Why does Sigma's 105 macro lens change with the focus aperture index when it is opened to the maximum aperture?

This phenomenon of macro lens is normal. Although the maximum aperture of the macro lens is f/2.8, after entering the macro section, the actual light input will be reduced due to the elongation of the lens barrel, and the aperture value displayed by some fuselage will be reduced, which is the characteristic of the macro lens.

Briefly explain the principle: when a macro lens with high copy ratio (such as 1: 1) is used for close focus, the lens group in the lens is in a special state, the distance from the lens optical center to the focal plane (the plane where the film or digital image sensor is located) is long, the imaging beam in the lens is reduced, and the light transmission area of the aperture section is reduced. Judging from the light reception of the focal plane, it is actually equivalent to the effect of aperture reduction.

Generally speaking, Nikon fuselage is a common phenomenon, while the aperture value display of Canon camera will not change. Because from the exposure effect, both Canon and Nikon's macro photography will face the problem of maximum aperture "loss", but Canon's body still displays the aperture value through the aperture size set by the lens, while Nikon's body displays the aperture value according to the light transmission effect.

Although the macro lens will lose the maximum aperture in the exposure effect, in fact, when shooting with automatic metering system, we don't have to make exposure compensation deliberately, and both of them will measure and expose according to the actual light receiving situation of the focal plane.

Reference:/digi/DC/20101-15/13093775194 _ 3.shtml.