Why can't the back cover of the newly bought Nikon D90 lens be screwed off? Tell me, master, I'm afraid that brute force will break the lens.

Then you can choose 60 d. Canon's camera shoots better and the picture quality is softer. Nikon's camera takes a better view. Sony cameras need not be directly considered, and the shutter life is almost 10W. I suggest you choose 60D, hoping to help you. The novice will send you a paragraph, and you will use: several key words of the camera. ISO and quality ISO are words with high exposure. When we buy cookies in the supermarket, we may see that our company has passed the ISO900 1 quality system certification on the packaging. This ISO is the abbreviation of International Organization for Standards. The International Organization for Standardization (ISO) has formulated the standards for biscuit management and film production, so there are several films on the shelves, namely ISO100,200 and 400, which are films with different photosensitive speeds. ISO sensitivity is the sensitivity of CCD (or film) to light. If ISO 100 film is used, if the camera can correctly expose within 2 seconds, then under the same light conditions, it only takes 1 second to use ISO200 film and 0.5 second to use ISO400 film. In the digital age, there are ISO options in the main menu of digital cameras, such as 100, 200, 400 or 800, just like those on film. Depending on the model, from ISO50 to 25600, the higher the number, the more sensitive it is. Fast food is popular for lunch and love, so we should do everything quickly. It is reasonable that we like high sensitivity. However, there is no free lunch. Although high ISO is fast, the image is rough and cannot stand fine magnification. Therefore, landscape photography needs the lowest sensitivity of the camera to get a fine picture. High ISO is usually used only as a last resort. People can't help themselves in rivers and lakes, and sometimes they have to, so high ISO image quality is one of the most important indicators of digital cameras. In dark places, such as dark rooms and midnight streets, even if the aperture is opened to the maximum at ISO 100, the shutter speed needs 1/4 seconds or even slower to properly expose. At this time, it is impossible to stabilize the camera without a tripod, and the photo will be burnt when the hand is shaken; Even with a tripod, as long as the subject turns his head, the photo will be posted. Flash can save the emergency, but it will destroy the atmosphere at the scene and make people look unnatural. Moreover, the effective distance of the small flash in the camera will not exceed four meters, and people and scenery that are a little farther away can't be photographed. What's more, flashlights are not allowed in some places, such as museums and theaters. We have no choice but to improve the sensitivity of digital cameras to ISO800 or even 1600. Small digital DC and digital SLR DSLR, which are also/kloc-0.0 million pixels, will not have much difference in resolution and image quality if they shoot at the lowest sensitivity (such as ISO 100 or 80), assuming the lens quality is the same. However, if the ISO is raised to 400 for shooting, the difference in image quality will be obvious. The image taken by DSLR is still relatively clean, which is not much different from that taken by ISO 100, while the image quality of DC is obviously degraded, with great noise, distorted color and lost details. If we continue to upgrade to ISO800, the image quality of small digital DC can only be described in four words, while the image quality of digital SLR is acceptable although it has declined. If it is further upgraded to ISO 1600, the image quality of most digital SLR will be greatly reduced, but it can still meet the needs of enlarging 10 inch photos. At this time, the picture quality of the small digital DC is poor, and it takes a brave heart to dare to watch it. In the case of equal pixels, the larger the CCD area, the better the imaging quality of high ISO. That is to say, when the CCD area is fixed, adding more pixels inside will lead to the decline of image quality. Therefore, the current digital camera should not simply add a few million pixels to the 6.5438+million pixels, but should work hard to improve the quality of CCD. It is urgent to reduce the noise level of high sensitivity (high ISO) and increase the exposure latitude. Travel photography 8 million pixels is enough. When choosing a digital camera, we should not only look at the pixel height, but also pay attention to the size of the camera CCD. It's 2008, and the resolution is enough. It's time to pay attention to the picture quality. Shutter In those years when photography was just invented, it usually took several minutes to take a photo. Most cameras don't need a shutter. At the beginning of exposure, take off the lens cover, then look at the watch. After five minutes, cover it and the photo will be finished. Later, the photographic speed of film became faster and faster (ISO became higher and higher), and the exposure time became one minute, several seconds,110 second or even one hundredth of a second. It's not fast enough to hold the lens cover by hand at this time. We need something that can accurately control the exposure time, and this thing is the shutter. Shutters include mechanical shutter, electronic shutter and electromechanical combination shutter. Definition: Shutter is a device for controlling exposure time in camera. By the way, this is the safe shutter speed. When shooting with 135 camera, the hand-held camera has a safe speed principle: the safe speed is the reciprocal of the focal length. If a 35mm lens is used, the shutter speed should not be lower than 1/35s, and if a 200mm lens is used, the