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

The explanation and analysis are as follows:

1, this phenomenon of macro lens is normal. Although the maximum aperture of macro lens is f/2.8, after entering the macro section, the actual light intake will decrease due to the extension of the lens barrel, and the displayed aperture value of some fuselage will decrease, which is the characteristic of macro lens.

2. Briefly explain the principle:

(1) When a macro lens with a high copy ratio (such as 1: 1) is used for short-range focusing, the lens group in the lens is in a special state, the distance from the optical center of the lens 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.

(2) From the light receiving situation of the focal plane, it is actually equivalent to the effect of reducing the aperture. Correspondingly, some fuselage will be displayed as apertures smaller than the set aperture, resulting in the phenomenon that "the maximum available aperture cannot reach the nominal aperture".

3. Generally, it seems that this phenomenon of Nikon fuselage is more common, 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.

4. 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 focal plane light.