Understand constant aperture and floating aperture

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

I. Definition To understand the constant aperture and the floating aperture, we must first understand the aperture, which is simply: aperture = focal length ÷ absolute aperture. If the zoom lens is set to 50mm, the aperture at this time is F8, which means the absolute aperture is 6.25mm；; When the aperture is adjusted to F4, the absolute aperture becomes12.5mm; ; When the aperture is adjusted to F2, the absolute aperture becomes 25mm again. Therefore, the smaller the aperture value, the larger the aperture of the lens, which is called large aperture.

Second, the constant aperture constant aperture means that the maximum aperture in the zoom lens will not change with the change of focal length. Like ef? 24-70mm？ 1:2.8 lens, when the focal length is 24mm, f is 2.8; When the focal length is 70mm, the aperture is still F2.8. This kind of zoom lens whose aperture does not change with the focal length is called a constant aperture lens. The production process of constant aperture lens, the diameter and weight of internal lens are more complicated, so the production cost is higher. 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 improve the shutter speed (such as sports photography). Finally, the lens group inside the lens is more complicated, which further improves the image quality. Therefore, constant aperture lenses are generally professional-grade high-end lenses, which are expensive.

Third, the floating aperture floating aperture, different from the constant aperture, is also called the non-constant aperture, which means that the maximum aperture changes with the change of focal length. Like EF-S 18- 135mm？ 1:3.5-5.6 lens, when the focal length 18mm, the maximum aperture is f3.5; ; When the focal length is 135mm, the maximum aperture is F5.6; as the focal length changes from 18mm to 135mm, the maximum aperture changes from 3.5→4.0→5.6, and the accuracy will be slightly wrong. This kind of lens is a floating aperture lens. The manufacture of floating aperture lens and the internal lens structure are relatively simple, and the production cost will also be reduced. However, it must be accepted that the maximum aperture decreases with the increase of focal length, so you may need to use a tripod to extend the exposure time in low light, or use a higher ISO, and also accept that the change of the maximum aperture will lead to the change of depth of field. However, as an ordinary lens, manufacturers are aware of these defects and will not upgrade the internal structure, that is, let consumers choose a higher-end 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, to cope with more complex shooting environment, higher shutter speed is needed, and at the same time, the guarantee of image quality should be considered, so a constant aperture lens is generally chosen. Some photographers also pursue image quality and shooting needs, and recommend constant aperture lenses, but you should know that the price is very expensive. For most photographers, the budget is limited, and the requirements for image quality and shooting are not high. It is enough to choose a floating aperture lens. In daily shooting, creation, travel records, etc. There is no need to use such expensive equipment. After all, the price comes first.