Introduction to photography: sensitivity (ISO)

Time:2021-3-10

When it comes to sensitivity, you may not quite understand what it is, but when it comes to ISO, I believe you are familiar with it. Every time I see our technology sharing, I often see the word ISO, so what is ISO? What’s the use of it? Let’s have a look~

1、 What is sensitivity (ISO)

In the film era, ISO sensitivity is an international unified index to measure the film speed standard used by traditional cameras. The sensitivity is determined by the purchased film, and the value is fixed, which is an attribute of the film itself. ISO 100, ISO 200 and ISO 400 will be marked on the package when purchasing film.

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Although digital cameras sense the intensity of incident light through CCD or CMOS, the concept of ISO sensitivity is introduced in order to unify the unit of measurement with the film used in traditional cameras. Therefore, the ISO sensitivity of digital cameras also reflects the speed of sensitivity.

In the digital age, the sensitivity of electronic sensors can be set freely within a certain range, which is much more convenient than the original film age.

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The sensitivity level is a multiple relationship, such as ISO 100, 200, 400, 800, 1600… The sensitivity difference between adjacent ISO is one time, the larger the number, the higher the sensitivity.

With the current digital technology, we can say:

Iso800 (2017) below for low sensitivity

Iso800 ~ 6400 (2017) belongs to medium sensitivity

ISO6400 (2017) or above is high sensitivity

2、 The influence of light sensitivity

1. Increasing ISO can increase the exposure of the picture.

The larger the ISO number, the more sensitive it is to light. Let’s take a look at the picture below. With the aperture and shutter unchanged, increasing ISO can improve the exposure of the picture.

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2. High sensitivity will bring noise problems

·So what is noise?

Noise, sometimes called noise, is mainly due to the fine particles produced in the process of receiving light and converting it into electrical signal output by electronic image sensor (usually CCD or CMOS). These particles are not found in the object and are usually caused by electronic interference.

The noise and noise are not increased linearly. Generally speaking, iso400, Iso800 and iso1600 are three turning points. You can see the following picture. When the sensitivity is below 400, the picture is usually very pure. When the sensitivity is above 400, the noise and noise will increase. If the sensitivity is further increased to 1600, the noise and noise will become more obvious. The pictures taken above 6400 basically have no use function.

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Of course, with the development of science and technology, the newly released high-end cameras still have strong image expressiveness in the case of 6400.

·How to avoid noise?

There are two cases will produce noise, usually using high sensitivity to shoot, in daily shooting, almost most of the noise is caused by the use of high sensitivity.

Another case is long exposure. Digital camera uses electronic sensor instead of traditional film to image, so the electronic sensor will heat under long-term exposure, and the improvement of heat will affect the sensitivity of the sensor, resulting in the formation of noise on the screen. So consciously avoiding these two points can effectively reduce noise.

Let’s summarize:

Sensitivity, expressed in ISO, the smaller the number, the lower the sensitivity, and vice versa (proportional).

The picture with low sensitivity is clear, and the picture with high sensitivity has a lot of noise.

3、 Three elements of exposure

So far, the three elements of exposure, aperture, shutter and sensitivity, have all been introduced. The three together determine the exposure of a photograph, also known as the triangle.

In order to achieve normal exposure, we need to adjust the parameters of the three, so that the elimination has to be that long.

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In the previous article:

The aperture values are: F / 1.0, f / 1.4, f / 2.0, f / 2.8, etc. the aperture will be reduced by one level or one gear with every increase of √ 2 times.

The shutter value is: 1, 1 / 2, 1 / 4, 1 / 8 and so on. The exposure of the two adjacent shutter speeds is twice the same. We often say the difference is one level.

·What is the relationship among them?

Suppose that every time the sensitivity is doubled, such as from ISO 100 to ISO 200, the sensitivity of the sensor to light will be doubled. At this time, for normal exposure, the shutter speed can be increased by one level or the aperture can be reduced by one level. Of course, in the actual operation, there is no need to calculate and force memory, real-time view of the light measuring ruler can accurately expose.

·What is the basis of the three settings?

Look at the picture below

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Exposure triangle not only affects the exposure value, but also brings three important effects

1. Aperture affects depth of field and lens sharpness.

2. The shutter affects the capture speed.

3. ISO affects sensitivity, high sensitivity brings noise.

The specific value of the three and the sacrifice value in the special environment depend on the specific shooting situation. In the previous article, we also introduced the values used in different scenes. The only thing left is to try to combine the three with your camera. Because the scenes and devices are different, the values are also different.

4、 Sensitivity setting of panoramic photography

In general, we suggest that low sensitivity should be used as far as possible in the outdoor or in the case of sufficient light, so as to ensure higher image quality and detail expression. ISO can be controlled within 100-200. Indoor, ISO can be increased accordingly, such as 200-400.

The above is just a general situation. If you encounter special situations, such as a dark venue with a large crowd, you should first consider the shutter speed. After setting a large aperture, you will find that it is still dark. At this time, you can only sacrifice the sensitivity. After all, compared with slight noise and shadowing, it’s better to deal with noise later.

Therefore, the specific situation of specific analysis, more shooting and more practice to know, do not rigidly adhere to the specific value.