Understanding of photographic shutter

These are all complicated. Let's just say it's a rule. I will send you the technical terms.

Focal length-the distance from the center of the lens to the focal point. The unit of focal length is usually expressed in mm, and the focal length of the lens is usually marked in front of the lens, such as f=50mm (this is what we usually call "standard lens"), 28-70mm (our most commonly used lens), 70-2 10mm (telephoto lens) and so on.

Aperture-A device used to control the amount of light passing through a lens.

Shutter-A device used to control the exposure time. Shutters can generally be divided into curtain blinds, lens blinds and steel blinds. Among them, curtain blinds can be divided into vertical curtain blinds and horizontal curtain blinds. Steel shutter can reach higher speed (the highest shutter speed can reach112000 seconds or more at present). Generally, the highest speed of the inter-mirror blade shutter does not exceed 1/500 seconds, but the biggest advantage of the inter-mirror blade shutter is that the noise generated during shooting is extremely low, which is very conducive to sneak shots and can realize synchronous flashing within the full speed range.

Shutter speed-the time when the shutter is open. It refers to the time (exposure time) when light sweeps across the film. For example, "1/30" means that the exposure time is 1/30 seconds. Similarly, "1/60" means that the exposure time is1/60s, and the shutter of1/60s is1/30s. The rest and so on.

Note: Some materials refer to shutter speed as shutter time. The two names are different, but they mean the same thing, both referring to the shutter opening time.

Depth of field-the relatively clear range of an image. The depth of field depends on three factors: the focal length of the lens, the distance between the camera and the subject, and the aperture used. The relationship between depth of field and the above three factors is: the longer the focal length, the shorter the depth of field; The shorter the focal length, the longer the depth of field (for example, under the same aperture and distance, the scene of 28mm lens is far greater than that of 70mm lens); (2) The closer the distance is, the shorter the depth of field is, and the farther the distance is, the longer the depth of field is (for example, under the same focal length and aperture, the subject scene at 10 m is far greater than the subject depth of field at 1 m); (3) The larger the aperture, the shorter the depth of field, and the smaller the aperture, the longer the depth of field (for example, under the same focal length and distance, the scene with the aperture of F 16 is far greater than the depth of field with the aperture of F4).

Sensitivity-indicates the speed of the photosensitive material. The unit of sensitivity is "degree" or "fixed". For example, "ISO 100/2 1" means a film with a fixed sensitivity of 100/2 1. The higher the sensitivity, the more sensitive the film is (that is, the less light is needed to shoot the same photo normally in the same shooting environment, that is, a higher shutter or a smaller aperture can be used). The sensitivity of 200-degree film is twice that of 100-degree film, and that of 400-degree film is twice that of 200-degree film, and so on.

Color temperature-Different pigments contained in different kinds of light are called "color temperature". The unit of color temperature is "K" (Kelvin). The color temperature we usually use is 5400 K-5600 K; The color temperatures that light types A and B can adapt to are 3400K and 3200K respectively. Therefore, different types of films should be selected according to the subject and environment, otherwise there will be color cast (unless the color temperature is corrected by color filter).

Attachment: Definition of color temperature "Suppose there is a black metal in an environment of minus 273 degrees (absolute zero). With the increase of temperature, black metal will emit light with different wavelengths, and the metal color temperature corresponding to colored light plus 273 is the color temperature of light. " For example, when a metal is heated to 2500 degrees Celsius, it will emit red light, and the color temperature of this red light is "2500+273" K, which means that the color temperature of this red light is 2773 K. The lower the color temperature, the higher the percentage of long-wavelength light (red and orange light). The higher the color temperature, the higher the percentage of short wavelength light (blue light and purple light). For example, the color temperature of sunlight at noon is about 5500 K; The color temperature of the flash is about 5600 k; The color temperature of the blue sky is about 20000 k; 100 watt light bulb's light color temperature is about 2600K K.

Exposure-the process by which light reaches the surface of the film and makes the film sensitive. It should be noted that by exposure, we mean film exposure, which is a necessary process for us to obtain photos. This is very different from what non-professionals call "exposure". What they call "exposure" refers to the accident that the camera leaks light and causes the film to fail.

Relative aperture-the ratio of the effective aperture (beam diameter) of the lens to the focal length. The larger the relative aperture, the faster the lens speed. Such as 1:2.8, 1:3.5-4.5, etc. Among zoom lenses, lenses with fixed relative aperture are generally called professional lenses, lenses with fixed relative aperture but between 1:2.8- 1:4 are called quasi-professional lenses, and the rest are called mass lenses.

