How to use your histogram in manual mode
While the exposure meter is a useful tool for getting the exposure right in Manual, also check the histogram display when you review the shot. This tells you if your exposure needs adjusting.
If the histogram is bunched to the left, and there’s a gap to the right, the shot is under-exposed. In this case you need to either use a slower shutter speed, a wider aperture or a higher ISO.
A histogram where there is a gap to the left, and the graph is bunched to the right, indicates that the shot is over-exposed. To correct this you need to use a faster shutter speed, smaller aperture or a lower ISO.
For an average scene, it is correctly exposed when the histogram is evenly distributed across the whole graph without large gaps to the left or right.