Five minutes to understand the three elements of contact.

As we know, photography is an art that runs out of light, and the success of exposure determines the quality of a photo.

How to control exposure becomes the basis of photography.

So what are the factors that control exposure?

These are the three elements of exposure: aperture (light input), shutter (light input time) and sensitivity (ability to feel light).

Aperture, in layman's terms, is the component on the lens that controls the aperture size. Aperture is expressed by the concept of aperture value, and the expression of aperture size is expressed by F value.

F value = lens focal length/aperture.

Of course, you don't need to remember the above formula, you just need to remember the aperture range of your lens. The aperture size is controlled by polygonal or circular blades in the lens, as shown in the following figure.

Since the value of the aperture f is reciprocal, the smaller the aperture value (denominator), the larger the aperture.

Common aperture numbers are arranged in the following order:

Two adjacent numbers are one gear, that is, f/ 1 is one gear higher than f/ 1.4, and f/8 is three gears lower than f/2.8.

It is not difficult to find the law by carefully observing their arrangement. Their interval is multiple, and the square ratio of two similar numbers is about 1: 2. Every time the aperture is increased, the illumination difference is doubled:

2 x 2.8 x 2.8 ≈ 4 x 4

2 x 5.6 x 5.6 ≈ 8 x 8

2 x 1 1 x 1 1≈ 16 x 16

Aperture not only controls exposure, but also controls depth of field and imaging quality. And these are not within the scope of today's exposure, we will talk about it later.

Shutter is a component in the camera that controls the exposure time. The shorter the shutter time, the shorter the exposure time and the less light entering.

This is easy to understand. Like the aperture, the shutter has gears. The common shutter file is 1s as the initial file, and the order of gradual acceleration is:

They took turns to open the shutters in half less time than before, so the exposure was reduced by half. That is, 1s shutter is one step higher than 1/2s shutter, and 1/250s shutter is two steps lower than 1/60s shutter.

Remember the change of aperture and shutter gear? We will use it later when we talk about the countdown rate.

There are two special shutter times,115s and1125s, which actually appear mainly to round off the following figures.

Sensitivity, also known as ISO, is a measure of the sensitivity of film (photosensitive element) to light.

The unexposed film will react chemically when it encounters light, and it will change at different positions according to the amount of incident light, thus leaving an image on the film.

Some photosensitive materials are very sensitive, and only a little light is needed to make them react chemically, while others are relatively slow and need a lot of light to make them sensitive. American Standards Institute (ASA) first established a set of standards about film sensitivity, which was later popularized and replaced by International Organization for Standardization (ISO), so we usually regard ASA 100 or ISO 200 as sensitivity parameters.

Like the aperture shutter, the sensitivity parameters also have gears, and the common sensitivities are:

By analogy, the interval is twice that of the first gear.

In addition to ASA scale, DIN scale is also commonly used in the movie era. The conversion between them and the sensitivity of corresponding film products is shown in the following table:

The higher the sensitivity, the better. The higher the sensitivity, the easier it is for impurities (particles/noise) to appear.

Understand the aperture shutter and sensitivity, and also know their relationship with exposure, so how to use their relationship to control exposure? This is going to talk about reciprocity.

Let's briefly sort out the relationship between exposure and light mentioned above.

The larger the aperture, the larger the light entrance aperture, the more the light entrance amount, the smaller the aperture, the smaller the light entrance aperture, and the less the light entrance amount;

The slower the shutter, the longer the light entering time, the more the light entering quantity, the faster the shutter, the shorter the light entering time and the less the light entering quantity;

The higher the sensitivity, the stronger the sensitivity to light, the lower the sensitivity, and the weaker the sensitivity to light.

As long as the exposure is the same, the exposure of photosensitive materials to light is the same, and the values of aperture shutter sensitivity can be replaced with each other. This interaction is called reciprocity rate (also called reciprocity law).

For example, take article 16 of sunshine as an example. When the sunshine is sufficient, use ISO 100 film. At this time, the aperture of accurate exposure should be f/ 16, and the shutter should be1125s (reciprocal of sensitivity).

Then, if the aperture is increased by one step to become f/ 1 1, and the incident light is doubled, then the shutter must be lowered by one step to become 1/250s to be the same as the exposure just now.

Similarly, if the aperture is reduced by one step to become f/22, the shutter should be increased by one step to become 1/60s.

That is, in ISO 100, f/ 16, 1/25s and f/ 1 1, 1/250s and f/22,/kloc.

Knowing the existence of reciprocity, we can control our aperture shutter combination at will. If you want a large aperture, you need to speed up the shutter speed. If you need to shoot deeper details, you need to lower the shutter.

Of course, the reciprocity rate is not omnipotent, and it also fails. When you use a short shutter (less than11000s) or a long shutter (more than 1s) for exposure, the reciprocity rate may fail.