The diaphragm is one of the three sides of the exposure triangle. It is used to control how much light reaches the sensor. It is measured in f numbers and controls the depth of field of our photographs, that is, the sharp areas of our shot. Y if we know how to take advantage of it, our photographs change.
Mobiles are changing the photographic technique. To begin with, they only have one diaphragm, so everything that we are going to say here can only be applied with mirrorless, SLR and other cameras that we find on the market. The diaphragm allows you to play with depth of field and sharpness. Let’s find out all its secrets.
Before knowing how to use it, we are going to explain in detail what exactly it is, how it works and how mobile phones have been able to do without it (and by the way remember that it is not something so new).
The diaphragm and its properties
It is used to regulate the amount of light that reaches the sensor. Today it is an iris-shaped mechanism -which opens and closes- that we find inside the lenses. In the early days of photography they worked with waterhouse diaphragmswhich were simple metal plates with small holes.
The most prized targets are those with the highest number of blades to get the iris, the diameter, as circular as possible and that influences both the bokeh (which is the quality of the out-of-focus areas) and the diffraction that we will see later.
And do you know where that scale comes from that is so rare for us in Literature? The best answer is found in the old photography books, specifically in the 4th edition of ‘Basic Photography. Introduction to professional photography’ by MJ Langford:
The higher the f-number, the smaller the light beam that will penetrate the lens, and the less luminous the image that will be formed. We now need a useful working series of f-numbers with which to indicate aperture control. The obvious choice will be this series: f1; two; 4; 8; 16; 32… The disadvantage is that each change involves halving the effective aperture, and as we saw, each time the diameter is halved, the illumination is also reduced by a quarter… From the el From a photographic point of view, it would be more useful to have a scale to be able to reduce the lighting by half. This can be done by adding more f numbers to the scale, at intermediate positions. Instead of a two-fold progression, we can increase each f-number by the square root of two, which equals 1.4. In this way, the scale is completed with: f1; 1.4; two; 2.8; 4; 5.6; 8; eleven; 16; 22; 32…
Is is the explanation for one of the recurring nightmares of photography students, and so it is necessary to always refer to the mnemonic rules such as: Small number, lots of light; large number, low light. And thanks to the square root of 2, any lens with an f11 aperture will let through the same amount of information.
What is the diaphragm for?
In addition to letting more or less light through, its main function is control depth of field, and again we have to resort to mnemonic rules: ‘Small number, shallow depth; big number, a lot of depth’.
So we can control the sharpness of our photographs. If we want it to come out clear from the grass we step on to the bottom of the mountains, we have to put a closed diaphragm, an f11. But if we want to isolate a face, we must do the opposite, which is to open the diaphragm, like an f4.
With the control of light and depth of field we have many possibilities. In the first place, to achieve a perfect exposure with the absolute control of the exposure triangle, in which we have three possibilities to obtain a perfect histogram, which touches both extremes and allows us a perfect reproduction of the lights, the shadows and the midtones. , depending on the existing light.
In second place, allows us to be creative when choosing the depth of field. Focusing the entire field is not the same as leaving only a part of it sharp. The final message will be totally different.
The only problem is that if we go to the extremes that the lens offers us, three problems appear that can be more or less serious depending on the quality of the lens and its relationship with the camera sensor.
As simple as that. An f2 aperture has less depth of field than an f8 aperture. We just have to adjust one end or the other. But it’s not like that. Opening the diaphragm a lot, letting a lot of light through, has its consequences. And closing it down a lot, setting an iris to a large number, causes other problems.
Diffraction, loss of sharpness and vignetting
To understand these issues, we need to understand how light passes through objective lenses and how the diaphragm influences them.
If we put an open diaphragm, the maximum of the lens, light will come from the entire diameter of the front lens, which is curved. Light, which is always transmitted in a straight line, has to change direction when hitting the converging plane of the lens. Therefore, the light that arrives from the ends travels more distance than the light that passes through the center. For this reason we only see a part in focus.
With the diaphragm closed, we only take advantage of the central part of the lens, and the light now travels, more or less, the same distance. Y so increase the depth of field.
If everything were so simple it would be wonderful, but the problems arise:
- Opening the diaphragm we have more vignettingdue to the greater distance that the light travels in the corners and less sharpness, due to the size of the aperture.
- closing it we will see the diffraction. As the light passes through such a small opening, the light rays that touch the diaphragm distort the light and we lose sharpness.
These problems they can be solved with good optical designs and especially through software. But it is better not to risk it if it is not necessary. The most advisable thing is to always use the medium diaphragms, in order to achieve the highest possible quality.
But the most important thing is to have the photograph you want, so we have to take risks or bet on the best lenses we can afford, rather than on a more expensive camera.
The problem of mobile photography and the diaphragm
All these games that we have seen here are impossible to apply in mobile photography. With exceptions, the diaphragm is fixed in phones. For two reasons, to compensate for the small size of the sensors and to avoid one more element that would increase the thickness of the terminal.
There is no choice but go yes or yes to computational photography and forget about the laws of optics. The diaphragm is open, but due to the small size of the sensors, we have a good depth of field. If we want to isolate a person we can only do it with a filter or after effect.
In this sense, mobile phones are like the first cameras that came onto the market. A fixed diaphragm and ready. It could only be photographed in good light. For many it may be a limitation, but we cannot forget that Stephen Shore did wonders with his Mickey Mouse camera.
Surely, over time, and with the larger size of the sensors, mobile phones will have diaphragms and computational photography will be one more resource, not the only option. So it is not convenient to forget everything that we have read here so that it does not catch us by surprise in the future.
George is Digismak’s reported cum editor with 13 years of experience in Journalism