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What is a diaphragm?

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, reduce the gap through which the light passes. If people did not squint, the sun would burn the retina of the eye with its strong light. At night you need to do the opposite – open your eyes wider to capture more light, while the pupils also expand. Eyes with large pupils have many animals that need to be seen well at night.

Often, the diaphragm is also called the ‘aperture’ or ‘aperture’ or ‘relative aperture’ or ‘number F‘. These concepts are strongly interconnected and for many photographers are synonymous. But among them there are some minor differences, described below.

The relative aperture of the lens 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 the f-number or the f-number.

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

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 you take the same lens, which has a relative aperture set 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 indicated on lens housing). There is no particular difference in the record.

Some lenses have an aperture ring on their body. On the ring there is usually a markup consisting solely of the diaphragm numbers (shown in the figure below). Virtually all modern lenses do not have such a ring, and the diaphragm controls are driven by electronics and camera controls.

Aperture control ring on the lens Nikon ED AF Nikkor 80-200mm 1: 2.8D (MKII)
Ring aperture control on the lens 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 “luminosity’ and ‘diaphragm’ are synonymous, but in reality there is a certain sacristy between them. Thus, the diaphragm is responsible only for the geometrical luminosity (the ratio of linear geometric indices). And not only the aperture, but also many other factors are responsible for the overall “true aperture” of the lens: the lens’s optical scheme, the percentage of reflection and light transmission by the lens, the fall of the aperture number when focusing at different distances, the percentage of light absorbed by the photo filter, etc. Details about the difference between the concepts of ‘diaphragm’ and ‘aperture’ can be found in the section on ‘T-stops‘.

The diaphragm is sometimes also called the объектив Lens aperture ’(Latin‘ Apertura ’-‘ Hole ’). Therefore, on many cameras, the aperture-priority metering mode is called ‘A‘ or ‘AV‘ – ‘Aperture Value’ – ‘Aperture Value’. Details on this mode are described in ‘P, A (AV), S (TV), M‘.

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

What is it, the diaphragm?
Under the mechanical part of the device diaphragm understand the changing round hole in the lens. Usually the hole opens and closes with the help of petals. In this case, petals are called petals of the diaphragm, and the diaphragm itself is called ‘iris’ (from English английского iris ’-‘ iris of the eye ’). The number and roundness of the diaphragm petals determine how round the hole will be. The rounding of the orifice plate is stronger, the better. Professionals often call a diaphragm simply ‘a hole‘, as this is a real, a kind of hole that changes its size and dispenses the amount of light.

What affects the diaphragm:
On the amount of light that a lens may miss for a while.
To control the depth of sharply depicted space (DOF)
On the brightness of the image in the optical viewfinder
On image quality, especially on its sharpness, aberration, vignetting, bokeh and various visual effects.

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