Today, I will talk about the most basic concepts and see how the aperture value is calculated.
01, the lens of the light capacity
Let’s start with the basic concept: the lens can be light, when taking pictures, the lens first light, and then the light will be transmitted to the CMOS imaging. The amount of light in the lens can affect the degree of light and dark photos.
The relative aperture of a lens is expressed as “effective aperture ÷ focal length”.
Yes, both focal length and aperture affect the amount of light that comes in, so now you know why telephoto lenses have smaller apertures.
Lenses are also marked with the size of the “relative aperture”, for example, 1:2, which means that the maximum relative aperture of the lens is 1:2 and it is a constant aperture lens. Another example is 1:2.8~4, which is generally the maximum relative aperture of 1:2.8 at the shortest focal length and 1:4 at the longest focal length.
The chart below shows the relative apertures of 1:4-5.6
Calculate the aperture so that you understand it thoroughly
- Aperture value F
The F value and the f/value we usually see are the “aperture factor”, which is the reciprocal of the relative aperture, i.e. “focal length ÷ effective aperture”.
If the relative aperture is 1:2.8, the corresponding aperture coefficient is F2.8 or f / 2.8, making the fraction a whole number, which is much simpler.
For example, for a standard 50mm lens with a maximum light diameter of 29.5mm, the maximum relative aperture is calculated as 29.5mm ÷ 50mm = 1:1.7 and the maximum aperture is calculated as 50mm ÷ 29.5mm = F1.7. This can also be interpreted as setting the light diameter to 1 and the focal length to 1.7.
The larger the F value, the smaller the aperture and the smaller the light intake; the smaller the F value, the larger the aperture and the larger the light intake.
- How to calculate the aperture value
Common aperture values are F1.4, F2, F2.8, F4, F5.6, F8, etc. Each aperture is 1 stop adjacent to the other, and the amount of light intake is 2 times the relationship.
F1.4 is 1 stop larger than F2 and the light intake at F1.4 is twice that of F2. In practice, F1.4, 10s, and ISO100 produce the same brightness as F2, 10s, and ISO200 and the same brightness as F2, the 20s, and ISO100.
In addition to apertures 1 stop apart, there are apertures 1/3 stops apart, such as F5.6, F6.3, F7.1 and F8, each with a 1/3-fold difference in light intake, allowing greater flexibility in exposure control. If you are shooting in shutter priority mode, the ISO is also set to a fixed value and only the aperture is automatic. If the camera currently gives you an aperture of F6.3, and you increase the exposure by 0.3EV using 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 and the aperture is now F1.7, to increase the aperture by one stop, we have to make the area of the illuminated circle 0.5 times larger. The area of the circle is πr², and to make it twice as large (2πr²), you have to make r √2 times larger, which is approximately the same as multiplying it by 1.414.
Thus, the aperture value becomes 50 ÷ (29.5 x 1.414), which is transformed to (50 ÷ 29.5 ≈ 1.7) ÷ 1.414. Thus, the difference in aperture values between adjacent stops (2 times) is 1.414 (√2) times. The difference in aperture values between adjacent 1/3 stops (1.3 times) is √1.3 times. f2 × √2 is F2.8, f2.8 × √2 is F4 and f5.6 √1.3 is F6.3.
