What is the meaning and trial method of EV value?

The meaning and expression of 1 exposure values

EV value is short for "exposure value". It is a digital representation of the required exposure of the film. We know that the same exposure can be composed of multiple exposures, such as1125, f8, 1/250, f5.6, 1/60, f 1 1. This number is the EV value. For ISO 100 film, the exposure of "t 1, f 1" is expressed by EVO (t2, f 1.4, t4, f2, etc. are also expressed by EVO). Based on "t 1, f 1", that is, EVO, the EV value increases1with each step of decreasing exposure; Every step of exposure increases, the EV value decreases by 1.

The value of 2.2. exposure values

One of the purposes of displaying EV value on the light meter is to use some cameras with aperture and shutter speed linkage, which makes it easier to adjust the exposure. This camera adjusts the exposure according to the EV value. When choosing the aperture or shutter speed, the two can be linked to keep the exposure constant. The second purpose is to make the judgment simpler and clearer when it is used to measure the contrast of a scene. Because as long as the difference between the bright and bright EV values is taken as the power of 2, its value is the light ratio. The EV value is also used to indicate the metering capability of the camera metering system. For automatic exposure, this photometric ability means the brightness range that can be used for automatic exposure.

(3) Photometric EV range is a parameter related to the performance of the camera's exposure meter, such as EV2- 16 (F2.8, ISO 21100), which means that the maximum aperture is 2.8 and the film sensitivity is ISO 21100.

(4) The exposure EV value refers to the range from the longest exposure time of the camera shutter with the largest aperture (except door B) to the shortest exposure time of the shutter with the smallest aperture, which reflects the exposure capability of the camera; For a camera with interchangeable lenses, because the maximum and minimum apertures of different lenses are uncertain, it mainly refers to the range of shutter time.

(5) The key difference between these two EV values is that the exposure EV value is often greater than the photometric EV value. Even if the brightness exceeds the range of photometric EV value and the exposure meter is still working, the photometric value obtained at this time is not accurate enough, so pay attention when shooting. For example, when the brightness is 1.4 and the shutter is 15 seconds (at this time, the EV value is -3, which is beyond the working range of the general exposure meter), although we can still see that the exposure meter is working in the viewfinder, the data at this time is not accurate enough. According to experience, the actual exposure should be enclosed in brackets in a large range. It is particularly noteworthy that when the maximum aperture of the lens used is smaller than the aperture value calibrated by the photometric EV value range, for example, when the photometric EV value range is EV2- 16 (F2.8, ISO 100), the maximum aperture value of the lens we use is F4.5 At this time, due to the smaller aperture, the amount of light that can be sensed by the built-in exposure meter will be reduced, which will make. That is to say, the upper and lower limits of the exposure meter do not follow the principle of reciprocity, mainly because the photosensitive range of the photosensitive elements in the exposure meter is limited. Similarly, when the brightness of the subject exceeds the upper limit of the calibrated photometric EV range, the readings displayed by the exposure meter will also lose accuracy.