shutter speed should not be lower than 1/200s, otherwise it may burn the picture. The last article about aperture said that all cameras are based on the principle of pinhole imaging: take a sealed box, drill a small round hole on either side, and then point the punched side out of the window. The scene outside the window, such as a tree, will generate a reflection of the tree on the inner wall of the box opposite the round hole. If we coat the inner wall with photosensitive material (film or CCD), this perforated box is a complete camera. This is a pinhole camera. Since the camera doesn't need a lens, why isn't there a small round hole in front of the camera but a few pieces of glass now? And these glasses (lenses) are so expensive! This is because the pinhole must be very small to image, which is also the origin of the pinhole camera name. Not if the hole is as big as the door. So we don't have an imaging door. The light in Kong Xiao is very weak, so I am patient with pinhole photography. It is common for a photo to be exposed for several minutes to several hours. Moreover, due to the diffraction interference of light, the photos taken by pinhole camera are not clear enough, just like looking at flowers in the fog. No one wants to spend hours taking a blurred photo. We should try to increase the amount of light. Is there any way to enlarge this small hole and generate a clear image? People immediately thought of the condensing function of convex mirror. Put the glass convex mirror on the big hole and the problem will be solved. That's true. This is how the camera lens was born. Today, all kinds of lenses of digital cameras are arranged and combined with several concave-convex mirrors, and then wrapped in plastic or iron. With the lens, the pinhole imaging hole-that is, the aperture below-is no longer a pinhole, but a hole. Solved the problem of light input. But sometimes the problem comes again: we don't need to have big holes all the time. For example, under the scorching sun on the beach in summer, there are sparkling flowers everywhere. In order to distinguish between human flesh and white sand, we need to squint carefully The lens is the eye of the camera, and the camera needs to squint at this time. Obviously, in order to cope with different light intensity, we also need to equip the lens with a device that can adjust the size of this hole, so that it can be reduced to a pinhole in strong light and a big hole in weak light. This device is the aperture. The English name of Aperture is aperture. A set of concave-convex mirrors with aperture gives birth to a complete lens. Definition: Aperture is a device to adjust the size of the lens entrance hole. Common aperture values are as follows: F 1, F 1.4, F2, F2.8, F4, F5.6, F8, f1,F 16, F22, F32, F44. The optical input difference between adjacent aperture values of every two blocks is twice. For example, if the aperture is adjusted from F4 to F2.8, the amount of light entering will double; From F2.8 to F2, it has doubled. Perhaps you have seen that the aperture value is opposite to the actual aperture size. The aperture is F 1 at the maximum light input and F64 at the minimum. For 135 camera, the minimum aperture of most lenses is F22. Since the aperture can be large or small, when is the best imaging quality of the lens? According to the above picture, the aperture is almost the same as the pinhole at the minimum aperture F22, and the digital camera becomes a pinhole camera. As I said before, pinhole imaging is not much better. When the aperture is maximum, the small hole becomes a gate again, and the imaging is poor. Therefore, according to the traditional golden mean of the Chinese nation, please remember that the lens has the best imaging effect (the clearest picture) in the medium aperture. If it is a 135 digital SLR, the medium aperture is F8 or F 1 1. Small digital DC depends on the specific model. If the optional aperture value is between F2.5 and F8, F4.6 in the middle is the best aperture. Time flies, the aperture of the lens is the faucet, which controls the size of the water flow (the amount of light entering). For the lens, we certainly hope that the larger the aperture, the better. It's like a faucet at home. Although we never turn it on when brushing our teeth and washing our face, in case of a fire at home, we will turn the faucet on at once. I hate why we didn't install a bigger faucet in the first place. The maximum aperture of the lens is not conducive to imaging, and we usually use the maximum aperture less; However, when the light is particularly weak and there is no tripod, such as shooting a street documentary at night, we will definitely use the maximum aperture without hesitation and regret why we didn't buy a large aperture lens at the beginning. But the large aperture lens is expensive and the weight is amazing. For example, Canon70-200mm has two versions, with an aperture of F4, 5000 yuan and a weight of 700g. F2.8 has an aperture of more than 8,000 and weighs1.500g.. This is because the aperture is relatively large, the lens is much larger and the processing is difficult. Whether it is worthwhile to pay twice as much money and sweat for a large aperture is a matter of opinion. Exposure To clarify the word exposure, let's go back to pinhole imaging. Imagine a dark and closed room with a small round window on one wall and a photosensitive material (white asphalt, large film or CCD) on the inner wall opposite the window. This is a big room camera. The room was dark before the small window was opened. When we open the small window, the light enters through the small hole and hits the film on the opposite wall, resulting in photochemical reaction (or photoelectric reaction, if it is