What is a diaphragm?

The diaphragm is simple. In a nutshell, aperture is a device in the lens that measures the amount of light.

Aperture device in the lens Nikon Nikkor 105mm 1: 1.8 (AI-S)

For a better understanding of the operation of such a device, I will give an example from life. When people look at the sun, they squint their eyes, that is, they reduce the gap through which light passes. If people did not squint, the sun would burn the retina with its strong light. At night, you need to do the opposite - open your eyes wider to capture more light, while the pupils also dilate. Eyes with large pupils have many animals that need to see well at night.

Often the diaphragm is also called 'luminous efficiency ' or 'aperture' or 'relative hole' or 'number F'. These concepts are strongly related to each other and are synonymous for many photographers. But among them there are small differences, described below.

Lens aperture Is the ratio of the effective aperture of the lens to the focal length of the lens. The reciprocal of the relative aperture is called f-number or aperture number.

The relative aperture of the lens is expressed numerically by ratio or fraction. For example, take a lens that has a relative aperture 16 times smaller than its focal length; as a result, the relative aperture can be numerically written in the following ways: 1:16 or f1 / 16 or f = 1: 16 or F 1:16, etc. d. There is no particular difference in the recording, and every photographer will always understand what is at stake.

If we take the number opposite to the relative aperture, then we get the number of aperture. Usually it is by this number that photographers directly understand the general term 'aperture'... If we take the same lens, which has a relative aperture 16 times smaller than its focal length, then its aperture number will be equal to 16. And numerically it can be written in the following ways: F16, F / 16, 16 (such a 'bare' number aperture is indicated on the lens barrel). There is no particular difference in recording.

Some lenses have an aperture ring on their body. There is usually a marking on the ring, consisting solely of aperture numbers (shown in the figure below). Almost all modern lenses do not have such a ring, and the diaphragm is controlled by the electronics and camera controls.

Lens aperture control Nikon ED AF Nikkor 80-200mm 1: 2.8D (MKII). Using the ring you can set the values ​​F / 2.8, F / 4, F / 5.6, F / 8, F / 11, F / 16, F / 22.

Usually the concept of 'aperture' and 'aperture' are synonymous, but in fact there is a certain sacristy between them. So, the diaphragm is only responsible for geometric aperture (the ratio of linear geometric indicators). And not only the aperture is responsible for the overall 'real aperture' of the lens, but also many other factors: the optical design of the lens, the percentage of light reflection and transmission by the lens, the drop in the aperture number when focusing at different distances, the percentage of light absorption by the photo filter, etc. More details about the difference between the concepts of 'aperture' and 'aperture' can be found in the section about 'T-feet'.

The aperture is sometimes also called the 'Lens Aperture' (Latin 'Apertura' - 'Hole'). Therefore, on many cameras, the metering mode exposure с aperture priority called 'A' or 'AV'-'Aperture Value '-' Aperture Value '. Details about this mode are described in the section 'P, A (AV), S (TV), M'.

Please note that the magnitude of the front lens of the lens and, in fact, the magnitude of the front light filter have no direct relation to the aperture ratio of the lens. Different lenses with the same focal length and the same maximum aperture can have completely different diameters for their front lenses. For example, take two class 50 mm F / 1.4 lenses: Nikon AF Nikkor 50mm 1: 1.4D и Sigma 50mm 1: 1.4 DG HSM EX... The first has a tiny filter diameter - 52 mm, the second has a huge one - 77 mm. But their aperture (practically - maximum aperture) will be the same.

What is the diaphragm?

By the mechanical part of the diaphragm device is understood a changing circular hole in the lens. Typically, the hole opens and closes with the petals. In this case, the petals are called diaphragm blades, and the diaphragm itself is 'iris' (from the English 'iris' - 'iris'). The number and roundness of the aperture blades determines how much the hole will be formed round. The stronger the rounding of the diaphragm opening, the better. Professionals often refer to the diaphragm simply as'hole'since it is real, a kind of hole that changes its size and doses the amount of light.

What the diaphragm affects:

1. The amount of light that the lens can let through over time.
2. To control the depth of sharply depicted space (GRIP)
3. The brightness of the image in the optical viewfinder
4. On image quality, especially on its sharpness, aberration, vignetting, bokeh and various visual effects.

