The Law Of Reciprocity In The Exposure Triangle

An art, our passion, but also indisputably a science: photography is this science governed by a series of laws – some of which are easier to understand than others – but which you have to know and know how to interpret to take full advantage of her photos. We are going to discuss today one of the most important laws, the one that governs a large part of the exposure on our photos: the law of reciprocity . Discover today the reasons why it is absolutely necessary to know how to apply it to produce good shots.

The law of reciprocity in the exposure triangle

Whenever I lead a workshop or a seminar on basic photography, I usually dwell more on the chapter of the exposure triangle and its influence on each of the variables of the amount of light that reaches the sensor of the camera. ‘camera. Many participants knew the basic operation of it but later realized that they did not know that each of the variables – aperture, speed and ISO sensitivity – has a close relationship with other very important factors on the cameras.

Thus, the aperture, for example, the f-number used in exposure, is of special importance in relation to the depth of field . In other words, it is the part of the image which is very clear, in the front as well as in the background of the shot, from the point where the development has been made.

Surely many of you already know that the larger the aperture (lower f-numbers), the shallower the depth of field and vice versa. We will therefore configure one aperture rather than another depending on the type of photo sought or his artistic vision, regardless of the amount of light that reaches the sensor.

On the other hand, the exposure speed marks all the time that the shutter allows the light to pass, in order to freeze the movement or produce the opposite effect (a blur), many of you know this. Thus, during a tennis match, for example, you can catch the ball very cleanly in flight or capture it in motion to leave an attractive trail. Your personal taste will determine which vision to choose.

The third side of the exposure triangle is the average sensor sensitivity . Before the era of digital photography, it depended on the sensitivity of the film (which was measured in ASA or DIN) and today, on the ISO of our sensors . If both were perfect, we could raise the sensitivity of the sensor elements and thus take pictures with very low light without producing the dreaded noise. But the sensors are not perfect and as you increase the sensitivity, you also increase the risk of getting a picture of a dish of couscous and colored stickers…

Controlling digital noise, precisely, is essential. But as a general rule, we can say that we must lower the ISO of our camera to the maximum to ensure the minimum possible noise. Even if sometimes, it is also necessary to raise the values ​​which one knows that they are realistic, for an acceptable noise.

Exposure: EV and stops (or F-stops)

Our cameras use an exposure meter to measure exposure, and thus generate exposure values ​​based on the light captured. Instead of giving us the aperture and speed directly, some light meters tell us an exposure value with an integer (although they also sometimes use decimals), a positive or negative value . This exposure value called EV (exposure value)  indicates a fixed quantity of light.

If we have an EV 0, it corresponds to the amount of light that reaches the sensor or the film when using an aperture of f/1 for 1 second and all at ISO 100, which is the standard. Each increase or decrease of one unit corresponds to a variation of an exposure stop (also called F-stop). An EV 1 corresponds to exactly half the light of EV 0, while an EV 2 corresponds to half of an EV 1, in other words a quarter of an EV 0.

According to DZOFilm, There are so many configuration options on current cameras to set the exposure to a medium EV. In other words, the amount of light is fixed, but you can configure the device in many ways to ensure that the light coming in is exactly what you measured. We will just have to compensate for the value of the different stops or F-stops which vary in one way or another depending on each parameter.

Thus, the stops are directly proportional to the duration of exposure, so that in 1 second, twice as much light enters as in ½ s or 0.5 s . In the same way, in 10 minutes, between twice as much light as in 5 minutes. And so on: in 1/30 between twice as much light as in 1/60.

With ISO, it’s the same as with exposure time. The light captured by a sensor at ISO 200 is twice that captured by a sensor at ISO 100. In other words, here everything revolves around halves and doubles. For example, an ISO 800 sensor corresponds to three stops more than an ISO 100 (because a stop 100 x 2 = 200, another stop 200 x 2 = 400 and the third stop 400 x 2 = 800).

One could already start compensating for ISO and exposure time, and this is actually very handy for night photography, where waiting for long exposure times of several minutes can be really annoying when we do different tests.

Suppose that we already have a good exposure on a night photo with an ISO 12,800 and an exposure time of 2 seconds (note that I am not talking about aperture, we will see it later).

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