Cameras are just machines that need input from their users.
When set to an auto exposure mode the camera relies upon the built-in metering system to determine exposure settings.
All built-in metering systems have to "assume" the world is "average tonality". This is often referred to as "18% gray". And it often works out pretty well because the typical subject or scene will have a wide range of tonalities that average out nicely.
But when you put an automated camera into a situation where tonalities are unusually bright or unusual dark, it is fooled into making incorrect exposures. This is where the user needs to override the automation and adjust things a little to get the exposure they want.
When a scene is dominated by very light tonalities, auto exposure will tend to underexpose and the user needs to increase exposure to make it correct. When a scene is dominated by very dark tonalities auto exposure will want to over expose, so the user needs to reduce exposure.
This happens with ALL cameras. Better cameras provide a method for the user to easily make these corrections to auto exposure. This is usually called "Exposure Compensation". I don't have a T6i, but I'm sure it has means of making these adjustments. Another response suggested using the Q button, which gives "quick" access to many of the camera's controls, including Exposure Compensation. But, before you can adjust it be aware that you cannot use Exposure Compensation in the camera's most highly automated modes... the scene modes (like "sports" or "landscape") or in the Auto+ mode (the "green box", which also prevents you from using some other features of the camera... it's basically a "point n shoot" mode, for folks accustomed to using their phones to take photos).
For the most automation with Exposure Compensation you might use the "P" or "Program" mode. In this you select the ISO and the camera sets both the lens aperture and the shutter speed. If you are using flash, the shutter speed will be limited to 1/200, so you may need to adjust the ISO accordingly. Exposure Compensation also works on Av (aperture priority), Tv (shutter priority) and M+ ISO Auto modes. These are all auto exposure modes... hence the Exposure Compensation to be able to override them. Exposure Compensation does not work in M (fully manual, no ISO Auto) exposure mode. If you think about it this makes sense, since there's no automation to override
Take a test shot after your set up without making any adjustment to Exposure Compensation. Then press the Q button and look for the Exposure Compensation scale on the camera's rear monitor screen. Move the indicator to the right 1 full stop and take another test shot. Repeat the process and move the indicator to the right an additional stop, for 2 stops of added exposure, then take another test shot. Now check out the images and see which you like or if you need to add more or fine tune the settings. There are 1/3 stops in between the full stops, which you can use if you wish to make more precise adjustment.
There are more details about using Exposure Compensation beginning on page 159 of you user manual. (If needed, the manual can be downloaded from the Canon USA website, in the support are for the T6i.)
Another "tool" your digital camera provides is a "histogram". This is a chart showing the tonalities in an image. The left side is the darkest tones, while the right side is the lightest. In most cases you don't want the histogram to be "piled up" against either the right or the left side. Ideally it will just barely reach both, but it's okay if it falls a little short of reaching one or the other side.
You don't need to use the histogram if you don't want to. You can tell much the same by viewing the images on your computer monitor. But learning to use the histogram on the camera allows you to better check your exposures in the field, so you can retake the shot, if needed.
Another thing you might find useful is Exposure Bracketing. Here you set up the camera to take a series of shots with some compensation you have pre-planned. For example, in a situation like yours where the unusually bright scene is causing under exposure, you can set the camera to take the first shot with no adjustment, a 2nd shot with one stop of + exposure and a third shot with two stops of + exposure. You aren't limited to full stops... can use +2/3 and +1-1/3 stop, if that gives sufficient increase to correct the under exposure. Later when reviewing your images, instead of just one per set up you will have a series and can choose the best exposure. The problem with this is it will fill up your memory cards and computer disks a lot faster, plus it takes time to prepare the camera for each series when there's a big difference. Oh, and don't forget to cancel it after each use, or you'll end up bracketing all your shots when it's not wanted.
People often use bracketing when they simply aren't sure and will set it to make one under, one unadjusted and one overexposure. In a case like yours where you know that underexposure is the problem you can combine bracketing with Exposure compensation and make the additional shots all over exposure, each with additional increment of increase. It can be 0, +1 and +2.... or it could be 0, +2/3 and +1 and 2/3. If you have the camera set to single shot you'll need to press the shutter release three times to take the bracketing shots. If you have the camera set to continuous shooting mode, you only need to press the shutter release once. The camera will automatically stop making exposures after the third is taken.
There's additional info about Bracketing on page 160 of the user manual.
I imagine this all sounds complicated, but it becomes easy once you learn to do it.
I hope this helps!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2), EOS M5, some other cameras, various lenses & accessories
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