enters in the same unit time, and the light entering amount of the upper level is exactly twice that of the lower level. For example, if the aperture is adjusted from f8 to f5.6, the amount of light entering will double. We also say that the aperture is enlarged by one level. For consumer digital cameras, the aperture f value is often between f2.8-f 16. In addition, many digital cameras can adjust the aperture by 1/3.

The English name of Aperture is aperture. Aperture is a device used to control the amount of light entering the photosensitive surface of the fuselage through the lens. It is also an extremely important index parameter of the camera, usually in the lens. Its size determines how much light enters the photosensitive element through the lens. We use the f value to represent the aperture size. Aperture f value = lens focal length/lens aperture diameter According to the above formula, to achieve the same aperture f value, the aperture of a long focal length lens is larger than that of a short focal length lens.

Aperture and shutter take precedence.

In addition to providing automatic mode, advanced digital cameras usually have two options: aperture priority and shutter priority, so that you can decide a certain aperture value or a certain shutter value on some occasions, and then match the appropriate shutter or aperture respectively to present different depth of field (sharpness) or effects.

Aperture priority exposure mode

After we decide the aperture f value ourselves, the camera metering system will automatically choose the appropriate shutter speed (which can be an accurate stepless shutter speed) according to the light conditions at that time. A digital camera with an exposure mode dial usually engraves the letter "A" on the dial to indicate the aperture priority mode (see Figure 4). Aperture prerequisite mode is suitable for photography that emphasizes the depth of field effect.

Because the focal length of digital camera is much shorter than that of traditional camera and the aperture of lens is small, it is difficult to produce narrow depth of field. Some digital cameras have a special portrait exposure mode, using built-in programs to blur the foreground and background.

Aperture type

Fixed aperture. The simplest camera only has a fixed aperture with a round hole.

Watthauser aperture. The initial variable aperture is just a series of circular holes with different sizes, arranged around a disk with a central axis; Rotating the disc can move the circular hole with appropriate size to the optical axis, thus controlling the aperture. John Vaught Hauser invented this kind of aperture in the middle of19th century.

The cat's eye aperture consists of a metal plate with an oval or diamond hole in the middle, which is divided into two parts. Two metal sheets with semi-elliptical or semi-diamond holes are arranged in opposite rows and moved relative to each other to form a cat's eye aperture. Cat's eye aperture is mostly used for simple cameras.

Iris-type aperture is composed of a plurality of overlapping arc-shaped thin metal blades, and the size of the central circular hole can be changed by the clutch of the blades. Some cameras can change the aperture of the diaphragm by rotating the ring on the lens barrel, while others automatically change the aperture of the diaphragm by controlling the micro-motor through the microprocessor chip. There can be as many as eighteen arc-shaped thin metal blades. The more arc-shaped thin metal blades, the closer the aperture is to a circle. By designing the shape of thin metal blades by computer, a nearly circular aperture can be obtained with only 7 thin metal blades.

Instantaneous aperture

The aperture of a SLR camera is instantaneous, and only when the shutter is opened will the aperture be reduced to a predetermined size. Usually the aperture is at the maximum position.

Double shutter aperture

The aperture of some simple cameras has shutter function. Most of these double shutter apertures are cat's eye apertures with two blades. Different from the simple cat's eye aperture, the double shutter aperture is usually completely closed: at the moment when the shutter is pressed, the double shutter aperture opens to a predetermined aperture, and then it is closed immediately after maintaining the aperture for a predetermined shutter opening time; In this way, the aperture also acts as a shutter.

aperture size

The smaller the value after f, the larger the aperture. The function of the aperture is to determine the amount of light entering the lens. The larger the aperture value, the more light enters. On the contrary, the smaller. Simply put, under the condition of constant shutter, the larger the aperture, the more light enters and the brighter the picture; The smaller the aperture, the darker the picture.

Focal length: the speed we discussed earlier is only one characteristic of all lenses, and the second characteristic of lenses is focal length. The focal length of the lens is basically the distance from the center of the lens to the clear image formed on the film plane.

The focal length of the lens determines the size of the image formed by the object photographed by the lens on the film. Assuming that the same subject is photographed at the same distance, the longer the focal length of the lens, the larger the image formed by the subject on the film.

For example, on the same camera, the height and width of an image shot with a 100mm lens are twice as high as those shot with a 50 mm lens. The height and width of the image produced by a 400 mm lens are four times that of a 100 mm lens, and so on.

Fixed-focus lenses (relative to zoom lenses to be introduced later) all have a certain focal length, which is determined by their optical systems. To be exact, the measurement of the distance from the center of the lens to infinity of a clear image projected on the film plane determines the length of the focal length. The infinity mentioned here refers to the distance setting when the lens focuses on a distant object (such as the horizon).

The focal length of a lens can be measured in inches, centimeters or millimeters. In this course, we use millimeters (mm) as the unit of lens focal length. 25 mm is approximately equal to 1 inch. So a 50 mm lens is about a 2-inch lens, and a 100 mm lens is similar to a 4-inch lens. As shown in Figure 2.33, generally speaking, the longer the focal length, the longer the lens barrel.