CCD), and the photo is born. This process is called exposure. In order to get correctly exposed photos, the exposure must be accurately determined. The so-called exposure is how much light is allowed to enter this closed room. If there is too much light, the photos will shine, and the night will become the day. If the amount of light is too small, the photo will turn black and white will turn black. Fortunately, we have two tools, aperture and shutter, to control exposure together. Exposure is a combination of aperture and shutter. It can be considered that the aperture (numerical value) is actually how big the small round window is, and the shutter (speed) is how long the window has been open. Assuming that the window is only open for 1/4 4 seconds, it is obvious that if the window is half open for 2 seconds, the film can be exposed correctly, because1/4 * 4 =1/2 * 2 =1,and the light input is the same. Similarly, if the window is fully open, the exposure time only needs 1 second. If the aperture of a lens is F4, and in photographic terms, the shutter speed of F4 is 1 sec, which is the correct exposure value, then F5.6 and 2 seconds, F8 and 4 seconds can also get accurate exposure pictures. Important conclusion: A correctly exposed photo can have n different combinations of aperture and shutter speed. To sum up the explanation of the above terms, there are three factors that can affect whether a picture is exposed correctly: aperture, shutter speed and ISO. Among them, aperture and speed jointly determine the amount of light entering, and ISO determines the photosensitive speed of CCD. If the light input is not enough, we can open the aperture or lower the shutter speed, if not, we can improve ISO. The disadvantage of large aperture is that the resolution is not as good as that of middle aperture. If the shutter speed is reduced, the picture may be pasted, and the quality of the picture will also decrease after the ISO is improved. Without a perfect plan, how to choose should be flexible. Focal light will converge to a point (focus) after passing through the lens. The focal length of lens is the distance from the center of lens (or lens group) to CCD, and the unit is millimeter (mm). For Quan Huafu 135 digital SLR camera and our commonly used 135 film camera (boxed film is used in supermarkets), the lens with a focal length of 50mm is called "standard lens", referred to as the title for short, and the photos taken are similar to those of the naked eye (the viewing angle is about 45). Strictly defined, a standard lens is a lens whose focal length is equal to the diagonal length of the negative (or CCD). The single 135 negative is 24x36mm, and its diagonal length is 43 mm according to Pythagorean theorem, so the title of 135 frame should be 43 mm. In practical application, we call all those with a focal length of 40-60mm titles. Early SLR cameras were all bundled with 50mm lenses, which may be the reason why they were called "standard lenses". Wide-angle lens (focal length less than 35mm) can make the camera "see wider" because of its large viewing angle; A telephoto lens (focal length greater than 70mm) allows the camera to "see farther", but the viewing angle is narrower. A telephoto lens is also called a telephoto lens or a telescopic lens. It can be inferred from the definition of focal length that the wide-angle lens is short, the telephoto lens is high and powerful. As long as you see those big, thick and long guys in the future, it goes without saying that it is a long-term concern. A lens with a fixed focal length is a fixed-focus lens. Before 1960, zoom basically depends on walking. After 1965, zoom lenses with adjustable focal length began to appear in the market in large numbers. The advantages of zoom lens are obvious. You don't have to go to change the focal length, just turn the lens barrel. However, zoom requires a complex optical system (its internal structure is mostly more than a dozen lenses), which brings two problems to the zoom lens: 1, which is bulky and heavy; 2. Imaging is often not as clear as the best fixed-focus lens. Optical Zoom and Digital Zoom We often see advertisements for digital cameras that say XX times optical zoom. Zoom multiple here = maximum focal length value/minimum focal length value. The optical zoom factor of the 28-280mm zoom lens is 280mm/28mm, that is, 10 times. The English name of Optical Zoom is Optical Zoom, which depends on the displacement of the lens to change the focal length. The larger the optical zoom factor, the more lenses, the larger the lens volume, the lower the imaging quality and the smaller the aperture. The bigger the optical zoom, the better. Generally speaking, as long as you are willing to spend a lot of money on careful design and production, with the current technical level, the optical quality of a lens with an optical zoom ratio of less than 4 times can approach or reach the average level of fixed focus, such as Canon Canon 70-200mmF2.8IS lens (market price: 2000 USD, weight 1.5kg). The optical quality of a lens with a zoom of more than 4 times is basically impossible to reach the level of a fixed focus lens. Since 1995, there have been large zoom lenses that are more than 10 times on the market. The bigger the optical zoom is, the more convenient it is, but the imaging will decrease accordingly. Panasonic FZ 18 digital camera, which went on sale at the end of 2007, has an optical zoom of 28-504mm, which is incredible 18 times. However, the measured resolution of the lens edge of this camera is quite poor. It seems that 18 times is close to the technical limit of optical zoom at present. I have only three words about digital zoom: liar. Digital zoom is only an electronic zoom, which can be