The secret of light and shadow behind mobile phone photos

Taking pictures is actually an agreement on light and shadow, and capturing the light and shadow at this moment is the purpose of taking pictures. If you want to do a good job, you must sharpen your tools first. This article will talk about the knowledge behind taking good photos.

The principle of mobile phone shooting is not complicated to explain, and it is basically an upgraded version of pinhole imaging. The light source allows light to enter the sensor within a set time through the aperture and stay for a specified time. After digital processing, the image is finally presented. Sounds boring, doesn't it? But be patient and look at me. (Not to save my face)

What the human eye can see directly is direct light or reflected light. When light strikes an object, it will be reflected, so that we can see the object. Usually, the surface of an object is not smooth. When light strikes a rough surface, it will reflect in all directions in disorder, which is also called diffuse reflection. Imagine that we open a hole in the wall, and the diffused light will enter the hole, thus showing a reflection on the wall at the other end. This is pinhole imaging, and this is the principle on which the camera is based.

There are many aspects that determine the quality of a photo, such as how much light enters the lens, how long it enters, and the degree of sensitivity. These are the three elements of a camera.

The size of the aperture directly affects the size of light entering the sensor.

The following are two pictures under large aperture and small aperture, and the difference is clear at a glance.

P.S. Most mobile phone devices don't support multi-level aperture, which means most devices are one aperture. So unless your phone supports it, this can be ignored. Come and hit me. Write it here as popular science knowledge ~

What determines is how long light is allowed to enter the sensor. The longer the shutter time, the more light enters, so the photo will be brighter. Shooting in a dark environment requires a long shutter, otherwise it is easy to be unclear; On the contrary, in a well-lit environment, a shorter shutter is needed, otherwise it is easy to overexpose.

The following pictures are two sets of photos under the shutter of 0.6S and 15S respectively. If the shutter is long, there will be enough light to enter the sensor, thus giving people the feeling of traffic.

Sensitive and easy to understand. The higher the sensitivity, the better the sensitivity to light, and the easier it is to brighten the object. However, it is worth noting that the higher the sensitivity, the more obvious the noise will be. If you want to use it well, you need to practice more.

Here is a video demonstration. It is really troublesome to upload videos on WeChat official account. Interested children's shoes, please contact WeChat. ...

After a beam of light passes through a lens, it will clearly appear on a plane, which is called focus and focal plane. For a single object, there is only one focal plane. In fact, our human eyes can be considered as clear imaging within a certain range, and it is impossible to be so harsh.

The key points on the mobile phone are different from those on the camera and need to be explained separately. Generally speaking, the front camera uses a fixed-focus lens, and generally uses a hyperfocal distance lens. In other words, as long as the photos taken with the front camera are clear from 0.5m to infinity. Some people may ask, I turned on the front camera and the picture changed a lot. Not zoom, just metering, 23333.

Let's talk about the zoom effect. The mobile phone is so thin that it is not practical to configure a zoom lens that needs to be stretched back and forth. So how is the zoom effect of the mobile phone achieved? Double shot! Generally, the telephoto and short focal length are used together, and the zoom effect we see is often that two cameras are used together, and the algorithm realized by software can work. Therefore, the zoom speed and zoom effect on mobile phones can show the hard power of mobile phone developers. Ahem, I didn't criticize some mobile phones ~

Nowadays, in order to facilitate everyone to shoot, mobile phones often add metering and focusing, which can be done by touching (unclear places). In addition, the user can lock the focal length by long press, thus achieving different focal length effects. The following two pictures are two pictures with different focal lengths, different parts and different artistic conception.

Next, let's talk about diffusion circle and depth of field effect.

Before and after focusing, light from gathering to spreading, and the image of a point from circle to point (focusing) and then spreading to circle. The circles before and after this focus are called diffusion circles, and the effect is shown below.

Let's continue to mention the concept of depth of field here. No precise definition was found on the Internet. As far as I know, the depth of field is:: the distance between objects that can be clearly imaged in the camera::. That is to say, when other conditions are fixed, the distance that can be clearly imaged when moving back and forth.

(1) The larger the aperture, the smaller the depth of field; The smaller the aperture, the greater the depth of field.

(2) The longer the focal length of the lens, the smaller the depth of field; The shorter the focal length, the greater the depth of field.

(3) The longer the object distance, the greater the depth of field; The shorter the object distance, the smaller the depth of field.

As mentioned above, the three elements of the camera, so what kind of purpose can be achieved through these three elements? The concept of exposure needs to be introduced here. If the photo is too dark, the user can't see the subject clearly. On the contrary, if the photo is too white, the user can't see the subject clearly. The correct way to capture light and shadow is that the subject can be clearly presented, which is exposure. The ideal exposure is:: 18% gray scale::.

18% ash does not refer to color ash, but refers to the reflectivity of 18 degree ash. In the design shooting, the reflectivity of the object close to black is judged as underexposed, and the camera will increase your exposure; When the reflectivity is too close to white, it will be judged as overexposure and the camera will automatically reduce the exposure.

