There can be a couple of reasons for this.
1) In the semi-automatic modes (Program, Tv, and Av) the camera has a feature called "Exposure Compensation". There's a button on the rear of the camera with [Av +/-] on it. This allows you to tell the camera that you want it to exposure the shot brighter or darker than the exposure that the camera would otherwise have selected based on metering.
Exposure compensation has no effect when shooting Manual because when doing Manual, the exposure indicator in the viewfinder (or the rear of the camera) will indicate the "0" position (center position) if your exposure matches what the light meter reads. If you wanted to over-expose or under-expose when shooting manual, you'd just adjust the exposure so the exposure indicator reads higher or lower.
Here's a video demonstrating how to adjust your exposure compensation: https://www.youtube.com/watch?v=ZBOo1aOb3IE
2) It can also be the result of the selected metering method and the subject. The camera has three metering methods... "Evaluative" metering is the default and uses the the full 63 zone metering system. "Center weighted" also uses the entire 63 zone metering system EXCEPT it gives the greatest weight to the meter readings detected in the central area (but not the center point) of the display. Finally there's "Spot" metering. This does not use the full 63 zone metering system and instead just meters the shot based on the light detect in in the very center of the display.
The meter is a "reflected" light meter. It cannot tell how much light is actually falling on a subject... it can only detect the light that reflects off the subject and into the camera. If a subject is dressed completely in black and there's a black background, then the camera detects very little reflection. This makes the camera believe there is not enough light so it will increase the exposure. It "presumes" the image, if thought of as a black and white image, should render as a middle-gray tone. This causes the camera to over-expose the blacks and anything in the image that was not actually black will be strongly over-exposed.
The reverse is also true... take a photo of a subject wearing white on a white snowy background and the camera, thinking the typical scene rendered in black and white should be a middle-gray will think there's too much light. It will compensate by decreasing the exposure and that will cause everything that wasn't actually "white" to be extremely dark and under-exposed.
Hand-held light meters (meters not built into cameras) have a sensor that you simply hold in the very light where your subject is located and it measure the light falling on that point. Those meters detect the true light... not just the light being reflected by the subject. They are extremely accurate -- just not nearly as convenient as the meter built into your camera.
Usually an external hand-held light meter is not necessary... the reflected meter built into the camera is fine as long as YOU remember that very black scenes and subjects tend to over-expose and very white scenes and subjects tend to under-expose. And that's one key reason why you'd actually want to use your camera's exposure compenation.
Tim Campbell
5D III, 5D IV, 60Da