Telefocus refers to: a long focal length lens refers to a photographic lens that is longer than the focal length of a standard lens. Telefocal lenses are divided into ordinary telephoto lenses and ultra-telephoto lenses. The focal length of ordinary telephoto lens is close to that of standard lens, while the focal length of ultra telephoto lens is much larger than that of standard lens. Take 135 camera as an example, the photographic lens with a focal length of 85 mm-300 mm is a normal telephoto lens, and the lens with a focal length of more than 300 mm is a super telephoto lens.

According to the structure of optical group, telephoto lens can be divided into ordinary telephoto lens, inverted telephoto lens and reflective telephoto lens.

Shooting with a long focal length lens has the following characteristics:

First, the angle of view is small. Therefore, the spatial range of the shooting scene is also small, and the shot image is larger than the standard lens at the same shooting distance, which is suitable for shooting the details of the distant scene and shooting the subjects that are not easy to get close to.

Second, the depth of field is short. So you can highlight the subject in a messy environment. However, it brings some difficulties to accurate focus adjustment. If the focus is slightly inaccurate when shooting, the subject will be blurred.

Third, the perspective effect is poor. This kind of lens has obvious characteristics of compressing spatial depth and exaggerating background.

When using a long focal length lens, it is usually used to shoot distant scenes. Due to the absorption of air and the influence of diffused light. Therefore, it is not easy to achieve very accurate focusing because of the small contrast of the captured image and the serious extinction of dust particles. When shooting with a telephoto lens above 300 mm, it is still difficult to gather all colors of light at one point, resulting in sub-spectral problems. Based on the above situation, sometimes the image quality of the shot works is not high.

Although the use of long focal length lens is influenced by many factors, its advantages are the main ones. Take long-distance photos of scenic figures, travel photos and portraits (when taking standard photos for affixing certificates, use a common telephoto lens of about 100mm to reduce image distortion, and use a lens of 105mm for the best effect), take photos of stands and sports competitions, take photos of zoo animals, take photos of wild animals, and take close-up photos with auxiliary mirrors.

When shooting with a long focal length lens, fast photographic film and fast shutter are generally used. If shooting with a 200m long focal length lens, the shutter speed should be above 1/250 seconds to prevent camera shake when shooting with a hand-held camera. Generally speaking, in order to keep the camera stable, it is best to fix the camera on a tripod.

Generally, the telephoto lens is only used in special circumstances, and the camera cannot be fixed with an ordinary tripod, otherwise the overweight lens will damage the camera base fixed on the tripod. Therefore, it is necessary to use a tripod that can support the lens holder to fix the camera. When there is no such special tripod, it can be fixed with two legs, one leg to fix the camera body and the other leg to support the telephoto lens. It is best to use a film faster than ISO400/27 degrees to ensure sufficient shutter speed.

Medium focus: Before introducing the wide-angle camera, first understand the focal length of the camera. In fact, when people talk about the focal length of a digital camera, they are not talking about the actual focal length of the digital camera, but about the equivalent focal length, that is, the focal length relative to the traditional 135 camera. From the principle of photography, the smaller the focal length, the wider the field of vision, and the wider the range of scenery that the photo can accommodate; The larger the focal length, the narrower the field of view, which means that you can shoot distant objects.

Wide-angle: In traditional cameras, wide-angle lenses over 28mm are very popular. However, due to the particularity of small photosensitive devices in ordinary digital cameras, the physical focal length of the lens needs to be very short to achieve a wide angle, which leads to high requirements for aberration correction and anti-glare coating. With people paying more and more attention to wide-angle shooting, now around 3000 yuan, cheap wide-angle digital cameras are becoming more and more popular. Ricoh is the originator of a portable wide-angle digital camera with high cost performance. From G3/G4 wide angle to RX/GX series products, 28mm wide angle is its biggest selling point. In addition, Canon, Olympus, Konica Minolta and other cameras have also introduced 28mm wide-angle cameras.

For most popular digital cameras on the market, the wide-angle focal length is generally between 35 and 38mm. The real wide-angle digital camera is actually a product with a lens focal length covering a wide angle of 28mm. Because the wide-angle field of view of 28 mm is wider than that of 35mm and 38 mm commonly used in digital cameras, the wide-angle field of view of 28 mm is 76 degrees, while that of 35mm is only 62 degrees, so it can produce very unique visual effects and accommodate a wider scene. This is also the reason why consumers are more optimistic about 28mm wide-angle digital cameras. The biggest feature of wide angle is that it can shoot a wide range, with exaggerated sense of distance and wide focus range. When using wide angle, you can enlarge the object in front of you and narrow the object in the distance, and the surrounding images are easily distorted. Wide-angle can also adjust any point in the image to the most appropriate focal length to make the picture clearer, which can also be called complete autofocus.

Wide-angle digital camera has short focal length, wide viewing angle and deep scenery, which is suitable for taking photos of large scenes, such as architecture and scenery.