How to treat the lens of SLR camera as a constant aperture?

1, as shown in the following figure: (By comparison, we can know more clearly the constant aperture framing method of SLR camera lens. )

2. Constant aperture means that the maximum aperture of each focal segment of the zoom lens is the same, such as 70-200/F2.8 (only one maximum aperture value), and non-constant aperture means that the zoom lens has different maximum aperture values on the focal segment, such as 70-300/4-5.6. Constant aperture is easier to control in use. Generally, constant aperture zoom lenses are relatively high-end, but non-constant aperture lenses are often lighter and more portable because of their small size.

3. Definition of constant aperture: It usually means that the zoom lens can use its nominal maximum aperture in all focal lengths. For example, the lens indicates 28-70 mm/2.8, which is a constant aperture lens. This means that the lens can shoot at a focal length of 28-70mm, and the maximum aperture value is F2.8 If the lens logo is 28-70 mm/3.5-4.5, it is a floating aperture lens, that is, the maximum aperture of 28mm focal length can be F3.5, and the maximum aperture of 70 mm can only be F4.5. On the old camera, the zoom operation is carried out when the aperture is set to the maximum. Constant aperture lens will not affect the exposure, and non-constant aperture lens will affect the final exposure because of the change of focal length.