IMF impact

As it turned out, the aperture affects not only the amount of light, but also the depth of field. The smaller the number F, the smaller the depth of field. The larger the F number, the greater the depth of field. This is one of the main techniques in photography to control the point of attention in the photo. It is very important to be able to manage GRIP for portraits where you need to focus on a person. Macro photographers are well aware what is DOF, they have to shoot on very tightly closed apertures to increase the depth of field. In general, where write about DOFwrite about blurred background. You can read the best way to take pictures with a blurry background in my article - Taking Pictures with Blurred Background.

Blurring the background at different apertures

Depth of field preview

Typically, modern cameras have the ability to focus with a fully open aperture. When a picture is taken, the camera’s automation closes the iris to the set value. To see how the images will look when the aperture is closed, you can sometimes use the aperture repeater. This allows you to look into the viewfinder (optical or electronic) without a picture how the picture will look when the camera closes the iris. You can read more about depth of field preview.

Aperture for picture enhancement

Aperture is understood to mean simply changing the aperture value. Using aperture control, you can achieve a sharper image from the lens. Basically, the sharpest image is achieved somewhere at the average aperture of a lens. At their largest aperture, lenses suffer from chromatic aberrations and vignetting. When closing the diaphragm HA and vignetting practically disappears. At very small apertures, lenses suffer from diffraction loss of sharpness. Also, when you close (decrease the aperture), not only the sharpness increases, but also the contrast of the picture. A large aperture allows sighting through the optical viewfinder without any problems, since the lens gives a lot of light and the entire frame is clearly visible through the peephole. You can only view with an aperture below F5.6 through the optical viewfinder in good lighting conditions. Also, pictures with a larger aperture can appear brighter and more saturated - this effect is associated with smoother transitions in pictures from dark to light areas.

Bokeh and aperture connected forever

Aperture greatly affects the bokeh pattern. Usually the best bokeh for the lens is achieved at the maximum open aperture. In this case, the physical hole itself is as round as possible. When closing the diaphragm, the diaphragm petals instead of a circle form different polyhedrons. These polyhedra are clearly visible in the blur zone. Very often such polyhedra are called nuts, washers and circular saws.

Since in cheap lenses there is a small number of aperture blades, usually no more than 5-6, then in the blur zone figures appear exactly like “nuts”. Those lenses that on closed apertures give the correct round luminous spots in the blur zone, for example, can be attributed to them. Nikon AF DC-Nikkor 105mm 1: 2 D Defocus Image Control or Tair-11A 2,8 / 135. In new lenses, it is very rare to find a large number of aperture blades, but now they make more rounded blades, which, even with a small number of them, give a round hole.

Below are my photographs taken with different cameras and lenses and taken at different values ​​of the number F. Shooting options (EXIF) for each photo are indicated in the bottom line.

Aperture in phone cameras and other small devices

The diaphragm is a mechanical part of the lens, it cannot be done programmatically. Almost all phones lack a physical diaphragm device. Many 'soap dishes' also lack a diaphragm. How to be? Usually the camera in such devices doses the amount of light only shutter speed and a variation of the ISO value, and the aperture value itself is constantly fixed at the maximum value. For example, on my Nokia 7610 it is indicated that F2.8, because the camera always shoots at F2.8.

How to adjust the aperture in the camera?

In cameras, it is responsible for the diaphragm F number (aperture number)... It shows how many times the diameter of the relative aperture is less than the focal length of the lens, on the lens it is written as f1 / 1.4 or f1 / 5.6, sometimes you can find the spelling f = 1: 6.3 or 1: 5.6, or f / 16, f / 3.2. Often, lenses or cameras only have one f-number, such as' 1.4 'or '16.0 .8.0'. Usually, the aperture number is written with a large letter 'F' without fractions, for example, F 1, and the relative aperture is more often written through a small letter 'f', for example f 11:XNUMX (there can be any spellings). The easiest way to adjust the aperture is by putting the camera in aperture priority mode. On the main control wheel of the camera, or in the menu of the camera, this mode is indicated by 'A' or 'AV'. To make it easy to remember, you can simply say: Aperture means you need to turn on the 'A' mode. Details about the creative aperture priority mode are written here.

