What is exposure? What are the classifications of photographic exposure? How to achieve accurate exposure?

Exposure is a photochemical or photoelectric physical process in which an imaging carrier is irradiated with light to form an image. Exposure needs to be done under the premise of complete metering. Exposure control is the basic skill of every photographer, and it is related to the success or failure of a photographic work.

According to the exposure equipment, exposure can be divided into:

• Film exposure. The so-called film exposure is a photochemical reaction process in which the reflected light from the scene is converged and projected onto the film through the lens to form a latent image.

• Photographic paper exposure: the photochemical reaction process of condensing artificial light through the negative film with the lens of the magnifier and forming a latent image on the photo paper.

• Sensor exposure: The reflected light from the scene is converged and projected onto the sensor through the lens to form the photoelectric physical change process of the digital image.

According to the degree of control of camera exposure, exposure is divided into:

• Fully automatic exposure. Also known as fully automatic shooting mode, this mode does not require photographers to set up.

• Program automatic exposure. Also known as program priority automatic exposure, it is a mode of automatic exposure with a program preset by the camera.

• Aperture priority automatic exposure. Also known as aperture priority shooting mode. The photographer independently sets the aperture size, and the camera automatically configures the automatic exposure mode corresponding to the shutter speed. This kind of aperture manual, shutter automatic aperture priority automatic exposure, is actually a semi-automatic shooting mode. Aperture priority is suitable for still-life subjects that require depth of field.

• Shutter priority automatic exposure, also known as shutter priority shooting mode. The photographer sets the shutter speed independently, and the camera automatically configures the automatic exposure mode corresponding to the aperture size. Shutter priority is suitable for moving subjects that prioritize speed requirements.

• Manual exposure, also known as manual shooting mode. The photographer completely sets the aperture size and shutter speed exposure mode independently. Manual exposure is suitable for shooting still or relatively still objects, such as still life, architecture, landscapes, etc.; it is not suitable for shooting moving objects, such as sports, documentary, and children's subjects.

• Bulb exposure, also known as slow exposure, a type of slow exposure. An exposure mode that continues to press the shutter button fully to start continuous exposure, and raises the shutter button to stop exposure.

• T gate exposure, also known as slow gate exposure. A kind of slow door. An exposure mode in which the shutter is fully pressed for the first time to start exposure, and the shutter is fully pressed for the second time to stop exposure. T gates are mostly used for shutters between medium and large format mirrors. The T gate is a hand-controlled long-time exposure shutter position, and the T mark is often used.

• Automatic depth of field automatic exposure, also known as depth-of-field priority automatic exposure. The camera automatically sets the exposure mode for the maximum depth of field.

• Auto Exposure Bracketing, also known as Step Exposure, Bracket Exposure, Jump Exposure, Multi-level Exposure. The camera function for continuous shooting of three or five shots of the same scene by increasing or decreasing the exposure.

• Prioritize sensitivity. Set the sensitivity value manually, the camera automatically configures the camera function or mode of the aperture or shutter value.

There are many factors that affect exposure. The main objective factors that affect exposure are: imaging carrier sensitivity, light source illuminance and scene brightness. The secondary objective factors that affect exposure are: the direction of the light, the distance between the light source and the scene, and the color temperature of the light.

The subjective factors that affect exposure are: the combination of aperture value and shutter value and sensitivity. The size of the exposure is determined by the product of the amount of light irradiated on the imaging carrier and the exposure time. EV (Exposure Value) represents the exposure value, which is determined by the brightness of the scene and the set sensitivity, and represents the combined value of the aperture and shutter based on the set sensitivity.

Note that exposure and EV value are two different concepts. Exposure is related to level of detail, and level of detail is related to image success or failure.

• How to achieve exposure control? The so-called exposure control is the process of controlling the amount of exposure by adjusting the light source illuminance, the brightness of the scene, the sensitivity of the film or photoreceptor, the shutter time of the camera, the size of the aperture, etc. The first purpose of exposure control is to match the exposure combination with the brightness of the scene to perform correct exposure, thereby reproducing the original appearance of the scene. The key to exposure control is to match the dynamic range of sensitivity, the light ratio of the film and the light ratio of the subject. Specifically, when the brightness of the scene is high, use a small aperture and high speed to reduce the exposure; when the brightness of the scene is low, use a large aperture and low speed to increase the exposure. The correct exposure can also be achieved by controlling the camera sensitivity and the light ratio of the shooting environment. For example, when shooting people, the light reflected by the reflector can be used to increase the exposure on the people's face.

• In theory, there is only one most accurate exposure under the same light conditions. But from the perspective of artistic personality, under the same light conditions, there are as many suitable exposures as there are photographers.