Photography starts with mobile phones (06) Basic exposure theory

Photography starts with mobile phones-content

1. Mobile phone is the best entry tool for photography.

2. The difference between mobile phone and SLR camera photography

3. What is a good photo?

4. Talking about the art of photography

5. Key factors affecting mobile phone photo taking

6. Basic theory of exposure

7. Understand photographic composition

8. Master the composition of portrait photography

9. Composition of actual combat scenes

The brightness of the photo is determined by the total amount of light received by the photosensitive element, that is, the exposure. Generally speaking, the correct exposure is the exposure when the brightness of the photo reaches the photographer's intention. If it is brighter than the correct exposure, we are overexposed; if it is darker than the correct exposure, we are underexposed.

Usually, the exposure of camera or mobile phone is accurate, and sometimes the scene is complicated and there will be deviations. Later, we can adjust by increasing or decreasing the exposure.

When taking pictures with the camera, you can hear a click after pressing the camera button. This is the sound made when the shutter moves (the mobile phone is an analog sound). So, what is a shutter? Literally means "shutter". Let's talk about traditional SLR cameras first.

This is a dynamic picture of the shutter of a traditional SLR camera.

The shutter is located between the mirror and the sensor. Press the photo key, the shutter opens, the light enters the photosensitive element, and soon the shutter closes, and the photosensitive is over. The middle time is usually called shutter time. Generally, it is less than 1 sec, and it is often expressed as reciprocal, such as116s, 1/250s, etc.

Shutter action is different at different speeds.

For example, when shooting the selected windmill, the photos taken at different speeds are very different.

Three photos with different shutter speeds, from left to right, are high-speed, medium-speed and low-speed shutters respectively.

Then, the mechanical shutter structure on the mobile phone is impossible to put down. Therefore, using an electronic shutter on a mobile phone can be understood as "taking a screenshot" directly on the sensor. When the mobile phone takes a picture, it will still make a "click" sound, simulating the sound of a mechanical shutter.

The main function of shutter is to control the exposure time. If you open the shutter for a long time, the camera will get more light; if you open the shutter for a short time, the exposure will be less.

Time solidification method-freezing the instantaneous state of moving objects

When shooting moving objects, such as sports competitions, the most common technique is to seize the wonderful moment of movement. When shooting, adjust the shutter speed to make the exposure time short enough to achieve the effect of time solidification.

24.0mm？ /4.0 1/ 1600 ISO 200

13 1.0mm？ /9.0 shutter 1/500s ISO 200

Slow down the time to solve-show the motion state of moving objects

When shooting a moving object, you can also deliberately slow down the shutter speed and show the motion state and trajectory of the shooting object through the slow shutter. For example, the track of cars at night, the drawing effect of streams, the atomization effect at sea and so on.

16.0mm？ /8.0 shutter for 53 seconds

Seawater at different shutter speeds

4.73mm？ /2.2 ISO 100 high-speed shutter solidifies the moment when the waves wash the beach.

16.0mm？ /9.06ISO 250 low-speed shutter captures the flow track of waves on the beach.

Safety shutter

When the shutter speed is reduced to a certain extent, the stability of the mobile phone or camera cannot be guaranteed when shooting by hand, and the photos taken will be blurred and shaken, so it is necessary to fix the mobile phone or camera on a tripod or stabilizer. Safety shutter refers to the reciprocal of the focal length of a full-frame camera lens. For example, if a focal length of 50mm is used, the safety shutter will choose a shutter speed above 1/50s.

Common representation of shutter speed

Shutter speed is expressed in numbers, and the larger the number, the longer the exposure time. When the shutter speed is lower than 1s, the greater the reciprocal value, the shorter the exposure time.

Common shutter speeds range from 30s to11000s. From slow to fast, they are 30s, 15s, 8s, 4s, 2s, 1s, 1/2s, 1/4s, 1/8s,1/kloc-. The shutter speeds of two adjacent gears are roughly two times different.

