How does a digital SLR camera measure light?

The so-called photometry actually refers to the aperture/shutter combination given by a digital camera according to the ambient light system and depending on a specific measurement method. Simply put, it is to measure the light reception of the object being photographed. Generally speaking, photometry mainly measures the brightness of the light reflected by the subject into the lens and then gives a certain combination of aperture shutter speed according to this brightness. This measurement method is also commonly called reflective measurement. According to the different placement positions of metering elements, metering methods can be divided into external metering and internal metering.

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External measurement:

Means that the optical paths of the optical measuring element and the lens are independent of each other to measure light. This metering method is widely used in various paraxial lens shutter cameras. Although there is enough sensitivity and accuracy, there are often deviations because of the different positions and photosensitive methods of the lens and photometric elements. At present, many consumer digital cameras use this metering system for metering.

Internal measurement:

Also known as TTL metering, also known as TTL metering. This metering method generally directly measures the amount of light entering the lens through the lens. Compared with external metering, this metering method can automatically correct the light when changing the phase lens, changing the shooting distance or adding a filter. At present, almost all SLR digital cameras and quasi-professional digital cameras adopt this metering method.

In internal measurement, there are two main schemes for placing metering elements: one is placed near the eyepiece in the framing light path, which is called TTL universal metering; Second, it is placed in the photographic light path, and the light is reflected from the auxiliary reflector or the blade surface of the film plane and focal plane shutter to the metering element for metering. This measurement method is called TTL direct measurement. Generally speaking, TTL universal metering system is similar to the metering system of traditional SLR cameras, with the characteristics of accurate color reproduction and elegant image, while TTL direct metering is mostly used in various consumer-grade quasi-professional cameras. Compared with TTL general photometry, this kind of direct photometry can better neutralize the problem of CCD color tolerance and avoid excessive image color contrast.

No matter which metering scheme is used, a professional digital camera is likely to have multiple metering modes. If these metering modes are classified according to the different areas measured by metering elements within the shooting range, they are mainly point metering, central part metering, central key average metering, average metering mode, multi-zone metering and so on. No matter which metering mode is adopted, the purpose is to hope that the photographer can determine the correct exposure more freely according to the actual environment.