Exposure combination refers to the combination of different apertures and shutters that can be used in the same shooting environment. For example, the shutter measured by the exposure meter is 1/30 seconds, and the aperture should be 5.6, so F5.6 and 1/30 seconds are an exposure combination. We can also use the exposure combination of F4 and 1/60 seconds instead, and we can also use the exposure combination of F2.8 and1125 seconds instead. In other words, these combinations are equivalent. However, it should be noted that although these exposure combinations are equivalent, that is to say, the exposure is accurate, but the depth of field obtained by different combinations is different.

Aperture (1)

The aperture size of the lens is also expressed by gears and by the value of F coefficient. However, it is worth noting that although the F coefficient is also halved or doubled between files, the value of the F file is not halved or doubled. The standard F coefficient sequence is: f/ 1, f/ 1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/1,f//kloc-0. Not all lenses have these gears, but generally only some of them, especially those small cameras, professional models and possibly other gears. The most important thing is to understand that these values represent the size of the aperture through which the aperture passes. The above aperture value sequence includes all gears. Compared with the front and rear apertures, each standard aperture can only let half or twice the light pass through the lens. Be sure to remember the values of these apertures, and you can easily increase or decrease the calculation.

Although the numerical aperture value is not written as a fraction, it is actually a fraction. Taking the aperture value as a fraction is also convenient to understand the relationship between aperture value and aperture size: the larger the aperture value, the smaller the aperture and the less light passing through the lens. For example, f/2 represents a fairly large aperture that can pass a lot of light, while f/22 is a very small aperture that can only pass a little light. They can be understood as fractions: 1/2 is much greater than 1/22.

Digital cameras display aperture values digitally, which can increase or decrease with the shutter speed 1/3. For example, you may find these F values: 4, 4.5, 5, 5.6, 6.3, 7. 1, 8, 9, 10 and 1 1. Just remember that there must be incremental changes in first gear, no matter where it starts. From f/4 to f/5.6 is the first gear, just like from f/ 1 1 to f8. Similarly, from f /5 (lower than f/5.6 1/3) to f/7. 1 (lower than f/8 1/3) or from f/6.3 to f/4.5, it is also a gear. Although they are not linear in value, the concepts of halving and doubling are equally applicable. It should also be mentioned that a few cameras can increase or decrease the 1/2 gear, while some cameras can set all gears through program control: 1/2 gear or 1/3 gear. Also, please read your camera manual carefully.

Aperture (2)

In modern lenses, only a few still have real manual aperture rings. Although the aperture ring generally has an obvious sense of pause when turning to each position, it can still be turned to any position. The feeling that the aperture ring stops is only convenient to calculate how many gears the aperture has turned, and it has no other meaning. Most large format cameras have a pointer or ruler to indicate the aperture value.

There are two terms used to indicate the change of pore size. "Turn it up" means to make the aperture bigger and the lens aperture bigger. This means that more light is allowed to irradiate the substrate through the lens. Another term "reduction" realizes the opposite process, making the aperture smaller and reducing the amount of light reaching the negative through the lens. So we can say: open the aperture from f/8 to f/4, or shrink the aperture from f/8 to f/ 16.

The combination of shutter speed and aperture can control how much light can reach the image sensor, and these two factors must be considered at the same time. They all work in the way of gears, and they are interrelated, which we call "reciprocal relationship". Once the total amount of light required for correct exposure can be determined, and any one of shutter speed and aperture size has changed, another value that should be set can be quickly determined according to reciprocity. Under the condition that the total amount of light shining on the negative film is unchanged, the change of shutter speed in the first gear is equivalent to the change of aperture size in the opposite direction. That is to say, if one of the shutter and aperture is halved and the other is doubled, the total amount of light (that is, exposure) irradiated on the film through the lens is the same.

Reciprocity is a very important concept, you must understand it.

You can understand that doubling the shutter speed and halving the amount of light passing through the aperture is exactly the same as halving the shutter speed and opening the aperture to let the doubled light enter the lens. Let's make two metaphors. For example, if you want to pour a liter of water, you can turn on the tap very big and fill it with a liter in an instant, or turn on the tap very small to let the water flow out drop by drop, which takes a long time. But there is no difference between the two methods, and eventually you can get a liter of water.

This is reciprocity. For another example, you need to earn 20 yuan. You can find a part-time job that can earn 4 yuan per hour for 5 hours; Or find a job that costs 5 yuan an hour and works for 4 hours. Or something else, working in 2 yuan is 65,438+00 hours per hour, or working at 65,438+00 yuan per hour is 2 hours. You can choose at will, but all you get in the end is 20 yuan.

