What's the use of film exposure?

In traditional film cameras, ISO stands for the standard of photosensitive speed; In digital cameras, the definition of ISO is the same as that of film, representing the photosensitive speed of CCD or CMOS photosensitive elements. The higher the ISO value, the stronger the photosensitivity of photosensitive materials. The calculation formula of ISO is S=0.8/H(S sensitivity, h is exposure). As can be seen from the formula, the higher the sensitivity, the lower the requirements for exposure. The exposure speed of ISO 200 film is twice that of ISO 100. That is to say, under the same conditions, the exposure time of ISO 200 film is half that of ISO 100 film. In digital cameras, the number of light sources and the value of picture brightness can be changed by adjusting the equivalent sensitivity. Therefore, the sensitivity has also become a value that indirectly controls the brightness of the picture.

In the traditional 135 film camera, the equivalent photosensitive value is a measure of the sensitivity of the camera film to light, which is usually expressed by ISO number. The larger the number, the stronger the optical rotation. Commonly used expressions are ISO 100, 400, 1000, etc. Generally speaking, the higher the sensitivity, the thicker the film particles and the worse the magnification effect.

The traditional camera itself has no sensitivity, because sensitivity is only a quantitative sign of the sensitivity of photosensitive materials to radiation energy under certain exposure, development and testing conditions. Anyone who has used a traditional camera knows that the most important index of film is sensitivity-in layman's terms, it is to measure how much light is needed to complete accurate exposure of film. We bought100,200,400 films in the camera shop, and the numbers represent the sensitivity. Sensitivity is generally expressed by ISO value. With the increase of this value, the sensitivity of the film to light increases, so that it can be shot in different light. Movies like ISO 100 are most suitable for outdoor shooting on sunny days, while ISO400 movies can be shot indoors or in places with poor light, such as early morning and dusk.

However, because the camera is different from ordinary cameras, its photosensitive device adopts CCD or CMOS, so there are corresponding requirements for exposure, and there are also problems of high and low photosensitive sensitivity. This is equivalent to a certain sensitivity of the film. In order to facilitate the understanding of digital camera users, digital camera manufacturers generally convert the sensitivity of digital camera CCD (or light sensitivity) into the sensitivity of traditional film, so digital cameras have the saying of "considerable sensitivity".

From the point of view of measuring film sensitivity, at present, the sensitivity of digital cameras is distributed in the middle and high speed range, the lowest is ISO50, the highest is ISO6400, and most of them are around ISO 100. For some digital cameras, the sensitivity is single, and the sensitivity tolerance of CCD is very small, which limits their use effect in the case of too strong or too weak light. Other digital cameras have a certain sensitivity range, but even within the allowable range, the shooting effect is different if the sensitivity is set high or low, so we should put the best sensitivity in normal shooting. Like traditional cameras, low ISO values are suitable for creating clear and soft pictures, while high ISO values can compensate for poor lighting environment.

When the light is insufficient, the use of flash is inevitable. However, in some cases, such as exhibition halls or performances, if it is not allowed or convenient to use flash, the brightness of photos can be increased by ISO value. Because of the adjustability of ISO value of digital camera, sometimes we can only reduce the use times of flash by increasing ISO value and increasing exposure compensation. Increasing the ISO value can increase the brightness, but it may also increase the noise of the photo.