doubled to 10,000 times with a slight change in software. Only optical zoom is the real zoom, and digital zoom is used by manufacturers to deceive lay consumers. Depth of field and aperture priority In layman's terms, the depth of field is the "acceptable clear area" extending back and forth from the focus of a photo. Compared with aperture and shutter, depth of field is difficult to understand because it is a concept based on subjective judgment. There is no absolute objective standard for clarity or ambiguity. Let's make an analogy: a group photo, if accurately focused, the faces of a row of people are clear, but the flowers in front of the crowd and the buildings behind the crowd are blurred. The clear area of this photo is limited to the crowd, so we call this photo shallow (small) depth of field. If you take a group photo with the minimum aperture F22, not only the people are clear, but also the flowers in front of the crowd and the buildings behind them are clear, and the clear area of the photo is very wide, so we say that this photo has a great depth of field. Landscape photography generally needs a large depth of field, because we want the scenery to be clear before and after. Portrait photography generally requires a small depth of field. We just want the beauty's face to be clear, and the branches and ugly men around this beauty should be blurred, so as to highlight the main body. Depth of field is directly related to whether the picture is attractive or not. The depth of field is determined by three factors: 1, aperture size; 2. focal length; 3, the distance of the subject. So you can't directly adjust the depth of field on the camera, only the depth of field preview button, small digital DC and some entry-level SLR don't even have the depth of field preview button, everything depends on estimation. Fortunately, it is not difficult to estimate the depth of field after mastering several principles. These three principles are: 1. The larger the aperture, the smaller the depth of field; 2. The longer the focal length, the smaller the depth of field; 3. The closer to the subject, the smaller the depth of field. The larger the aperture, the smaller the depth of field refers to the actual aperture size. In the third section, we said that the aperture value is opposite to the actual aperture size. For 135 camera, the minimum aperture of most lenses is F22, when the depth of field is the largest (if the focal length and shooting distance remain the same), and the depth of field is the smallest at F2.8 (if the maximum aperture of this lens is F2.8). It is easy to understand that the longer the focal length, the smaller the depth of field. Wide-angle lens has large depth of field, while telephoto lens has small depth of field. In fact, if the ultra-wide-angle lens of 17mm uses a small aperture below F8, you can focus anywhere you want, and the photos you take are very clear. On the contrary, a lens with a depth of field of more than 200mm is very small and needs to be carefully focused. If possible, it is best to shoot with a tripod. These telescopes have a small viewing angle and are difficult to stabilize. If you don't pay attention to the key points, you will float to the outside of the cloud nine. The closer to the subject, the smaller the depth of field. If you are close enough to the puppy's face, sometimes the tip of the nose is clear but the eyes are blurred, and the depth of field is very small. Therefore, special attention should be paid to focusing when shooting at close range, such as taking pictures of flowers and plants with a macro lens. Since the depth of field is so important, we must find out the influence of the above three factors on the depth of field, take a few photos with different apertures at different focal lengths, play them back immediately, and be familiar with the changes in the depth of field of the picture. Finally, without the preview button of the camera, just look at the focal length and aperture, and you can know the depth of field of the picture by heart. Here we can finally talk about aperture, which is the most important word in photography. Aperture has three functions: 1. Controlling the light input directly affects whether the picture can be exposed correctly, which is the key to successful shooting; 2. Control the depth of field. The smaller the aperture, the greater the depth of field. Although the focal length and shooting distance will affect the depth of field, the change of focal length and shooting distance will also affect the composition. If the composition is determined, the only weapon we can control the depth of field is the aperture. 3. Aperture affects the clarity of the picture. Any lens has the best image (the clearest picture) at the middle aperture, and the resolution is poor at the maximum aperture and the minimum aperture. So the influence of the aperture guy on the photo is really too great, which will affect the whole body. As we said before, a correctly exposed photo can have several different combinations of aperture and shutter speed. How to choose depends on your shooting intention. If you want the minimum depth of field, set the maximum aperture F2.8 If you want the maximum depth of field, set the minimum aperture of F22 directly. If you want the highest resolution, set the F8 medium aperture. This leads to the most important concept of photography: aperture priority. Aperture priority is to manually define the size of the aperture, and the camera will determine the shutter speed that can be exposed correctly according to this aperture value. Aperture priority is English, and the capitalized A or Av on the main dial of the camera stands for aperture priority shooting mode. Kneel for the king