Aperture (3)

In photographic terms, you can use a slow shutter to add a small aperture, or you can use a fast shutter to increase the aperture. The exposure obtained by the two methods is the same. But (note, this is very important) the imaging effects of the two methods on the image sensor are different. The reasons for the different imaging effects will be introduced in later chapters on depth of field and control of moving objects. From this point on, as a photographer, you should start to learn how to judge whether the photos you take are as good as you want.

In practice, reciprocity means that once you get a specific exposure through scene photometry (which will be discussed later), you are free to choose the combination of shutter and aperture to get the correct exposure. Suppose that the appropriate exposure value for a scene is115 seconds, and f/ 1 1. You can increase the shutter speed to 1/8 seconds (double the shutter time for changing gears) and reduce the aperture to f/ 16 (also reduce the light by half for changing gears). Because115 seconds and f/ 1 1 are completely equivalent to 1/8 seconds and f/ 16. Similarly, you can also use 1/4 seconds and f/22, 1/30 seconds and f/8, 1/2 seconds and f/32, and 1/60 seconds and f/5.6. All these combinations can make the same amount of light shine on the image sensor and get the correct exposure. Knowing this, we can choose to use the aperture size or shutter speed we need and make adjustments based on reciprocity.

In fact, this reciprocal relationship is easier to realize on the camera than what we discussed here. When you change the setting of aperture or shutter, record the changed gear number and change it to another setting with the same gear number. As long as one is added and one is reduced, the same exposure can be guaranteed. For example, suppose that 1/250 seconds and f/4 are correct exposure value. Then increase the No.4 shutter to115 seconds (increase the shutter time) and reduce the No.4 aperture to f/ 16 (decrease the aperture), so as to get the same exposure. If the shutter and aperture of your camera have the same increment, such as 1/2 or 1/3, then you only need to count the shutter dial and aperture ring of several gears.

speed

In fact, the choice of shutter speed and aperture depends largely on the setting of camera sensitivity when you shoot.

The sensitivity of digital camera to light is expressed by "sensitivity", that is, ISO value. More light is needed to complete the photoelectric conversion process when imaging at low ISO value. On the contrary, a high ISO value requires less light to accomplish the same thing. It should be noted that, generally speaking, with the increase of sensitivity, the imaging quality will decrease.

As I mentioned before, sensitivity is also expressed in the form of "gear", and the concepts of magnification and halving are also used, just like shutter and aperture.

Again, the more thoroughly you understand the concept of "blocking", the easier it is for you to deeply understand exposure. The expression of sensitivity is a linear sequence, each ISO value represents a gear, and the relationship between each gear is doubled or halved. For example, from ISO50 to ISO 100, there is a difference of one gear, from ISO200, there is a difference of two gears and from ISO400, there is a difference of three gears. These values can be easily calculated by simply doubling or halving. For another example, what is the difference between ISO 1600 and ISO25? Yes, maybe you can see that I'm trying to confuse you through reverse calculation, but if you understand this calculation well, you won't be confused and get the correct result. So, starting from 1600, we can calculate 800,400,200,100,50,25, a total of six gears.

Knowing the relationship between ISO values is very helpful for you to choose the right sensitivity setting at the right time. If you already know the exposure required for correct exposure when using a certain sensitivity, you can easily change the exposure when using another sensitivity for the same exposure, as long as you know the relationship between the two sensitivities.

Actually, it's very simple. Just increase or decrease the gear. For example, if the correct exposure parameters of ISO 100 are115 seconds and f/8, what should the correct exposure parameters be if ISO400 is set? Calculate the difference of ISO value: starting from 100, double it to get 200, and then double it to 400, thus increasing the ISO value by two steps. Therefore, changing the exposure parameters of ISO 100 in two steps is the correct exposure parameters of ISO 400. We changed the exposure parameters to: 1/60s, f/8. Of course, you can change it to any equivalent exposure value, for example,1/60seconds f/8 is equivalent to1125s f/5.6 or1/250seconds f/4 or1/500s f/2.8.

The standard sensitivity is usually in increments of 1/3:

O 25 32 40 50 64 80 100 125 160 200 250 320 400 500 640 800

Not all cameras have so many sensitivity settings, but keep in mind that doubling or halving the ISO value means a difference of one gear. ? 2000 is two grades slower than ISO 8000, but two grades faster than ISO 50. It can be concluded from the above sequence that ISO 64 is 2/3 files slower than ISO 100.