Influence of shutter speed on photo effect

Shooting the same scene with different shutter speeds has different effects. For example, when shooting waterfalls, the photos taken by the high-speed shutter are full of vitality, and the photos taken by the low-speed shutter fully show the sense of water flow and create a quiet atmosphere.

Shutter speed 1/400 seconds

Shutter speed 0.5 seconds

Aperture is a device used to control the amount of light entering the photosensitive element of the fuselage through the lens. When the shutter is pressed, the aperture mechanism in the lens will open. The aperture size and shutter speed * * * both determine how much light the photosensitive element receives, and at the same time * * * also determines the exposure value of digital photos.

Due to space constraints, the CMOS size of mobile phones is smaller than that of cameras such as SLR, so the amount of light entering is less. In order to solve this problem, mobile phone manufacturers will choose large aperture cameras in addition to large-size CMOS. For example, the two cameras behind Huawei Mate 10 all use F 1.6 aperture.

As we know, the aperture value is generally expressed by the f value. The larger the F value, the narrower the opening, the smaller the aperture and the less light entering. Conversely, the smaller the F value, the wider the opening, the larger the aperture and the more light entering.

Different aperture sizes in SLR lenses

In addition to the amount of light, the aperture value also affects the background blur effect of mobile phone manufacturers who like to make an issue. Theoretically, the larger the aperture value, the more obvious the blurring effect of the background, and the smaller the aperture value, the smaller the blurring effect of the background.

When shooting at full aperture, the focus range is narrowed, and the background is effectively blurred as a result. Manual exposure (F 1.4,1160)

However, it should be pointed out that unlike cameras, the focal length of mobile phone lenses is often short, so the depth of field created by taking macro photos only with cameras is usually weak. Therefore, the aperture value of the mobile phone camera does not completely correspond to the aperture value of the camera lens.

Take the mainstream f/2.2 aperture of mobile phones in recent years as an example. Its blur effect is only equivalent to the f/ 13 or f/ 14 aperture of a full-frame camera, so if you only use the lens aperture to take macro photos, the background will only be blurred a little. However, the blurring effect of the so-called comparable SLR, which is the main focus of mobile phone manufacturers, is mostly realized by software algorithms, so there will be embarrassing situations such as incomplete edge tailing and unnatural blurring effect, which is more like adding Gaussian blurring effect after a P-picture. Some mobile phones shoot separately through multiple cameras, and then synthesize by software to imitate the effect of lens blur.

Different from the SLR lens, the mobile phone camera uses an electronic aperture. Its structure is different from the mechanical aperture of traditional lens. It is difficult to set a mechanical device to change the size of the light entrance hole in such a small space of the mobile phone camera. The physical aperture of a mobile phone is a constant circular aperture. If you want to set the aperture size, you can only pass the electronic aperture. Electronic aperture is a software algorithm, in the set shutter time, constantly taking multiple photos, transparent processing, and then synthesis.

The so-called depth of field is the range that can still be clearly seen before and after the focal length is aimed at a certain point. It can decide whether to blur the background to highlight the subject or shoot a clear background. Simply put, a small depth of field can take photos with blurred background, and a large depth of field can take photos with clear background.

When shooting a portrait, you want to blur the background and highlight the portrait, so you need to choose a small depth of field. When shooting scenery, I hope that the far and near scenes are clear and distinguishable, so I want to choose a large depth of field.

In portrait photography, large aperture is used to obtain shallow depth of field, blur background and highlight people. F 1.4,1160seconds

In landscape photography, a small aperture is needed to obtain a large depth of field, so that far and near scenes are clear. F 1 1， 1/320s

There are three factors that determine the depth of field: lens focal length, shooting distance and aperture size.

The relationship between aperture and depth of field is that the larger the aperture, the smaller the depth of field, and the smaller the aperture, the greater the depth of field.

