How to choose digital camera lens

Lead: the picture taken by the camera. Xue Muqiao's several questions about China's modernization road: State-owned photography shops such as Tiananmen Square and Beihai Park only take one shot, and cooperative photography teams can freely choose the shot. ? Professional explanation: the lens referred to in film and television is not a physical or optical lens, but a lens that carries images and can form a picture.

How to choose the focal length of digital camera lens 1

The focal length is another key factor of the lens. First, understand what focal length is. The lens of a digital camera can be regarded as a set of lenses. When the parallel light passes through the lens, it will converge to a point, which is called the focal point, and 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 a fixed-focus lens; The lens whose focal length can be adjusted and changed is a zoom lens. In the field of photography, the focal length mainly reflects the lens angle. For the traditional 135 camera, a lens with a viewing angle of about 50mm is close to the human eye and does not deform when shooting, which is called a standard lens. Generally covering the range of 40mm-70mm. A lens with 18mm-40mm is called a short-focus lens or a wide-angle lens, a lens with 70mm- 135mm is called a medium-focus lens, a lens with 135mm-500mm is called a telephoto lens, a lens with more than 500mm is called a telescope head, and a lens with less than 18mm is called a super-wide-angle lens or a fisheye. This division of scope is only people's habit, and there is no strict definition. The CCD of a digital camera is generally much smaller than that of 135 film, so the focal length of the lens is much shorter at the same viewing angle. For example, for a digital camera using a 0.33”CCD, the lens used is about 13 mm, and the viewing angle is roughly equivalent to the standard lens of a 50 mm 135 camera ... Because the CCD specifications used by digital camera manufacturers are different, everyone adopts it. Equivalent to the focal length of a 35mm camera (i.e. 135 camera)? Statement.

Usually, there are F and F in the technical parameters of digital cameras, F stands for the maximum aperture value and F stands for the focal length. For example, mark f=8-24, which is equivalent to 38mm- 1 15mm of a 35mm camera.

zoom

Zooming ability is also the focus of digital camera lens design. The zoom of digital cameras is generally divided into optical zoom and digital zoom. Optical zoom, like the traditional optical imaging camera, realizes zoom through the telescopic combination of lenses, which is also a real zoom.

However, the optical zoom lens is very expensive and complicated. Complex zoom lenses are generally too large for miniaturized digital cameras. So at present, the optical zoom of mainstream digital cameras on the market is mostly about 3 times.

Digital zoom can be said to be exclusive to digital cameras and a brand-new concept. Its basic principle is to interpolate the shot scene data through the arithmetic unit in the digital camera, so as to enlarge the object and achieve the zoom effect. It seems that this zooming method can present more details, but in fact it is only a simple enlargement of the original image, and it will not increase the clarity of the image. Because the extra pixels are actually not taken in by the lens, but are calculated by software interpolation. The digital zoom of digital cameras is now 2 times, 3 times or 4 times common in the market. Is it marked? This digital camera has 3x optical zoom and 2x digital zoom? It can be seen that they are important indicators of digital camera lens performance. It seems that in order to attract more attention, digital camera manufacturers often emphasize that the zoom ability of their products can reach 6 times, and some even claim that they can reach 12 times. This is actually the result of multiplication of optical zoom and digital zoom. For example, a digital camera with 6-fold zoom capability actually realizes the so-called 6-fold zoom of 3-fold optical zoom and 2-fold digital zoom. 12 times zoom capability is equivalent to 3 times optical zoom and 4 times digital zoom. The picture can indeed be enlarged by 6 times or 12 times, but the actual definition of the image still depends on the optical zoom ability of 3 times.

3. Shutter and aperture

In photography, the original quality of the captured image comes from the control of exposure, which is affected by both aperture and shutter. Exposure is the process of letting light shine on photosensitive film or film and capturing images. Exposure is the time integral value of illumination on the photosensitive surface of photographic photosensitive materials. If overexposed, it will make the shot image bright and lose many details and colors; However, if the exposure is insufficient, the tone of the photo will be very dark, and the captured scene image will still be unclear.

Digital cameras, like traditional cameras, project the captured images onto the CCD photoreceptor of digital cameras through the lens, and the photoreceptor records the image information on the camera's memory card through the digital-to-analog converter. This process is the same as the exposure process of a traditional camera. In order to obtain rich images, it is necessary to control the amount of light projected on the CCD photoreceptor. Aperture and shutter are used to control exposure.

1) shutter

Shutter is a device used to adjust and control the time when light passes through the lens and reaches the photosensitive surface. Generally speaking, the shutter determines the time to take an image. It blocks in front of the photosensitive element and is usually closed. It will only open when the shutter button is pressed, and its opening time is determined according to the set shutter speed. The shutter speed range of a general camera is: 4 seconds, 2 seconds, 1 second, 1/2 seconds, 1/4 seconds, 1/8 seconds,1/5 seconds,/kloc-0.

If the camera shakes due to instability during the opening of the shutter, the captured image will become blurred. This is why the camera should be stable when shooting, and why it is easier to shake when the shutter speed is too slow. So there is a concept of safe shutter speed. Shutter speed generally cannot be slower than safety shutter: safety shutter = 1/ lens focal length.

For example, the focal length of the lens is 50 mm, and the safety shutter is 1/50 seconds, that is, the shutter speed above 1/60 seconds can avoid the image blur caused by hand shock. Of course, the safety shutter is only a reference parameter. The larger the shutter speed range, the better, so that the camera can adapt to more shooting ranges.

2) Aperture

Aperture is a mechanical device used to control the amount of light entering the photosensitive surface of the fuselage through the lens. Usually in the lens, this effect is achieved by controlling the size of the lens aperture. When the external light is weak, the aperture becomes larger; Otherwise, turn down the aperture. The expression of aperture size is usually expressed by f value. The formula is: f value of aperture-= lens focal length/lens aperture diameter.

The complete aperture value is F 1 from small to large. F 1.4, F2.8, F4, F5.6, F8, F 1, F 16, F22, F32, F44 and F64.

The smaller the f value of the aperture, the larger the aperture will be, and the more light will enter in the same unit time. And the amount of light entering 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 be doubled, so it can be said that the aperture is opened by one level. For mass digital cameras, the aperture f value is often between F2.8 and F 16. In addition, many digital cameras can adjust the aperture by 1/3.

The larger the aperture range of the camera, the stronger the shooting ability to adapt to different light environments. But in the aperture range, we usually pay more attention to the maximum aperture value and less attention to the small aperture value. This is because if the shooting environment is too bright, besides closing the aperture, we can also speed up the shutter speed and shorten the exposure time. However, if the light in the shooting environment is too weak, you can't slow down the shutter speed at will, because once the shutter speed is slower than the safe shutter speed, the camera will easily blur the image due to hand jitter. For example, if the focal length of a camera is 50, then its safe shutter speed is 1/50 seconds. If the shutter speed is1125 seconds in good light, the aperture value of the camera is F8. However, if the light is weak, it is necessary to extend the shooting time in order to keep the exposure and ensure the quality of the shot image. The aperture value of the corresponding camera should also be adjusted from F8 to F 1 1. At this time, it is necessary to test the light transmission performance of the lens. Therefore, the larger the aperture value of the camera, the better the transmittance of the lens.

It can be seen that shutter and aperture directly affect exposure, and the characteristics they can achieve directly reflect the level and characteristics of lens manufacturing, which is an important factor to measure lens quality. It is also an important parameter index to measure digital cameras.