Crop Corner

1) You're using a 200mm lens with a maximum aperture of f2.8. 2) You want to shoot the scene at f8, because there are elements of the scene you want to be in focus that are at different distances from the camera. 3) Two elements in the scene are far enough apart that both will not be in focus at f2.8, but they will be in focus at f8. When you focus your lens on an element in the scene, you're seeing the scene through the viewfinder at its maximum aperture of f2.8. The camera is designed in this manner for several reasons. Most importantly, it allows the viewfinder to operate at its brightest setting to facilitate your viewing the scene. It also allows the camera's autofocus mechanism to obtain enough light to work effectively. If your lens has a maximum aperture of f4, you would view the scene through the viewfinder at that setting, which is one stop darker. If you have lenses with different maximum apertures, try looking through them at the same subject to see if you can discern the one-stop difference in brightness.