When you shoot using Manual mode the camera will meter using it's internal "reflected" light meter and it will offer advice on the exposure but it will not change exposure settings for you.
This means that, depending on your metering mode and subject, you'll point the camera at the subject, half-press the shutter, and take a meter reading. The scale at the bottom of the viewfinder window. It is marked with a -3..2..1..0..1..2..3+ with a "-" side and "+" side. There's an arrow indicating what the camera thinks of your exposure. For example if the arrow is under the "-1" side, then it means the meter believes you are underexposed by 1 stop. You can either adjust the aperture, or the shutter speed, or the ISO until the meter is indicating the "0" position ... that's where the camera thinks you'll need to be to get a correct exposure.
But these meters are just tools and they can be fooled (and it's not a defect.)
In "Evaluative" mode, the camera analyzes the the light in several "zones" all around the frame and tries to indentify the best exposure based on those zones.
In "Center weighted" it works exactly as "Evaluative" (all zones are metered) but it gives much stronger "weight" to the meter readings from the zones near the center of the frame and much less weight to the zones outside the center area.
In "Spot" metering mode the camera meters only in an area near the center AF point (it does not follow your selected focus point ... it uses the center point regardless of your selection.) No other points are considered. This means if your camera was in "spot" mode and you metered a bright object at the center of the frame, the camera would like underexpose your shot because the bright object makes the camera believe it needs to reduce light intake and since the camera isn't allowed to factor in any other zone, the camera will not realize that the images is underexposed in other zones.
Bright or mostly "white" subjects will fool a reflected light meter into underexposing the shot. This is because the meter is designed to anticipate an average level of reflectivity (I think it's about 12% for your camera) but some objects will reflect considerably more light back to the camera's light meter than other objects (white reflects quite a bit... black doesn't reflect much). The camera doesn't "know" what it's metering... it only knows how much light is being reflected back.
In situations where you really want more accurate exposure, you would visually look for the brightest area in your composed frame and also the darkest area in your composed frame and you would use "spot" metering to manual meter both points... and then use the exposure halfway between those two metered points (there are hand-held light meters that do this automatically.) A portrait photographer using a hand-held "incident" light meter doesn't just meter the subject's face... they meter the "shadow" side of the face and the "highlight" side of the face and use both readings to find an exposure that captures the range needed.
Tim Campbell
5D III, 5D IV, 60Da