When shooting people in the snow scene, the camera thinks that the scene is too white, which will reduce your exposure and the characters are underexposed. And you shoot people in a black coal yard, and the camera thinks that if the scene is too dark, it will increase your exposure and the characters will be overexposed.

The camera itself cannot distinguish all kinds of light, and the exposure standard is 18%. If the picture reaches this level, it will be considered as correct exposure. Measurement is to make the measurement area reach this standard.

Single camera has more metering modes than mobile phone. Here is a brief introduction.

The principle of average photometry is to divide the picture into several areas, each area is measured independently, and then the average photometry value of the whole picture is calculated according to this.

In the case of sufficient light source, it is very suitable for shooting atmospheric scenery and people.

Although the overall situation is considered, the central area will be weighted and highlighted. This is very useful when shooting faces and highlighting the center.

Let's look at three groups of pictures, which are the photos taken under average metering, central key metering and spot metering.

The following video was shot on Huawei Honor. The shutter and aperture of the mobile phone are fixed. With the change of metering area, the camera can only adjust the ISO sensitivity to make the metering area reach 18% gray level.

Here is a video demonstration. It is really troublesome to upload videos on WeChat official account. Interested children's shoes, please contact WeChat. ...

What looks very advanced is actually a:: grammatical sugar::. Nominally, exposure compensation is a separate function provided by the camera, which can brighten and darken photos. The greater the exposure compensation, the brighter the picture, and vice versa.

I took two pictures below, which were taken under different exposure compensation conditions. It is obvious that the colors of the two pictures are obviously different. But we didn't see the data related to exposure compensation in the shutter information. Is Google Pixel lazy? In fact, exposure compensation is just a grammatical candy. The camera will dynamically adjust the shutter+aperture+sensitivity according to your exposure requirements, achieving a shiny effect. Therefore, there can be no exposure compensation in the shutter information, and it will be easier to misunderstand if there is exposure compensation. Is the shutter and sensitivity information before or after compensation?

The shutter information of the two pictures is f1.81125 4.46mm ISO118, f1.81. I personally guess that on mobile phones, adjusting exposure compensation is generally achieved by controlling sensitivity.

A simple video was recorded to show the change of exposure compensation in the photo.

Here is a video demonstration. It is really troublesome to upload videos on WeChat official account. Interested children's shoes, please contact WeChat. ...

Imagine us sitting by the fire. At first, the temperature was low and the color of the fire was red. Then the temperature gradually increased and the fire gradually turned blue. It seems that the color indicates that the color value is related to the temperature of the problem. According to Wikipedia, when a standard black body is heated and the temperature rises to a certain extent, the color changes, which is called color temperature. Because the standard black body is heated, the color will change when it is defined, so the color temperature is expressed in Kelvin unit (K) here.

The higher the color temperature, the bluer the color, and the redder it is anyway.

The picture below comes from the white balance of photography-Zhihu. The following two pictures are high color temperature (cloudy day) and low color temperature (dusk). On cloudy days, cold light refracts more and the color is blue, so the color temperature is higher. At dusk, there will be more warm light, yellow and lower color temperature.

At this point, let's continue to explain the white balance ~ our eyes can make up various color temperatures by themselves, but the camera can't. We can't tell which light source we are shooting under. Then tell the camera what kind of light source it is currently in, and the camera corrects the color temperature, which is white balance.

The following video demonstrates the video of Samsung Note8 setting white balance. In the video, we can see that the smaller the k value, the bluer the displayed video. (Here, the value of k tells the camera the current color temperature. The smaller the value of k, the more compensation in the direction of high color temperature, so the picture will be blue. )

Here is a video demonstration. It is really troublesome to upload videos on WeChat official account. Interested children's shoes, please contact WeChat. ...

High dynamic range, high dynamic range sounds metaphysical, but it's not complicated. HDR mode is actually not a shooting technology, but a post-technology.

When we are taking pictures, it is often difficult for beginners to cope with the complex lighting environment. For example, backlight shooting, night scene shooting and so on. It's easy to take overexposed or dark photos. Is there any stupid way? Yes, the answer is the subtitle Duck -HDR.

There is something wrong with the above sentence. HDR mode, through the fusion of multiple photos, can achieve the rain and dew effect that natural light can't achieve.

The above two pictures were taken with Huawei Honor P20 mobile phone. They shot with HDR mode enabled and HDR mode disabled respectively.

Now the mobile phone is very powerful. Unlike the movie era, we can easily take a photo with the right exposure. The background knowledge and tips mentioned above are only auxiliary, and taking pictures will always be your light and shadow perspective there.

Finally, the four elements of a real camera are gradually thrown out.

Let's see what we can do with our previous knowledge.

I wish you all happy shooting, happy shooting ~

Scatter flowers! ! !