In Live View mode on your Rebel T6i/750D, you can vary the type and amount of data that appears on the monitor along with your subject. Press the Info button to cycle from the default display mode, shown on the left in the figure, to one of the alternative displays shown in the figure.
Press Info one more time to hide all shooting information. In Movie mode, some data changes to show movie‐recording options instead of still‐photography settings, and the Histogram display shown on the right is not available.
The chart labeled histogram is a tool you can use to gauge exposure. Note that when you use flash, the histogram is dimmed. The histogram can't display accurate information because the final exposure will include light from the flash and not just the ambient lighting. In addition, the histogram dims when you use M (Manual) exposure mode and set the shutter speed to Bulb, which keeps the shutter open for as long as you hold down the shutter button. The camera can't predict how long you're going to hold that button down, so it can't create a histogram that will reflect your final exposure.
Also note the little box labeled Exposure Simulation. This symbol, which appears in the more advanced display mode as well as in the mode that includes the histogram, indicates whether the monitor is simulating the actual exposure that you'll record. If the symbol blinks or is dimmed, the camera can't provide an accurate exposure preview, which can occur if the ambient light is either very bright or very dim. Exposure Simulation is also disabled when you use flash in Live View mode.
You can make these additional tweaks to the Live View display:
Display a grid. To assist you with composition, the camera can display one of two styles of grids on the monitor. For still photography, enable this feature via the Grid Display option. In Movie mode, look for the option on Movie Menu 1.
Adjust the exposure‐data shutdown (Metering Timer option). By default, exposure information such as f‐stop and shutter speed disappears from the display after 8 seconds if you don't press any camera buttons. If you want the exposure data to remain visible for a longer period, you can adjust the shutdown time, but only if the Mode dial is set to P, Tv, Av, or M. Make the change via the Metering Timer option, which is in the Live View menu for still photography and on Movie Menu 1 for Movie mode. The metering mechanism uses battery power, so the shorter the cutoff time, the better.