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Why the larger the F, the smaller the aperture? how to calculate the numbers such as F2\ 2.8\ 4? There are often friends who do not understand the nature of the aperture. Today I will talk about it from the most basic concept and see how the value of the aperture is calculated.

01, the light-entering ability of the lens, let's start with the basic concept: the lens can enter the light, and the amount of light can affect the light and shade of the photo.

People use "relative aperture" to express the amount of light in a lens. The larger the relative aperture, the more light in. Relative aperture = effective aperture / focal length.

Yes, both focal length and aperture affect the amount of light in. Now you know why the aperture of telephoto lenses is so small.

The effective aperture is the diameter of the entry hole in the lens.

If the focal length of a lens is 50mm, the diameter of the largest light hole is 29.5mm, and the calculated value of the maximum relative aperture is 29.5mm / 50mm=1:1.7.

The lens is also marked with the size of the "relative aperture" in the format of "effective aperture: focal length".

For example, 1:2 is written on a lens, which means that the maximum relative aperture of the lens is 1:2, which is a constant aperture lens. Or, for example, the maximum relative aperture of 1RV 2.8 (at shortest focal length) and 1:4 (at maximum focal length).

02. Aperture value F the F value and f / value we usually mentioned are "aperture coefficient". The aperture coefficient is the reciprocal of the relative aperture, and the aperture coefficient = focal length / effective aperture.

If the relative aperture is 1:2, then the aperture coefficient is 2:1, which is 2, usually written as F2 or f / 2.

The larger the F value is, the smaller the aperture is, the smaller the light intake is; the smaller the F value is, the larger the aperture is, the greater the light intake is.

03, aperture value conversion large aperture lens is more popular with users, and the price is higher. If I am a manufacturer, can I write a maximum aperture for the lens at will?

The answer is no. After reading the first two paragraphs, I believe you also understand that any aperture value is calculated.

The common aperture values are F1.4, F2, F2.8, F4, F5.6, F8 and so on. Each of the above-mentioned aperture is adjacent to one gear, and the amount of light input is 2 times.

F1.4 is one gear larger than F2, and the light intake of F1.4 is twice that of F2.

In the actual shooting, the brightness of F1.4, 10s, ISO100 is equal to F2, 10s, ISO200, also equal to F2, 20s, ISO100.

Why is F1.4 twice as much as F2? Why not twice as much as F1 as F2?

The light input of F1.4 is twice that of F2, which means that the area of the incoming circular hole of F1.4 is twice that of F1.4. The area of the circle is π × r ². To double the area of the circle (2 π r ²), it is necessary to change r into 2 times the original √, √ 2 ≈ 1.414. 1. 4 × √ 2 ≈ 2, that's where it comes from.

And so on, 2 × √ 2 ≈ 2.8 × √ 2 ≈ 4 10 4 × √ 2 ≈ 5.6 × √ 2 ≈ 8...

In addition to the aperture of the adjacent first stop, there are also the aperture of the third stop of the adjacent one, which can satisfy the more subtle exposure adjustment, and the conversion relationship between them is × √ 1.3.

When you turn on the camera to adjust the aperture, F5.6 often does not directly become F8, but in turn changes to F6.3, F7.1, F8.

Do you understand?

This article is from the official account of Wechat: photography Research Society (ID:shoujiPhotography), by Zhang Xiaozhang.

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