Canon T3i Viewfinder
Canon EOS Rebel T3i Viewfinder
One feature removed since the T2i is the pair of horizontal infrared sensors that lay just below that camera's viewfinder. These were used to detect your eye as it approached the viewfinder, thereby allowing the T2i to deactivate its LCD information display automatically. The T3i lacks this capability, and so the display must either be disabled manually, or can be automatically disabled when the shutter button is half-pressed.
Like the Canon T2i, the T3i viewfinder uses a pentamirror design. The advantage of a pentamirror is that it is lighter and less expensive; however, it also delivers less light to the viewfinder eyepiece, thanks to less efficient internal reflection than can be achieved with an all-glass pentaprism design. As a result, the Canon T3i's viewfinder image is not as bright as the EOS 50D, which at around $1,100 is Canon's lowest-priced model to use a true pentaprism.
The illustration below (courtesy Canon USA) shows all the elements in the Canon T3i's viewfinder. The extensive information available appears beneath the image area in a small strip, and reports all of the major camera settings, such as shutter speed, aperture, flash mode, EV compensation, etc. The information display is unchanged from that of the Rebel T2i.