'Light' and 'dark', 'fast' and 'slow' lenses

The maximum aperture value determines how much the lens can be used in poor lighting conditions. Lenses with a large aperture are called 'bright' or 'bright', usually an F value should be below 2.8. That is, lenses with maximum apertures F1.4, F1.8, F2.0, F2.2, F2.5, F2.8 are called fast or just bright. Everything below F1.4 is called super fast. Super fast lenses include Nikon 50mm f / 1.2 AI-S Nikkor or Canon Lens FD 55mm f / 1.2 SSC. Lenses that have an aperture value of F / 2.8 to F / 5.6 are called ordinary. medium-aperture lenses, these lenses can be attributed Nikon 24-85mm f / 2.8-4DAFIF Nikkor or Nikon 300mm f / 4.5 Nikkor-H Nippon Kogaku Japan Auto Non-AI. Lenses with a maximum aperture less than F / 5.6 are called low-aperture or 'dark'. These lenses include MS MTO-11 1000mm F10.0. By the way, it is very difficult to make a fast zoom, in more detail here.

Different holes for different values ​​of the number F

Since aperture affects speed excerpts, then the lenses are still divided into fast and slow. A fast lens means that you can use it to shoot an image with a short shutter speed (with 'fast' shutter speed). And under slow, that it can be used to take a photo with a long ('slow' shutter speed). If you fix the ISO value, it depends on the aperture excerpt, and the brighter the lens, the faster it is. And the darker the lens, the slower it is.

The difference in aperture ratio

The difference in aperture and other photographic variables is usually measured in feet. When changing the aperture by one stop excerpt will change in twice... Also, if you change the aperture by one stop, you can change the ISO by two instead of the shutter speed. It is very important to note that the difference in aperture values ​​is not linear, but quadratic. Take two apertures F / 5.6 and F / 2.8, it would seem that the difference in geometric aperture is 5.6 / 2.8 = 2 times, but this is not true. On aperture affects the area of ​​the circle formed by the diaphragm, and not its diameter. The number F is associated only with the diameter. To calculate the difference in area you need to take the squares of the diameters. Therefore, it turns out that the difference in aperture ratio between F / 5.6 and F / 2.8 is (5,6 * 5,6) / (2,8 * 2,8) = 4 times. Here is such a trick. How to remember this? There are two ways out, either by dividing the squares of the F numbers, or by first dividing the F numbers and then squaring the result. Why am I bored with calculations - but because often amateur photographers have no idea how many times one lens is 'lighter' or 'darker' than another lens.

Also, experienced photographers know about the so-called aperture series of numbers, in which every two adjacent numbers F differ by one stop.

A number of numbers F: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 46, etc.

Golden Rule:

Aperture and shutter speed are bound by the golden rule. To keep correct exposure with the same ISO, you must either close the aperture and increase the shutter speed, or, conversely, open the aperture and reduce the shutter speed.

Close, open, increase decrease - no need to be confused

Everything is very simple. Closing or decreasing the aperture means increasing the F number. The aperture was F2.8, when it was closed, it became F5.6, it was closed even stronger, it became F16.0, etc. For example, there is the phrase 'covered the hole by two stops', it is deciphered as follows: 'made the number F large and reduced the area of ​​the hole by 4 times'. The main thing is not to get confused, when the aperture opens, the F number decreases. And when the diaphragm closes, the F-number increases. For example, the aperture was F32.0, when it was opened, it became F8.0, when it was opened even stronger, it became F5.6.

What to do - nothing is clear

If you have a DSLR, turn the camera backwards so that you are looking into the lens, press the shutter button (take a picture) and you will see the hole in the lens close and open - this is how the aperture works. If you peered into your lens and did not see anything, then below is a slow-motion video, where you can clearly see how the aperture works during shooting. In the video, the petals close to F / 16 and form a very 'small hole':

1. The method of operation of the device aperture on Nikon digital SLR cameras and its effect on video recording
2. Nikon 'E' Lenses with Electromagnetic Iris Control
3. An interesting aperture on Nikon digital SLR cameras
4. G-type and Non-G type lenses (with aperture ring and without aperture ring)
5. Work with old Nikon lenses such as AI, AI-S, NON-AI, PRE-AI, AI-Converted, which transmit or do not transmit aperture value to the camera

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Conclusions

Diaphragm Is a luminous flux dispenser that affects exposure, GRIP, brightness of the optical viewfinder and image quality. In general, if you don’t shoot at different values ​​of the number F, you don’t really know what it is :)

Material prepared Arkady Shapoval. Training/Consultations | Youtube | Facebook | Instagram | Twitter | Telegram