Curves and colour wheels in Video by Zaweba

Sliders get a shot most of the way. Curves and colour wheels finish the job. In Video by Zaweba both live in Inspector, then Colour: a master curve with separate red, green and blue curves, and three colour wheels for shadows, midtones and highlights. They give you finer control over contrast and colour than the basic sliders, and they are where a lot of the character of a grade comes from.

How a curve works

A curve maps the brightness going in to the brightness coming out. Along the bottom are the tones of the original picture, darkest on the left and brightest on the right. Up the side is what each tone becomes. A straight diagonal line changes nothing. Drag a point up and those tones get brighter; drag it down and they get darker.

The master curve changes brightness for all three colour channels together. The classic move is a gentle S: pull a point in the darker quarter down a little and a point in the brighter quarter up a little. The darks get darker, the lights get lighter, the middle stays put, and the picture gains contrast without the harsh look of pushing a contrast slider hard.

The red, green and blue curves work one channel at a time. Lift the blue curve in the shadows and the dark parts go cool. Lower it in the highlights and the bright parts go warm. That split, cool shadows and warm highlights, is a familiar look, and the channel curves let you shape exactly where it starts and stops.

What the wheels add

The three wheels do something similar in a more direct way. Each one tints a range of tones. Push the shadow wheel towards blue and the shadows take on a blue cast; push the highlight wheel towards orange and the highlights warm up, while the midtone wheel steers the middle where skin and most detail sit.

Wheels are quicker for broad moves and easier to judge by eye. Curves are better for precise work, such as pinning one tone in place while changing another. Many people balance a shot with the wheels and then refine contrast with the master curve.

Correction jobs they handle

Curves fix colour casts that change across the tonal range. A shot can have neutral shadows but green highlights, perhaps from mixed lighting. A slider moves everything at once, but a small dip in the green curve near the top fixes just the highlights.

They also rescue flat footage. A shot that sits in a narrow band of greys can be stretched with the master curve so the darkest part becomes properly dark and the brightest properly bright, without losing detail in the middle.

Checking your work

Small curve moves go a long way. Watch the RGB parade in View, then Scopes while you adjust. When something in the frame should be neutral, a white wall or a grey road, its red, green and blue traces should line up at the same height. If one channel sits higher, that is the cast to correct. The Colour page puts a parade and histogram right beside a large viewer, with a before and after split, which makes curve work much easier.

Mistakes to avoid

Steep curves posterise. If a section of the curve becomes very steep, a narrow band of tones is spread across a wide range, and smooth skies or skin can break into bands. Keep the curve smooth with few points.

Don't fight the basic sliders. If exposure is wrong, set it with exposure first and then shape with curves. Bending a curve to fix bad exposure leaves you with less room for everything else.

Pushing the wheels too far is easy because the change builds slowly as you drag. Turn the colour section off with its bypass switch now and then, and compare.

Curves and wheels in Video by Zaweba are worth an hour of play on a single clip. Once they make sense, every grade gets quicker and more deliberate.