F2.4

F22

Here, the photographer focuses on the alarm clock in front. The top left picture uses a large aperture of f/2, correspondingly, the depth of field is small, and the other two distant alarm clocks are out of focus. In the picture on the right, a lens with the same focal length is used in the same position, but the aperture is changed to a smaller f/ 16. More parts of the transformation became clear. Changing the aperture size is the best way to change the depth of field, because other aspects of the image will not be affected.

In fact, the focal length also affects the depth of field. 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. The shorter the focal length of the lens, the greater the depth of field. The picture on the left below uses a 135mm lens, and only the close-up subject is clear. In the picture on the right, the same position, the same aperture, set the focus at the same position as before. A 50 mm lens will make the subject in the background clearer. Observe the influence of the change of focal length on the viewing angle (the number of scenes contained in the picture) and the size of things in the picture. The clear range of the two photos is obviously different.

The farther the lens is from the subject, the greater the depth of field. In the picture on the left below, the focus distance is about 1 m, and only a narrow range from near to far is clear. In the picture on the right, we use the same focal length and aperture, but retreat to a place 3 meters away from the subject to focus on her again, and more places in the picture become clear. The effect of retreating is similar to that of shortening the focal length. More scenes are displayed in the picture, and more parts of the image become clear.

The focal length of the mobile phone camera is fixed, and the physical focal length is only 4-5mm, so it is difficult for the mobile phone camera to take photos with shallow depth of field. At this time, some skills are needed-try to get close to the subject and let the subject and the background form a certain distance. Although the effect is not so obvious on professional cameras, the large aperture does bring more interesting elements to mobile phone cameras.

The English expression of sensitivity is ISO, and the common values are 50, 100, 200, 400, 800, 1600, 3200, etc. Sensitivity is the sensitivity of photosensitive elements to light.

The relationship among exposure aperture, shutter and ISO can be compared with the example of filling a swimming pool with water. The bottom area of the swimming pool is like the area of the photosensitive element, the aperture is like the water inlet pipe, the shutter is like the water inlet time, and the required exposure is like a swimming pool filled with water.

Assuming that the swimming pool is filled with water, the thicker the water pipe, the shorter the time. Similarly, to complete the exposure required for a photo, the larger the aperture, the shorter the shutter time.

The same aperture, the same shooting scene, the lower the sensitivity, the longer the exposure time and the slower the shutter speed. The higher the sensitivity, the shorter the exposure time and the higher the shutter speed.

ф o200 shutter speed1500 seconds,

Due to the higher sensitivity ISO, a higher shutter speed can be used, so that the moment when the model jumps is fixed in the picture.

Sometimes in dark scenes, in order not to turn on the flash to destroy the atmosphere, high sensitivity is often used to shoot.

F4 1/800/800s ISO 6400

With the increase of sensitivity, the noise of photos not only increases, but also the clarity of details and the color saturation of the picture are seriously affected. The photosensitive element amplifies the charge signal through the signal amplification circuit, and at the same time amplifies the interference signal together, which is the main reason for noise.

iso 100

iso25600

For hand-held shooting in dark conditions, we must first ensure the clarity of the picture, and then consider the loss of image quality caused by high sensitivity.

Color temperature is a measure of temperature. In physics, when a black body is heated to emit light, the color that appears at different temperatures is the color temperature, and the unit is K (Kelvin).

Color temperature values corresponding to different light sources.

Color temperature is widely used in life. When we choose lighting, we choose lighting equipment with different color temperature values according to the color of the light we need.

So what is the use of color temperature in photography? If the color temperature of the shot scene is different from the color temperature set by the camera or mobile phone, the color cast of the shot photo will appear. For example, white objects in the scene (such as white walls, clothes, etc. ) will be cast in the photo. The camera or mobile phone needs to correct the color shift, and this process is white balance.

In general, the camera or mobile phone is set with automatic white balance, which is accurate in most cases. In some scenes, photographers can set manual white balance to achieve different creative effects.

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