Generally speaking, given a source frame with both bright and dark parts in the scene, this is about what a histogram should look like. The levels could probably be left alone here. This is generally what you should be striving for. The source was a clean frame an old DV source.
A pure black frame should look like this. Note that the graph is flush left and one column only as absolute black. If there were noise, there would be other bumps in the graph.
This is a black frame that has some noise in it. Notice the little "mound" at the bottom of the graph. This "mound" is dark noise and can be seen if you crank the brightness up. This is common for analog video captures. The source was from a raw digital cable capture.
This is a black frame that is way too high (actually dark gray) and has an absurd amount of noise in it. This was one of the black frames between a scene transition from an old DV source. If this "black" frame was really clean and just too high, it would look like the red line. The mounds on either side of it are black noise and can easily be seen if you crank the brightness up.
This is a dark scene from and old DV source. The black levels are way too high. There is also an absurd amount of black noise. To fix this frame, bring the black levels slider to roughly where the red line is. The black noise will be squashed and removed from the video.
This frame has black levels way too high. There is a minor amount of black noise. Notice that there is also white noise at the right side of the graph. To help find where the white video ends and the white noise starts, probe the frame and read off the numbers of areas that you know to be bright white. Adjust levels until those areas are bright white. The white noise will automatically be smashed and removed from the video.
This is a histogram from a frame with washed out colors from an old DV source. Colors will suffer greatly if blacks are too high and whites are are really light gray. To fix the colors using the levels filter, slide black and white pointers under the red lines respectively. Afterwards, color saturation can be tweaked using the other filter if need be. Do NOT just crank up chroma settings hoping to fix a situation like this. It will only bleed out and still look bad.
This histogram shows reasonably good black levels (low noise), but they are way too high. White levels could also be slightly higher to properly balance the frame.
This histogram shows another super noisy and too high black level. There is also some white noise and white levels are too low.
This histogram is similar to the last but with a dark scene and essentially empty white levels. Boosting gamma levels may make this video much more watchable.
This is a raw capture from digital cable where the black levels are dominant and correct for a night scene.
Here the black levels are about right but slightly too high. There is an empty space on the white levels that will make colors look dim and not vibrant. This was from a raw digital cable capture from a TV station that should have known better.
Here the black levels are about right and could probably be left alone. Again, white levels are squashed. This was from a raw digital cable capture from a TV station that should have known better.