Who can give me a detailed explanation about the data knowledge of digital SLR camera lens?

As we all know, the aperture ring of ordinary zoom lens is always marked with two sets of data. What does this mean? Novices often ask this question. It actually shows you that the aperture of this lens will change with the change of focal length, such as AF Zoom Nikkor1:4.5-F5.6/80-200mmd lens. That is to say, if we keep the maximum aperture of this lens at 4.5, it will be F4.5 at 80mm With the increase of focal length, the aperture actually changes gradually. 200mm end, although the aperture, we still take the zoom lens of 70-2 10mm/F4.5-5.6 as an example. When you take a hand-held photo at 70mm with F8 aperture, assuming the shutter speed is1125 seconds, according to the principle of shutter speed and focal length, you can take a very clear photo without the negative influence of "hand shaking". But when you push the focal length to the end of 2 10mm and the light is unchanged, the aperture will automatically change to F 1 1, and the camera will automatically change the shutter speed to1/60s according to the change of the lens luminous flux. At this time, there will be a little problem. If you still have the camera, there is a reason. It is difficult to take clear photos (it takes more than 1/250 seconds to overcome the influence of "hand shake" or use a stable camera such as a tripod). Many people look at a lot of blurred photos and think that the zoom lens they use is "a dog's head or a rotten head", and they are cheated by the cheap lens of the lens manufacturer. There is indeed a misunderstanding of lens manufacturers. It is a natural feature that some photographers don't understand that the floating aperture lens will "float automatically", which is a misunderstanding, and it is also the fundamental reason why some photographers often have unsatisfactory photo effects or film paste after using the floating aperture zoom lens. Therefore, the floating aperture lens has a famous nickname: "Silent Black Boy", which will silently murder your movie or picture effect.

The main purpose of designing a floating aperture lens in this way is to reduce the diameter and weight of the lens and reduce the manufacturing cost, but the biggest disadvantage of the floating aperture lens is that you have to endure the pain that the lens aperture decreases with the increase of focal length. You can't shoot by hand in weak light, but use high-sensitivity film and the depth of field changes. Therefore, some enthusiasts always want to burn a fixed-focus lens or a constant-aperture zoom lens if they have money, which is also reasonable.

The relative aperture of the aperture can also be called focal power, that is, the ability of the lens to allow light to enter, which refers to the light hole on the lens. The larger the aperture, the more light enters and the shorter the exposure time, and the smaller the influence of camera shake on definition. Even if the light is weak, the lens with large aperture can shoot (such as F 1.2, 1.4, 2, 2.8, etc. ), but it is difficult to shoot a lens with a small aperture (such as F4.5 or above).

The aperture size has a great influence on the required lens diameter. When the focal length is short, the lens is very close to the film, and the loss of light energy is very small, so a small aperture is enough. Long focal length, on the one hand, the light path is long, on the other hand, the long focal length has a small viewing range and a large loss of light energy.

If you want the same amount of light to reach the film, the diameter of the aperture should be increased accordingly. The basic formula is: under the condition of constant focal strength (aperture), the focal length must be doubled and the lens diameter must be doubled. For example, for a lens with a focal length of 100mm, as a normal standard lens with a focal length of 50mm, to reach F2.8, the aperture diameter must be twice that of the normal standard lens, the diameter of the 200mm lens must be four times that of the normal standard lens, and the diameter of the 400mm lens must be eight times that of the standard lens. This is the fundamental reason why the lens diameter of telephoto lens with the same relative aperture must be several times larger than that of wide-angle lens, and it is also the main principle of manufacturing constant aperture zoom lens.

