Seven related knowledge of aperture

Lead: What is the aperture? When asked this question, most of us know that the aperture is like a gate to control the amount of light entering the lens. When the exposure time is fixed, the larger the aperture is, the smaller the aperture value is and the more light enters. Knowing that a large aperture is easy to shoot a shallow depth of field and so on. In the process of learning photography, have you encountered any other knowledge about aperture? This time, let's sort out some key points and review our lessons together!

Seven related knowledge of aperture: aperture, aperture value and aperture series.

Aperture is like the pupil of a camera, which controls the amount of light entering by enlarging and narrowing, so as to adjust the amount of light shining on the negative film or photosensitive element. Generally speaking, the aperture size refers to the aperture size, which is often expressed in the form of f/ 1.4 and f/2. Usually we call it "aperture value". The larger the number behind, the smaller the aperture. But have you ever wondered why these numbers? Similarly, the larger the number, the smaller the aperture, www.fsbus.com

In fact, these numbers are the ratio of the focal length of the aperture to the aperture of the aperture, which is more called "focal ratio" academically. When the focal length is fixed, if the aperture is large, the aperture is also large, and the aperture value is naturally small. On the contrary, the aperture value is large. Considering the concept of exposure series, our common full-scale aperture values are f/ 1, f/ 1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/1/. Now, due to the progress of science and technology, some cameras can also achieve half or one-third aperture adjustment, so everyone has seen f/3.5, f/6.3 and other aperture values. Observing these full-scale aperture values, maybe many friends already know the difference between them? 2, but what about this? How did you get it? This requires taking out a pen and paper and using a mathematical formula to calculate. Friends who are interested in further study can refer to the following calculation method.

First of all, let's list several formulas that are known at present. Assuming that the focal length does not change to n and the aperture radius is r, then f=n/2r. On the premise that the shutter and sensitivity are unchanged, we visually represent the exposure as the aperture area of the aperture, so let the exposure be t, T=2? R2(2π squared).

When the aperture value f= 1, the aperture radius of the aperture is r 1 and the exposure is T 1. We want to know what the f value should be if we want to reduce the exposure by half compared with T 1. The key to reducing exposure is to reduce the aperture to reduce the aperture area. So, let the exposure be halved to T2, when the aperture radius is r2 and the aperture value is f? .

Under the condition of fixed shutter and sensitivity, according to the change of aperture size, a unit for measuring the amount of light entering is reduced by half between different levels. For example, the difference between f/ 1.4 and f/2 is one level, and the amount of light entering f/2 is half that of f/ 1.4; The difference between f/5.6 and f8 is one level, and the light input of f/8 is half that of f/5.6, and so on. Of course, the zoom adjustment of the aperture in the camera can be more detailed now, and it can be adjusted by 1/2 or 1/3, so many cameras will have other F values besides the full-scale aperture value.

The exposure series (T-stop) is a generalized calculation unit of exposure, which includes three control elements: aperture, shutter series and sensitivity, and is the total exposure.

Large aperture and shallow depth of field

Many friends like large aperture, just like the feeling that it is easy to shoot beautiful shallow depth of field, and they can receive light well even in the case of insufficient light. However, a large aperture is not everything. Sometimes the depth of field is too shallow and it is easy to lose focus. Because the depth of field is too shallow, if the subject moves slightly after focusing, it is easy to cause the subject to move out of the depth of field, leading to the problem of inaccurate focusing.

In addition, when the lens aperture is opened to the maximum, that is, in the "open aperture state", the image quality may be slightly loose and not sharp enough, and usually it can be reduced by about 3 levels to obtain the best image quality.

Small aperture and light diffraction

Since the aperture is too large, it has too many disadvantages. If the aperture is too small, there will be a relative problem, that is, the diffraction of light. When light passes through a small aperture, it is easy to cause fluctuations to overlap, forming bright and dark ripples, which will affect the image quality. Generally, when the aperture of a lens is less than f/ 1 1, the resolution will gradually deteriorate, and when the aperture is around f/22, the image quality will be very poor. In addition, a small aperture will make the black spots and dust on the photosensitive element more likely to affect the picture, so it is generally not recommended to reduce the aperture to below f/ 1 1.

Find the best aperture range

As mentioned earlier, the image quality is not good when the aperture is opened to the maximum, and it is not good when the aperture is too small. So where is the available aperture range of this lens in your hand? According to the experience of some senior photographers, in general, the best image quality can be obtained by reducing the aperture by about 3 levels, and it is best not to reduce the aperture f/ 1 1, but for myself, I will still know my acceptable range through my own attempts. When testing, you can find a scene with more details to shoot, shoot a tripod on the photo frame, and capture the same picture for comparison. The shot image can be enlarged to 100% on the screen. Check whether the image quality of every detail is OK, and then get the best aperture value from EXIF data.

