Where is the Canon 5d aperture preview button?

The second button in the lower right corner of the lens holder. According to the query of Canon official website, the Canon 5d aperture preview button is the second button on the lower right of the lens bayonet. The aperture preview button is used to adjust the lens aperture to the set target aperture value. When you use this button to adjust the aperture size, you can find that the viewfinder will darken instantly.