Full auto mode (the green box) locks out many changes that would be permitted in other modes... one of which is exposure compensation. If you had dialed in exposure compensation in another mode, it would be ignored when you switch to auto mode.
I do not own an 80D but I have compared the reflected meter readings with my various Canon bodies against what my dedicated Sekonic meter reads and have found them to all be very accurate.
1 photographic stop represents a doubling or halving of the amount of light. If you have to dial the exposusre down by 3 stops, then that means you have to dial it down to 1/2 of 1/2 of 1/2 (or 1/8th) of the exposure that the camera wants to use. That's a huge difference.
You can test the camera on a bright sunny day.
In mid-day sunshine, the sun pumps out a very reliable amount of light. A correct exposure for the Sun based on the "Sunny 16" rule says that that IF (and only if) you use f/16, then the shutter speed should be the INVERSE of the ISO setting.
In other words if you use manual exposure, set your aperture to f/16, set your ISO to 100, then the shutter speed should be 1/100th sec. And in doing that... if you "meter" the scene with those settings dialed in, the built-in meter should show the digital needle near the center point.
CAVEAT: Keep in mind this is a "reflected" light meter. If you point the camera at a scene which is dominated by darks, the meter will read less light being reflected (because darks don't reflect as much) and the camera may indicate an under-exposure to encourage you to boost the exposure a bit. Conversely if you point the camera at a scene dominated by light colors, the opposite can happen. In other words, if the camera metering differs by a small amount (maybe a 1/3 of a stop) then I wouldn't worry too much. But if it differs by 3 stops (which is what you are saying seems to be happening) then that's a defect and needs to be addressed.
There are a few other issues you could check but I think are unlikely... I had wondered if the aperutre blades on your lens were stopping down as they should. You can dial in a high f-stop value then look at the front of your lens and press your depth-of-field preview button and make sure you see the aperture blades constrict.
You aren't using any filters on the front of the lens? I can only imagine that would result in the camera under-exposing but you are saying it is over-exposing so I doubt it's that.
Are you using "Evaluative" metering or some other mode such as "Spot" or "Center weighted", etc.?
Tim Campbell
5D III, 5D IV, 60Da