Number of diaphragm blades and spots

The beautiful scattered scene with large aperture is the image beauty pursued by many photographers, which can have great effect whether it is used in sketch, portrait or night scene, and the extreme of scattered scene lies in the "creamy" spot, round shape and vignetting effect at the edge. Whether you can shoot such a beautiful lens depends mainly on the number of aperture blades of the lens itself. There are many aperture blades, even if the aperture is reduced, you can still create a circular shooting spot. However, the number of diaphragm blades is not as large as possible. Too much will affect the running speed of the lens and increase the production cost of the lens.

At present, there are 5 ~ 9 lenses on the market. According to the different shapes of the diaphragm blades, even if the number of diaphragm blades is the same, it may not produce the same scattered light spot effect. Some lens specifications will specify the use of "circular aperture blades", which means that this lens can shoot a circular lens that is 2-3 levels smaller than the maximum aperture.

In addition, the number of diaphragm blades is also related to the number of star awns extended by strong light. When the number of aperture blades is odd, the photographed star will be twice as many as the number of aperture blades, and if it is even, it will be the same as the number of aperture blades. According to the predecessors of photography, the stars photographed by even aperture blades are thick, while those photographed by odd aperture blades are thin and clear.

Constant aperture

When you buy zoom lenses, you may have noticed that some lenses have the function of "constant aperture". In general zoom lens specifications, the maximum aperture is always marked with a range, such as F/3.5 ~ 4.5, but some zoom lenses can keep f2.8 Why is there such a difference? So it's because of the low zinc content? There are two kinds of wood lamp groups: the branches move with zoom and are completely fixed. Generally speaking, the lens group that moves with zoom is usually a large zoom range or a wide-angle zoom lens, and their manufacturing cost is relatively low. Under the premise of constant aperture, the aperture value f will change with the change of focal length. (As mentioned in the previous article, the aperture value is the ratio of focal length to aperture. )

Relatively speaking, the constant aperture may be a fixed aperture design in the zoom lens, so the aperture value will not be affected by the change of focal length. A typical example is F2.8 with 70 ~ 200 mm, and there are also some zoom lenses with constant aperture, which move with the movement of the lens group, but they also keep the aperture constant by changing the aperture size, such as constant F4.0 At 50mm, the aperture is 12.5mm, and at 60mm, the aperture is enlarged to 15 mm.

Aperture, autofocus and rangefinder

What is the direct relationship between aperture and autofocus? Why do some camera focus systems often write the statement "central f/2.8 cross autofocus point" or "central 33 point can be used when the aperture is lower than f/5.6 and higher than f/8"?

First of all, understand a concept, that is, when a general camera carries out autofocus, it is always carried out under the maximum aperture of the lens. No matter how much aperture the user sets before pressing the shutter, the camera will focus on the maximum aperture first, and then quickly approach the specified aperture size. So when it comes to autofocus and aperture, "aperture" usually refers to "the maximum aperture of the lens", not the aperture value we usually set.

At this time, looking back at these descriptions of the focusing system, we can see that it mainly refers to the opening aperture of the matching lens. The explanation of the two sentences in the previous example can be as follows: "When the lens with large aperture f/2.8 is connected, the central focus will start cross focusing to enhance the focusing accuracy of the central focus. (Editor's Note: After all, the large aperture is relatively shallow, and it is easy to be out of focus. ) or "when the aperture of the lens is between f/5.6 and 8, you can use the central 33-point autofocus point. This kind of narration is especially made to emphasize the focusing ability when the camera is connected with a large aperture lens, or when some super telescopes are connected and a rangefinder is used.

Focusing with a large aperture is difficult because the depth of field is too shallow, and it is easy to lose focus or not sharp enough. Therefore, many cameras will particularly enhance the focus accuracy of the center for large apertures. If you shoot with an aperture AF such as f/2.8 or a larger aperture AF, the success rate of focusing with the center will be higher.

In addition, because of the relationship between focal length and design, the maximum aperture of some super telescope lenses may be only F 5.6 ~ 8, or if rangefinder is used, the original open aperture series of the lens may be reduced (Editor's note: the principle can be thought of through the aperture value formula mentioned above, and it should be calculated! This formula can also be used to convert the maximum aperture value of the lens after using the rangefinder. Some advanced models do not emphasize the focusing ability at small aperture. After all, many cameras often lose focus due to insufficient brightness under a small aperture, which should be one of the considerations for photographers to use binoculars in outdoor ecology, sports events and so on.