What video scopes show and how to read them

Video scopes measure your picture so you can judge exposure and colour with numbers rather than by eye. In Video by Zaweba, View > Scopes shows four: a waveform for brightness across the frame, an RGB parade for the balance of red, green and blue, a vectorscope with a skin line for colour and saturation, and a histogram for how the brightness is spread. Each answers a different question about the same frame.

Your eyes adapt, and your screen may be too bright, too blue or set to a vivid mode. Scopes don't adapt. That's why colourists keep them open all the time.

The waveform: how bright, and where

The waveform plots brightness from bottom (black) to top (white), laid out across the width of the frame. Something bright on the left of the picture appears high up on the left of the waveform. A sky shows as a band near the top, a dark foreground as a cluster near the bottom.

Read it for limits first. Anything flattened against the top line is clipped white and has lost detail; anything squashed on the bottom is crushed black. A well-exposed shot usually uses most of the height without piling up hard against either end. Faces in normal light tend to sit somewhere around the upper middle, which is a useful quick check.

The RGB parade: colour balance

The parade is three waveforms side by side, one each for red, green and blue. On anything that should be neutral, such as a white wall, grey concrete or black shadows, the three should line up at the same height. If the blue column sits higher than the others in the shadows, the shadows are blue. If red rides high in the highlights, they're warm.

That makes it the best tool for fixing a colour cast. Adjust temperature and tint, or the colour wheels for shadows, midtones and highlights, until the neutral parts line up.

The vectorscope: hue and saturation

The vectorscope is round. The centre means no colour at all; the further a point is from the centre, the more saturated it is, and the direction tells you the hue. A grey image is a dot in the middle. A colourful one spreads outwards.

The skin line is the most useful part. It's a line from the centre towards the direction where human skin tones fall, across skin of every shade. Isolate a face with a mask and check where it lands: on or near the line looks natural, far off it looks sick or sunburnt. It's also a quick way to see if a grade has become too saturated, as the trace stretches far from the centre.

The histogram: the spread of tones

The histogram counts how many pixels sit at each brightness, from dark on the left to light on the right. It's the familiar graph from photo editing. A heap against the right edge means clipping; a heap against the left means crushed shadows. It doesn't show where in the frame anything is, which is what the waveform adds.

Using them together

A practical order: set the range with the waveform, balance the colour with the parade, check skin and saturation with the vectorscope, and glance at the histogram for an overall feel. View > Colour page also puts a histogram and RGB parade readout beside its big viewer, which is handy when stepping through every shot in a film.

Scopes don't make creative choices for you. A moody night scene is meant to sit low on the waveform. They simply make sure the choice is deliberate.