Introduction to Photographic Notes _ Understanding Exposure

There is a saying that photography is the art of exposure. In fact, exposure has two meanings:

Exposure control has three elements: aperture, shutter and sensitivity.

Sensitivity, usually called ISO, refers to the sensitivity of a camera to light. The upper and lower limit of the value is 100, and the maximum value can exceed 3 million. Usually the upper limit of a camera is 25600.

The higher the sensitivity, the stronger the camera's ability to capture light, and the more suitable for shooting in dark environment. Usually use low sensitivity during the day and high sensitivity at night. The lower the sensitivity, the better the image quality, but the slower the shutter. The higher the sensitivity, the more noise and the worse the image quality, but a faster shutter can be used.

If the aperture and shutter remain the same, the ISO will double and the exposure brightness of the photo will double. In other words, the improvement of photo brightness and ISO increases linearly.

Shutter speed is the exposure time of the camera, that is, the opening and closing time of the shutter curtain in the camera. Usually expressed in seconds, 1/60 is 1/60 seconds.

The faster the shutter is, the more suitable it is to capture fast-moving objects, such as racing cars and birds. Sometimes, if the photographer is moving himself, at this time, it is also necessary to increase the shutter speed. If the shutter speed is very slow, such as 1s or above, it is called long exposure, which is suitable for shooting night scenes or drawing flowing clouds into filaments (the second meaning of exposure is artistic creation).

Under the condition of constant aperture and sensitivity, the shutter speed is doubled, the photo exposure brightness is reduced by half, the shutter speed is doubled, and the photo exposure brightness is doubled.

In the lens, the aperture is used to control the amount of light passing through. Half is represented by f: F= focal length of lens/effective aperture diameter of lens. On the camera, the aperture is expressed as f/8. The larger the number, the smaller the aperture, and the smaller the number, the larger the aperture. Numerically, the interval of each aperture is 1.4 times (root number 2). The following is a list of apertures:

The aperture is larger and the brightness of the photo is doubled. Other parameters being the same, f/2 is twice as bright as f/2.8.

In addition, the aperture can not only control the luminous flux, but also control the depth, that is, the blur degree of the photo.

From the above introduction, we can know that if we want to get better image quality, we need to use lower sensitivity. If you want to increase the shutter speed without increasing the sensitivity, you need to use a large aperture, especially in dark scenes. This is why lenses with large apertures are more expensive.

Determine the ISO before taking photos, for example, try to use ISO 100 during the day and ISO 3200 or ISO 6400 at night.

Choose the aperture and shutter according to the shooting subject. Use a large aperture when shooting portraits and a small aperture when shooting landscapes. For example, ISO 100, aperture f/8, shutter speed 1/40s. If you want to increase the shutter speed to 1/80s, you should use f/5.6 or adjust ISO from 100 to 200.

Usually when shooting, we don't manually adjust all three exposure parameters. More often, we rely on the camera to complete some automatic settings, and then we make adjustments. There are three camera exposure modes.

Manually set the sensitivity and aperture value, and the camera automatically measures the shutter speed. Suitable for shooting landscapes, portraits, daily shooting, etc. Setting the aperture manually can control the depth of field. When shooting portraits, use a large aperture; Use a small aperture when shooting scenery.

On the camera, the Aperture priority mode is generally marked as file A (the English translation of aperture is aperture).

Manually set the sensitivity and shutter speed, and the camera automatically measures the aperture value. Suitable for shooting sports scenes, such as sports competitions, birds and animals, etc.

On the camera, the Shutter priority mode is generally marked as S file (the English translation of shutter is shutter).

Set the sensitivity manually, and the camera automatically determines the aperture and shutter speed. Shoot complex scenes, such as the stage. In this way, all parameters are controlled by the camera.

On the camera, the automatic mode of the general program is marked as P program.

The camera can automatically determine some exposure parameters. What is the basis of camera settings? The answer is that the camera can measure light. Through photometry, the camera can judge the light of the current shooting environment, which is used as the basis for setting exposure parameters. There are mainly the following measurement modes:

The camera obtains the photometric judgment value according to the photometric mode, and then sets the exposure parameters controlled by the camera according to the specific exposure mode.

Sometimes, the exposure parameters automatically set by the camera can't satisfy us. For example, when the backlight is used, the face area is normal, but the background light is too strong, and the whole photo will be very bright. At this time, adjustments need to be made, such as reducing exposure or increasing exposure. At this time, exposure compensation is needed.

Exposure compensation is usually based on step size, and 1/3 or 1/2 is the unit of increase or decrease. General 1/3 is more common. Generally speaking, exposure compensation is to make photos brighter or darker. Exposure compensation and exposure mode are used in combination.

On the camera, the Exposure compensation is marked with EV (the English translation of exposure is exposure, and the EV here is estimated to be exposure value).

The final effect of exposure, we can directly observe the photos taken through the naked eye. However, sometimes, this method lacks accuracy. At this time, histogram is needed.

Histograms never reflect the exposure of photos. The brightness of photos is divided into 0 to 255***256 values. The higher the value, the higher the brightness. Where 0 represents the darkest area of pure black, 255 represents the brightest pure white, and the middle number is gray with different brightness. People further divide the brightness into five areas, namely black, shadow, midtone, highlight and white.

