Diaphragm (optical)

Different openings of the diaphragm

The diaphragm is a device that gives the lens the ability to regulate the amount of light that enters the camera. It is usually a disk or system of fins arranged in the objective of a camera, in such a way that it limits the amount of light that reaches the photosensitive medium in the camera, usually in an adjustable way.

The progressive variations in aperture or pupil of the diaphragm are called aperture, and are specified by the f-number, which is the ratio of the focal length to the effective aperture diameter.

Aperture in photography

Diaphragm of 20 foils within a 150 mm Schneider lens

Naked diaphragm

Evolution

The diaphragm is the part of the camera that determines the size of the opening of the camera towards a light source. In its most elementary form, seen in the oldest cameras, it was nothing more than a fixed-diameter perforated plate. This type of opening is found in the first photographic instruments, called "pinhole cameras". Unfortunately, since this system did not have a built-in lens, to get a sufficiently sharp image, the diameter of the hole did not usually exceed 0.5 mm. Due to the size of the aperture, the camera's exposure time was much longer, ranging from a minimum of 5 seconds to a maximum of a whole day.

Later, a system consisting of a group of perforated plates with different diameters in the shape of a circular disc was adopted, which allowed the aperture to be varied by changing said discs in front of the objective. Finally, the diaphragm evolved towards its current structure, which consists of a set of fins, generally metallic, that move inwards or outwards, forming a polygonal hole with its ends, whose diameter defines the opening value; the most modern use fins with a rounded profile, which produces a more harmonious effect in the bokeh of the resulting image. This new system modernized the world of photography, but at the same time brought some problems, such as the circles of confusion generated by the existence of a lens, thus affecting the depth of field of the image.

Aperture and f-number

The ability that has a goal to let the light pass is called luminosity. One of the ways to indicate this luminosity is by means of the letter f which is defined as the division of the distance between objective and image (which is the focal distance of the objective if we focus on infinity) by the diameter of the effective opening. This relationship leads to a normalized scale in progression 2{displaystyle {sqrt {2}}}: 1 - 1.4 - 2 - 2,8 - 4 - 5,6 - 8 - 11 - 16 - 22 - 32 - 45 - etc. The jump from one value to the next is called step. The minimum value that the number f can have is 0.3. Although this value is unattainable in practice.

Strictly speaking, the focal length is the length at which the optical center (the single node of a thin lens with the same power as the objective) is of the image it forms. When the lens is focused "at infinity" this distance becomes the focal length. Therefore, when focusing at other distances, the luminosity changes, reducing it. This variation can be considered negligible when we focus at long distances. In cinema, this variation is taken into account and two numbers are used to indicate the luminosity: f and t. The f is calculated and is used to determine the depth of field. The t is the f-number theoretically corresponding to the actual measured luminosity, where light absorption by the objective lens is also taken into account. These numbers usually do not differ by more than a third of a step.

Aperture and depth of field

Aperture greatly affects depth of field. The more closed the diaphragm is (higher f-number), the greater the depth of field. The more open it is (smaller f-number) the smaller the depth of field. This behavior is due to the fact that the angle formed by the ends of the diaphragm with the point in the focused plane with a closed f will be a narrow angle and will allow the positioning of the circles of confusion in positions that are further away from the focusing plane both in front and behind. increasing the space that looks appreciably sharp.

If the aperture is wide open (f/2), many more light rays will enter the camera with subject information than the camera can clearly absorb. In this way, more than one point of reality corresponds to a single point of the image formed in the focal plane, thus creating an effect of less sharpness in the majority of the image. It also gets a lot more light.

Today the phenomenon just explained is not considered by many people as a failure, but rather it is looked for in some photographs. Thanks to the circles of confusion, the depth of field decreases, and at the same time the focused space in the image. This fact helps give more importance to the subject in the foreground.

Aperture and sharpness

The diaphragm also has direct consequences on the sharpness of the image. With small apertures (high f-number) diffraction increases, negatively affecting it. With large apertures, greater sharpness is obtained, but in a more limited area, due to the loss of depth of field. Although in theory greater sharpness should be achieved with large apertures, in practice this is not the case, as at larger apertures (small f-number), lens design limitations known as aberrations, especially chromatic aberrations, dominate over wide aperture and sharpness worsens.

For older lenses, a rule of thumb to get good sharpness is to set the aperture to intermediate positions from f/5.6 to f/11, which also provides good depth of field. Newer lenses often have multiple elements and special lenses that correct aberrations, allowing for greater sharpness at lower f-numbers, typically between f/5.6 and f/6.3. The optimal value of the f number for maximum sharpness is a characteristic of each objective that manufacturers do not usually publish, but that can be obtained by testing different apertures. The optimum point of sharpness of a lens is called the “sweet spot”.

Appropriate shutter speed for each focal length

The greater the focal length (focusing on distant objects) the camera seems to move more, so that this movement is not reflected in the photograph, the shutter speed should be increased even more.

In order to know the appropriate speed for each diaphragm so that the photographs do not come out blurred, you just have to remember this rule: the maximum speed at which we will take the photo (without a tripod) will be 1/focal distance that we use. For example, if you are using a 50mm it is not recommended to use speeds lower than 1/60.

When the camera is told not to work automatically, that is, to not calculate the speed and aperture according to its criteria, it can normally be told various options such as working completely manually, giving priority to diaphragm or speed.

Aperture Priority

When the camera is told to work with the aperture-priority semi-auto option, it is being set up so that based on the aperture the photographer chooses, it will calculate the necessary speed and ISO to compensate for the exposure. In this way, the light emitted by the photographed object is the same as that reflected by the camera sensor.

In the case of Canon cameras, this option is given by the letters Av (Aperture Value), while in Nikon we find it in the letter A.

Priority to speed

When the camera is told to work with the speed-priority semi-auto option, it is being set up so that, based on the shutter speed the photographer chooses, it will calculate the appropriate aperture and ISO to compensate for exposure.

In the case of Canon cameras, this option is given by the letters Tv (Time Value), while in Nikon we find it in the letter S.

Relationship between parameters

As we have seen, the amount of light that reaches a camera sensor is determined by three parameters: the aperture, the shutter speed and the ISO. All of these are governed by a series of numerical values.

• Diaphragm opening is scaled by the f/ values, which may vary according to the type of camera target. As a general rule, in each step to the right of this scale half of light is obtained from the preceding value.
• The shutter speed is scaled in seconds and fractions of seconds:

… 15” - 8” - 4” - 2” - 1” - 1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - …

As in the opening of the diaphragm, in each step to the right of this scale, half the light input is obtained with respect to the previous value.

• Sensitivity is scaled according to ISO values.

… 25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400 - 12800 - …

In each step to the right of this scale, that is, by increasing the ISO value, we obtain a greater entry of light with respect to the previous value.

Types of diaphragms

Depending on age or manufacturer, a diaphragm may have fixed or adjustable values. In some cases, such as Waterhouse or rotaries, we can vary its diameter very little or not at all, since they contain an aperture already determined by the manufacturer and type of objective.

Today most diaphragms have adjustable values both manually and automatically. Known as an "iris-type diaphragm", they can be continuously adjusted from minimum to maximum aperture depending on lighting conditions.

These modern diaphragms are called iris because of their resemblance to the human eye. The more light it detects, the more it closes, minimizing its diameter and thus allowing less amount to pass so that the image does not burn or overexpose. On the other hand, in low light conditions, like the human eye, the diaphragm expands proportionally to it, leaving us with a brighter image.