Photography knowledge: Are constant aperture and floating aperture still stupid?

When choosing a zoom lens, we will find that there is a constant aperture and a floating aperture on the lens. What do they mean? What is the difference? How to choose?

I. Definition

To understand constant aperture and floating aperture, we must first understand aperture. Simply put, aperture = focal length/absolute aperture. If the zoom lens is set to 50 mm and the aperture is F8, it means that the absolute aperture is 6.25mm；; ; When the aperture is adjusted to F4, the absolute aperture is12.5 mm; ; When the aperture is set to F2, the absolute aperture is 25 mm. Therefore, the smaller the aperture value, the larger the aperture of the lens, which is called large aperture.

Second, constant aperture.

Constant aperture, that is, the maximum aperture in the zoom lens will not change with the change of focal length. When the focal length is 24mm, f is 2.8; When the focal length is 70mm, the aperture is still F2.8. This zoom lens does not change with the focal length and is called a constant aperture lens. For example, EF24-70mm 1 ~ 2.8 lens, f is 2.8, focal length is 24mm, and the aperture is still F2.8 when the focal length is 70mm..

In the production process of constant aperture lens, the diameter and weight of internal lens are complicated, so the production cost is high. At the same time, because the aperture is unchanged, the zoom lens can perform better in more complicated shooting environments (such as news photography), or it is necessary to open the aperture to increase the shutter speed (such as sports photography). The lens group in the lens is more complicated and the image quality is further improved. Therefore, constant aperture lenses are usually professional-grade high-end lenses, which are expensive.

Third, floating aperture.

Floating diaphragm is different from constant diaphragm, also called unstable diaphragm. It means that the maximum aperture changes with the change of focal length. For example, EF-S18-135mm1:3.5-5.6 lens has a maximum aperture of F3.5 when the focal length is 18mm, and F5.6 when the focal length is135mm. When the focal length is changed from 18mm to 135mm, the maximum aperture is changed from 3.5 to 4.0 to 5.6, with a slight error in accuracy. This lens is a floating aperture lens.

The manufacture of floating aperture lens and the structure of internal lens are relatively simple, and the manufacturing cost will also be reduced. However, the use of floating aperture lenses must be accepted. With the increase of focal length, the maximum aperture becomes smaller, so it may be necessary to use a tripod to extend the exposure time or use a higher iso when shooting in low light. At the same time, we must accept the change of the maximum aperture, which leads to the change of depth of field. However, the floating aperture lens is a normal lens, and manufacturers know that these defects will not improve the internal structure, which allows consumers to choose a higher and more expensive constant aperture lens. Therefore, the floating aperture lens is cheap and the image quality is slightly worse.

Fourth, how to choose?

For professional photographers, in order to cope with the more complicated shooting environment, higher shutter speed is needed, and at the same time, the picture quality should be considered, so a constant aperture lens is generally used. Some photographers also pursue image quality and shooting needs, but they also recommend fixed aperture lenses, but you should know that the price is very expensive.

For most photographers, the budget is limited, the quality and shooting requirements are low, and a floating aperture lens is enough. Daily shooting, writing, travel records, etc. do not need such expensive equipment. After all, value for money comes first.

Do you know the constant aperture and the floating aperture now?