Abstract: Among the filters used in SLR cameras, there is a commonly used filter called subtracter, which is also called medium gray density mirror, neutral grey mirror or grey mirror and nd mirror. The main function of the subtractor is to reduce the amount of light entering the camera. Among the gears of condenser, ND4, ND8, ND8, ND32 and ND 1000 are common, which play a great role in different fields. Next, this paper will briefly introduce the role of the variable diameter mirror and the use of several gears of the variable diameter mirror. Come and have a look with me! First, the role of variable diameter mirror.

We can literally see the function of the reducing mirror. The main function of the subtracter is to reduce the amount of light entering the camera.

For example, when shooting water in the wild, we need to shoot it with a shutter of 1 s if we want the water to appear as soft as cotton wool. Then the problem comes. If the light is strong, the shutter speed is still around 1/30 seconds to achieve such a long exposure time, that is, using the minimum aperture of the camera and the lowest ISO shooting. At this time, the only way is to use one or even two ND8 lenses to weaken the light entering the lens.

Second, reduction mirrors are used much more.

1, the gray filter in ND, the number of files is calculated according to the n power of 2, therefore, the common number of files are

2,4,8, 16,32,64, 128,256,5 12, 1000,2000? 10000

For each additional level, the medium gray filter reduces the exposure by one level, and so on. The nd medium gray filter with 1000 level can reduce the exposure by 10 level.

2. At present, the common ND neutral gray filters that are easy to buy are numbered ND4, ND8, ND8, ND32, ND 1000.

(1) Taking ND4 as an example, it can reduce two-step exposure and slow down the shutter speed by four times. That is, if the shutter speed is115 seconds, the shutter speed can be slowed down to 1/4 seconds after using the neutral gray filter in ND4.

(2) Taking ND8 as an example, it can reduce the third exposure and slow down the shutter speed by 8 times. That is, if the shutter speed is115 seconds, the shutter speed can be slowed down to 1/2 seconds after using the gray filter in ND4.

(3) Taking ND 16 as an example, it can reduce the exposure of four gears and slow down the shutter speed by 16 times. That is to say, if the shutter speed is 115 seconds, the shutter speed can be slowed down to1second after using the gray filter in ND4. By analogy, it is easy to calculate.

3. In fact, ND8 or ND 16 is the most commonly used way to shoot running water in woods or forests.

4. When shooting running water under bright sky, ND 16 is still not enough. At this time, ND64 or ND 128 will be enough.

5. For some special shooting needs, even more than ND 1000 is needed.