Like so many other concepts in photography, “law of reciprocity” is a concept you want to run away from. But, although it sounds very complicated, I invite you not to do it 😉 because the reciprocity law in photography it is basic to understand light and how exposure works.
Without light there is no photography and, without mastering it, it is impossible to have good results. Stay with me and I will try to explain it in a very simple way. Although before, I would like to recommend this mega guide on lighting in photography so that you can delve into the subject or refer to it whenever you need to clarify concepts or seek advice on light and photography.
The exposure triangle
we can’t talk about reciprocity law without first talking about the exposure triangle, since the law of reciprocity is closely linked to it.
The exposure triangle It is made up of the three variables that influence the exposure of a photograph: shutter speed, aperture and ISO.
diaphragm opening
Controls the amount of light we let through the lens to the sensor. The aperture of the diaphragm is directly related to the depth of field. At lower aperture values (f/1.4), more light passes through the sensor and less is the area in focus in the image. At higher aperture values (f/22) less light passes into the lens and the larger the area in the image is in focus.
diaphragm opening
shutter speed
Controls the time that the shutter of our camera remains open. At high speeds, the shutter opens and closes more quickly and therefore less light enters the sensor. Conversely, at slow speeds, the shutter stays open longer and therefore lets in more light. In addition, the shutter speed is related to the ability to freeze the movement of the image (high speeds) or that it is imprinted (slow speeds).
Mario explains it clearly in his video:
ISO
It is the sensitivity of the sensor to light. The higher the ISO value, the more sensitive it is and, therefore, the more capable it is of capturing light. On the contrary, the lower the value, the less sensitive it is and the less ability to capture light. The ISO is also associated with the concept of noise. The higher the sensitivity, the more noise in the image, the lower the sensitivity, the images with less noise and, therefore, less noise.
If you have it clear so far, let’s move on to the next point. If not, I recommend that you spend a few minutes on it. It will not only be useful for you to understand the topic that concerns us today, which is the law of reciprocity, but also It is the essential recipe to understand and take photography.
From blurring the background of a portrait, getting a well-focused landscape, freezing a drop of water, or achieving a silky effect… everything is based on these three concepts of the exposure triangle.
What is the law of reciprocity?
Now that we have the concepts of exposition Cooler, let’s get down to business. The law of reciprocity is very simple, it is simply based on the logical principle of compensate exposure or find the balance between all variables to achieve correct exposure when shooting in Manual Mode.
The formula of the law of reciprocity (by way of curiosity) is:
Exposure= Intensity (I) x time (T).
Where the intensity would be the diaphragm opening, and the time the shutter speed.
->Imagine that you want to fill a glass of water. The more you open the faucet, the faster it fills, so you need less time to fill it. The tap opening is our diaphragm opening (or intensity), the time it takes to fill the glass, our shutter speed (or time).
On the contrary, if we open the tap less, we will need more time to fill it to the top. In this relationship so simple between opening and time the law of reciprocity is based.
In short, the longer the tap is open, the less opening we need to fill it, or the larger the tap opening, the less time we need to fill it. Simple, right?
Now a more photographic version through a couple of very simple examples 🙂 :
Example 1: Let’s say I’m taking a portrait and I feel like blur plus the background. what i will do is open the diaphragm to be able to work with less depth of field as we have seen before. So far so good, the “problem” is that if I open the diaphragm I let in more light and therefore my image will look burnt or overexposed.
The law of reciprocity tells us that, logically, we must compensate the exposure to make it correct. In this case, if I’ve let in more light through the aperture, what I need to do is subtract light through one of the other exposure variables I have: shutter speed and/or ISO.
Example 2: Let’s say this time what I want is freeze motion of a falling drop. In this case, I will seek to work at speeds as high as possible. If I increase the speed, as we have seen before, less light will enter the sensor. What should I do in this case? Exactly, compensate by letting in more light through the other exposure variables, in this case: aperture and ISO.
Example 3: I want to photograph a landscape with the maximum sharpness possible, so I am interested in keeping the ISO to minimum. This implies that little light will enter the sensor and that to correct the exposure, therefore, I will have to play with the other two exposure variables, in this case aperture and shutter speed.
Another essential ingredient, the steps in photography
I hope that so far you have the general image of what the law of reciprocity is; It is simply find balance in your exhibition by adding or removing light. Now, to apply it correctly, we need to go a little beyond the general idea, we have to do it through what we know as the steps.
So far, we have been talking about adding or removing light. The amount of that light that is essentially what the steps are. When we talk about going up a step means increasing the light input, and going down a step means decreasing the light input.
Surely this concept sounds familiar to you if you have read it at some point. The steps are the different exposure values. Each step lets in half as much light as the previous one. In the following scheme you can see the complete steps (many cameras offer intermediate values to adjust and refine the exposure more).
If you look at both graphs we see, for example, that in the case of the shutter speed, the step from 1/250s to 1/500s reduces the light input by half and vice versa, if we go from 1/500s to 1 /250s we double the light.
Or, if you look at the aperture steps, going from f/22 to f/16 would mean doubling the light input. Instead, making the step from f/16 to f/22 would mean having half the light.
Practical exercise on the law of reciprocity
There is no better way to gain agility with the steps than practicing, that’s why I suggest you solve the following exercises to go catching him the trick to the law of reciprocity and the steps. The idea is to start from an original exposition that I propose to you, and play as if we were varying different values. You dare? here they go:
Correct original exposure: ISO 100- f/2 -1/500s*
- ISO 100 -f/2.8- ¿1/?
- ISO 100- ¿f/?- 1/1000s
- ISO (?)- f/5.6- 1/500s
- ISO 200- f/4-¿1/?
- ISO 200- f/1.4- ¿1/?
*solutions at the end of the article 😉
By solving them, you will realize that, at exposure level values like these: ISO 400 f/5.6 1/500s and these: ISO 200 f/4 1/500s are the same. In this fact lies the law of reciprocity, in seeking balance in the exhibition by adding or removing light through the steps.
Don’t be in a hurry to answer and take a look at the different step charts. Think about how many steps there are from one value to another, if they have subtracted or added light, and how you could compensate the exposure to leave it at 0 again 😉
Conclusions
If you have just started in photography, you will have finished this article with your head like a hype 😉 it is not for less, because in this article I have talked about the soul of photography in terms of technique. If you master everything that I have discussed in it, everything, everything, everything you want to achieve with an image, you will be able to do it. I’m not exaggerating.
So do not get overwhelmed with all the concepts and try to break them down little by little, understand them well, through this article or the many others that we have on the blog.
Even so, I hope that this explanation has been clear to you and that it has helped you understand somewhat diffuse concepts such as steps, the exposure triangle, and of course, the law of reciprocity 😉
*Solutions:
- 1/250s
- f/1.4
- ISO800
- 1/125s
- 1/500s
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