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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.

High-energy warning, the article is dry, drink more water!

01, the light-receiving capacity of the lens, let's start with the basic concept: the lens can take light, when taking pictures, the lens can take light first, and then transmit the light to the CMOS for imaging. The amount of light in the lens can affect the light and shade of the photo.

People use "relative aperture" to express the amount of light in a lens, and the relative aperture is "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 lens is also marked with a "relative aperture", such as 1:2, which means that the maximum relative aperture of this lens is 1:2, which is a constant aperture lens. Or, for example, the maximum relative aperture at the shortest focal length is 2.8, and the maximum relative aperture at the longest focal length is 1:4.

The following picture shows a relative aperture of 1:4-5.6,

02. Aperture value F the F value and f / value we usually see are "aperture coefficient", which is the reciprocal of relative aperture, that is, "focal length / effective aperture".

If the relative aperture is 1v2.8, the corresponding aperture coefficient is F2.8 or f / 2.8, which turns the fraction into an integer, which looks much simpler.

For example, for the standard lens of 50mm, the maximum light diameter is 29.5mm and the calculated value of the maximum relative aperture is 29.5mm / 50mm=1:1.7. The calculated value of the maximum aperture is 50mm / 29.5mm=F1.7. It can also be understood that if the light diameter is set to 1, the focal length corresponds to 1.7.

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. How to calculate the aperture value the common aperture values are F1.4, F2, F2.8, F4, F5.6, F8 and so on. Each aperture is adjacent to one stop and the amount of light input is twice as large.

F1.4 is one gear larger than F2, and the light intake of F1.4 is twice that of F2. In practical application, the brightness of F1.4, 10s, ISO100 is equal to F2, 10s, ISO200 is also equal to F2, 20s, ISO100.

In addition to the adjacent 1 stop of the aperture, there are adjacent 1 step 3 of the aperture, such as F5.6, F6.3, F7.1, F8, the difference in the amount of light input of each stop is 3 times, can be more flexible to control the exposure. If you use shutter priority mode to shoot, ISO also sets a fixed value, only the aperture is automatic by the camera. The current aperture value given by the camera is F6.3. If you increase the exposure value of the 0.3EV with exposure compensation, the camera will set the aperture to F5.6.

So how are numbers like 1.4, 2 and 2.8 calculated?

If we are taking a picture with the 50mm lens above, it is now aperture F1.7. If you want to increase the aperture, it is necessary to change the area of the circle of light to 0.5 times that of the original. The area of the circle is π r ². If the area is doubled (2 π r ²), r will be changed to 2 times the original √, which is about equal to multiplying 1.414.

Therefore, the aperture value becomes 50 / (29.5 × 1.414), and the deformation results in (50 / 29.5 ≈ 1.7) / 1.414. As a result, the aperture values of the adjacent gear (2 times) differ by 1.414 (√ 2) times. The diaphragm values of the neighboring 1x3 (1.3x) have a difference of 1.3x √. F2 × √ 2 is F2.8 √ 2 is F4 Magi F5.6 √ 1.3 is F6.3.

Do you understand?

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

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