The horizontal axis represents the brightness value of 0-255. The vertical axis represents the number of pixels corresponding to the brightness in the photo. This function image is called histogram. The height of the column in the histogram represents how many pixels in the picture are that brightness, so we can actually see the distribution and proportion of brightness in the picture. For example, in the histogram below, the peak is to the left in the middle (shaded area), indicating that there are many dark gray or dark parts in the picture.

The correct exposure method is to make the light in the photo balanced, not overexposed or underexposed, that is, neither too bright nor too dark. We should not overexpose high-light parts, such as the sky and architectural lighting, but also try to keep dark parts, such as the ground, from losing details.

Sometimes, you can't do it in a photo, you can use HDR technology. But in general, photos should not be overexposed, and they can be slightly underexposed. The following are descriptions of different shooting themes.

Try to use low ISO (such as ISO 100) and small aperture (such as f/8) in landscape shooting, and use first gear and automatic aperture mode. If you need a long exposure, you can use a ND mirror (reducing the amount of light entering the camera can extend the exposure time). For long exposure, you need to use a tripod (tripod can provide stability and is essential for slow door shooting). The aperture can be reduced, but a small aperture will reduce the image quality.

At the same time, the filter should be used reasonably: ND mirror extends the exposure; CPL mirror eliminates reflection; The gradient mirror can adjust the light ratio between the sky and the ground.

Travel photography, this kind of "a trip here" photo, the key is to take a clear picture of people and scenery. This requires the use of a small aperture, because a large aperture will lead to blurred background. First gear is recommended, and aperture is preferred. Note that the shutter speed should not be too slow, otherwise you can't see clearly at night (because ISO is too low). Consider raising the aperture slightly, but be careful not to blur.

You can choose global metering or spot metering during exposure to ensure that your face is clear as far as possible. When taking such photos, try to avoid scenes with large light ratio.

You must choose a large aperture and a telephoto lens when taking portraits.

Because portrait shooting needs a large aperture, aperture is the first. F/ 1.4, f/ 1.8, etc. Can be used. If a lens is used, you can use the telephoto end of the lens, such as 100mm end and 135mm end. It is preferable to use an aperture.

The metering mode can use evaluation metering or spot metering to meter the face. Try to use the lowest ISO sensitivity. In order to ensure the shutter speed, generally speaking, the shutter speed can be chosen as the reciprocal of the focal length of the lens. For example, for an 85mm lens, the shutter speed should be above 1/80s.

White balance is also important. Generally, you can choose automatic white balance, and you don't need too much saturation contrast when setting colors. Just use the portrait mode (the portrait mode of the camera is specially optimized for portraits to avoid blushing or turning blue).

Night shooting can be divided into small scenes and big scenes.

A small scene is to shoot a part, such as a lantern. This requires the use of a large aperture, which is used for priority exposure, and the sensitivity is controlled according to the shutter speed, as long as the shutter speed is guaranteed to be within the safe shutter range (no virtual shooting, no double shooting shutter range). ISO can also be set to automatic.

Shooting a big scene means shooting a wide range of night scenes. In this case, the key lies in the rational use of exposure time. If we need to shoot by hand, we should open the aperture as large as possible, and then adjust the sensitivity to keep the shutter speed around 1/60s and keep it stable. If it is a long exposure, use a small aperture and tripod to adjust the sensitivity to the lowest ISO 100 (it is not recommended to use the extended sensitivity of 50, just use the original minimum value of 100), and use a small aperture (such as f/ 16) to automatically measure the shutter speed. If it is too slow, you need to manually set the shutter speed with M gear. If the shutter time exceeds 30s, you need to release the cable to manually control the exposure time.

In addition, in the shooting time, the good night photo was taken from sunset to 30 minutes after sunset (30 minutes after sunset).

Starry sky shooting can be divided into starry sky (Milky Way) shooting and stellar orbit shooting.

Starry sky shooting needs to follow the 500 principle, that is, exposure time * focal length.

Shoot the starry sky with high sensitivity (such as ISO6400, ISO 12800, etc. ), maximum aperture (such as f/2.4), wide angle.

When shooting the orbit of a star, it takes a long exposure time, which can be superimposed or single shot.

Using shutter priority mode (TV file), the sensitivity can be automatically selected (especially when the scene changes greatly), and the shutter speed is guaranteed first.

Generally, the shutter speed of shooting running and sports competitions needs to be above 1/500s; It takes more than11000 to shoot birds; It takes more than 1/2000 to shoot a racing car.

In general, if you use the camera's high-speed continuous shooting function to shoot, you should turn on the focus-chasing autofocus. At this time, the focusing performance of the camera is particularly tested.

Portrait (small depth of field), landscape and travel photography generally use one file, with aperture first and shutter not too slow.

When metering, shoot the scenery with global metering; When there are important points, such as portraits, global metering or spot metering should be used.

When shooting, it is easier to deal with it if it is slightly underexposed, but if it is overexposed, the photo will basically be scrapped.