What is the reciprocity law of aperture and shutter?

Exposure is of course the most critical factor to determine the imaging quality, which is determined by aperture, shutter speed and ISO sensitivity.

If only one parameter is changed alone, the exposure of the photo will definitely change and the picture effect will also change. This is because each of them will have different effects on the picture. For example, the aperture can affect the depth of field, the shutter can determine the shape of moving objects in the picture, and the sensitivity value can directly affect the image quality. If you want to keep the total exposure of the picture unchanged and change an effect of the picture, you need to change one of the three corresponding items. After this change, the values of the other two items will definitely be affected, and at least one of the two items will also change.

If you want to get a small depth of field, you must enlarge the aperture, that is, the overall exposure increases. In order to get the original exposure, you need to increase the shutter or decrease the sensitivity. There are four ways to change the aperture size without changing the exposure. For example, the aperture is f/8, the shutter speed is1125 seconds, and the sensitivity is ISO200. In order to reduce the depth of field and change the aperture to f/4, how to change the shutter speed and sensitivity to keep the original exposure unchanged?

The first method: increase the shutter speed to 1/500 seconds (decrease by two steps), and the sensitivity value remains unchanged.

The second method: keep the shutter speed unchanged and reduce the sensitivity to ISO50 (by two steps).

The third method: increase the shutter speed to 1/250 seconds and reduce the sensitivity to ISO 100 (both items are reduced by one level).

The fourth method: increase the shutter speed to11000 seconds (three steps less) and increase the sensitivity to ISO400 (one step more).

When the total exposure is constant, there is an exchange relationship among aperture, shutter speed and sensitivity, and the changes of first-order aperture, shutter speed or sensitivity have the same influence on exposure. This is the relationship between them, and this is the "reciprocal" relationship in photography.