Constant aperture zoom lens can also be called stepless zoom lens, and its design requirements are very high. The maximum adjustable focal length of the lens determines the lens diameter. If you want to build a zoom lens with a constant aperture of 1: 2.8/28-200mm, you need to bring the light energy corresponding to 1: 2.8 to the film or photosensitive layer with 200mm energy, which determines that you must use a large aperture lens to maintain high power, but the use of a large aperture lens brings about a decline in aberration quality. In order to control the quality of aberration, we should adopt high focusing intensity. In this way, the optical system required for the design becomes huge and complicated, and the manufacturing cost of the lens will inevitably increase greatly. Many people propose to make a top-quality "one mirror travels all over the world" lens. For a long time, people have been asking whether it is possible to manufacture a lens with a focal length of 18-200mm, a constant aperture of f 1.4 and an anti-shake of 1: 65438+. I think this is a common sense mistake. Judging from the lens manufacturing technology trend of the world camera lens manufacturers at present and for a long time to come, it can be said that this is a beautiful fantasy that we can't realize at all. This is not because manufacturers have the ability to manufacture but deliberately do not manufacture in order to maximize profits. If so, it can be made for the national interests such as military industry. In fact, lens manufacturers are very willing to show people the highest technical level of lens manufacturing they can achieve, such as Nikon's "Eagle Eye" AI 58 mm/f1.2; Canon's 50mm/F 1.0 is an extreme example in the history of camera lens manufacturing, but why haven't we made a constant aperture lens with a focal length of 18mm to 200mm and F 1.4? This is due to the insurmountable difficulties and obstacles in manufacturing technology. Where is the difficulty? It is difficult to keep the high focal strength of 1.4 from 18mm to 200mm, not to mention how complicated the lens has become because of such technical standards. It is scary to see that the lens diameter reaches or exceeds 208 mm. If the outer diameter of the lens is greater than 350mm, you can imagine: Who can carry this super-heavy lens? What cameras are there in the world at present? It's really made. Who can afford its astronomical price?

There is only one exception to this super lens. Some large astronomical telescopes and the famous Hubble telescope in the world can achieve such manufacturing technical indicators at any cost. But its appearance is extremely huge and complicated, and the manufacturing cost is piled up in billions of dollars.

So it is not difficult to understand that there is such a big difference in price, volume and weight between constant aperture lens and floating aperture lens. Take the fixed focus standard lens as an example. The prices of F 1.2, F 1.4 and F 1.8 are often several times different. The fundamental reason is not that the half-step aperture increases, but that the larger the aperture, the larger the lens diameter will be. With the increase of lens diameter, the manufacturing difficulty and cost will increase greatly.

Why do fixed-focus lenses and telephoto lenses with constant aperture weigh more? This is not the intention of the manufacturer. In fact, how to reduce the weight of lens has always been the direction of manufacturers' long-term efforts. But because of the above reasons and technical difficulties, it has always been a very difficult thing to solve.

After understanding the above principles, you will naturally know the advantages of fixed focus lens and constant aperture lens in practical application. Simply put, compared with ordinary zoom lenses, fixed-focus lenses and constant-focus lenses have at least the following advantages:

1, the fixed-focus lens and the constant-aperture lens can maintain stable focal strength and depth of field at any focal length, and maintain relative shutter speed, especially indoors or in dim conditions with poor light. When the floating lens becomes difficult to shoot with a tripod, the fixed-focus lens and the constant-aperture lens can also keep shooting by hand;

2. Fixed focus lens and constant aperture lens are convenient for free use of depth of field, especially constant aperture lens, which will not change the aperture and then change the depth of field like floating lens;

3. Fixed focus lens and constant aperture lens can maintain and use high shutter speed, which is of decisive significance when shooting dynamic objects in spotlight or using telephoto lens. Imagine this: if you are taking a stage photo or returning to the birds at dusk, if you secretly shot a wonderful documentary scene in the dark, you are absolutely not allowed to turn on the flash. At this time, if you don't have a high-quality fixed-focus lens or a constant large-aperture zoom lens, I estimate that a large number of photos taken can basically be thrown directly into your trash can or deleted in batches.

4, the convenience of metering, of course, this is not an advantage, because now, whether it is a fixed focus or a zoom lens, the metering technology in modern cameras can automatically adjust the metering amount according to the change of light. But this is really a big advantage of the old camera.

Of course, the price of enjoying these conveniences is that the money paid needs to be greatly increased. The difference between zoom lens and constant aperture lens is generally more than several times, and some are even as high as ten times. This is something that poor peasants and middle peasants like us dare not even think about.

At present, the popular zoom lens with compact design is widely used. Its price is relatively cheap and the lens is very light. The key secret of its design lies in the use of "floating aperture", that is, with the increase of focal length, the relative aperture of the lens decreases continuously, which brings serious loss of lens light, reduction of application scope, increase of sensitivity and subsequent increase of manic points, at the expense of certain depth of field adjustment or increase of tripod.

Having said so much about the shortcomings of the floating aperture lens, do you already want to smash the ordinary zoom lens in your hand? Wait, the floating aperture lens also has its own great advantages:

1. Compared with professional fixed-focus or constant-aperture lenses, the floating-aperture zoom lens is light, flexible and easy to carry, especially when going out for sightseeing or climbing a 4000m mountain with heavy load. When you are over 500 years old and still crazy about photography, you will definitely realize this.