Effects apply to the whole frame by default, which is usually wrong. Most of the time the thing you want brighter, softer or darker is one part of the picture.

What a mask is

A shape that limits where an effect applies. Inside the shape the effect is on, outside it is off, and the transition between those two states is the part that decides whether it works.

Two shapes cover almost everything: a rectangle for anything aligned to the frame or to a screen, and an ellipse for faces, lights and anything organic.

A step chart: flat stretches, then a jump to a higher baseline after one video breaks out.

Feather is the setting that matters

A mask with no feather has a hard boundary. The effect is at full strength on one side of a line and absent on the other, and the human eye is extremely good at spotting exactly that kind of edge. A hard-edged mask always looks like a mask.

Feathering blurs the boundary so the effect fades in across some distance. Enough feather and the boundary becomes invisible; the viewer sees an area that is brighter or softer without seeing where it stops.

How much: more than you think. As a starting point, feather across roughly a tenth of the mask's own size. A small ellipse over a face wants a soft, wide falloff, not a crisp oval.

The exception is when you WANT the boundary visible: a censoring blur over a document should look deliberate, because a soft-edged blur over sensitive information reads as an accident and invites people to squint at the edges.

What masks are good for

Darkening a distraction. A bright window, a lamp, a doorway pulling the eye away from you. A soft elliptical exposure reduction there is invisible and effective.

Lifting a face. The reverse. A gentle exposure lift on a face that is slightly under, without brightening the whole room, which would also lift the noise in the shadows.

A vignette you control. The built-in vignette is centred and symmetrical. A masked darkening lets you put the falloff where the frame actually needs it.

Local sharpening or softening. Softening the background slightly to imply depth, when you did not have a lens that would do it optically.

Censoring. A blur or pixelate over a name, a key, a face. Covered properly in how to blur sensitive info in a screen recording and blurring one object, because censoring has requirements the other uses do not: it must not be reversible, and it must not drift off the thing it is hiding.

The moving subject problem

A static mask works when the thing it covers does not move. The moment your subject shifts, a static mask is covering the wrong region.

Three options, in increasing order of effort:

  1. Make the mask larger and softer than the movement. Often enough for a talking head who sways a little.
  2. Keyframe the mask position so it follows. See keyframes; two keyframes and an ease is usually all it takes.
  3. Cut the shot at the point the subject moves and mask the two halves separately.

For censoring specifically, options one and two are the only acceptable ones, because a mask that slips for four frames has failed at its whole job.

Where they are in Zella

Feathered rectangle and ellipse effect masks sit behind the Advanced tools toggle in the editor toolbar, with motion keyframes, the curve editor, live luma scopes and Color Match.

Blur callouts are the separate, simpler path: a callout you place on the canvas that blurs what is under it, which is the right tool when the job is censoring rather than shaping the image.

When not to reach for a mask

If the whole frame genuinely needs the change, do not mask it. A masked grade applied to eighty percent of the frame is a global grade with an extra failure mode.

And if you find yourself building four masks on one clip, the shot is fighting you. Relight it, reframe it, or accept it. Masks are for the one thing in an otherwise good frame that is not cooperating.

Apple's Final Cut Pro guide covers shape and colour masks in a professional context, where the same feathering rule applies.

Common questions

What is feathering and why does it matter? It softens the mask edge so the effect fades in rather than starting at a hard line. A hard-edged mask always looks like a mask, because the eye is very good at spotting exactly that boundary.

How much feather should I use? Roughly a tenth of the mask's own size as a starting point. The exception is censoring, where a visible boundary is correct.

What if my subject moves? Make the mask larger and softer than the movement, keyframe its position to follow, or cut the shot and mask the halves separately. For censoring, only the first two are acceptable.