The eraser is the tool that costs you the second revision

Deleting and hiding look identical until Thursday

Two people cut a product out of its background. One selects the background and presses Delete. The other selects the product and clicks Add Layer Mask. On screen the results are the same.

On Thursday the client says the shadow under the product was nice, could we keep a bit of it. The first person starts again from the camera file. The second person picks up a soft white brush, paints for eight seconds, and sends it back.

This is the single skill that separates people who can edit from people who own a copy of Photoshop.

What a mask actually is

A layer mask is a greyscale image attached to a layer, the same pixel dimensions as the layer, with one value per pixel from 0 to 255.

• Black hides. 0 means that pixel of the layer is fully invisible.
• White reveals. 255 means fully visible.
• Grey is partial. 128 means half opacity, pixel by pixel.

Nothing is deleted. The pixels are all still sitting there, and the mask is just a stencil deciding what gets through. Paint the stencil white and they come back.

Say it once as a sentence and it will stay: *black conceals, white reveals*.

Two things to know immediately. Press \ (backslash) in Photoshop to see the mask as a red overlay on the image, or Alt/Option-click the mask thumbnail to see the mask on its own in black and white — that is how you find the ragged edge you cannot see against the picture. And press X to swap your brush between black and white, because masking is nothing but painting and unpainting.

The five things you do to masks

1. 1Paint them. A soft round brush at 30 to 50 per cent opacity, built up in passes. Hard brush for hard edges, soft for hair and shadow.
2. 2Gradient them. Drag a black-to-white gradient across a mask on a Curves layer and you have darkened a sky exactly the way a photographer's graduated filter does, with no line where it stops.
3. 3Feather them. A mask with a one-pixel transition looks cut out with scissors. Two pixels of feather is usually the difference between fake and not.
4. 4Invert them. Ctrl/Cmd+I. You almost always select the easier thing and then invert.
5. 5Disable them. Shift-click the thumbnail to switch a mask off and check what you have hidden. Do this before you deliver.

Masks on adjustments are the real workflow

The moment that makes it click: every adjustment layer arrives with a mask already attached.

So darkening one face in a group photo is not a selection problem and not a dodge-and-burn problem. It is: add a Curves layer, pull the curve down, press Ctrl/Cmd+I to fill the mask with black so the darkening is hidden everywhere, then paint white with a soft brush over that one face. Too strong? Lower the layer opacity. Wrong face? Paint black.

Nothing was changed in the photograph. Every part of that is reversible for as long as the file exists.

Two more masks with confusing names

Clipping masks

A different thing with a confusingly similar name. A clipping mask makes a layer show only where the layer directly beneath it has pixels. Alt/Option-click the line between two layers in the Layers panel, or Ctrl/Cmd+Alt+G.

Use it when you want a texture, a colour fill or an adjustment to affect exactly one layer and nothing under it — for example, a Hue/Saturation layer clipped to a cut-out shirt so you can recolour the shirt and only the shirt. Photopea, Krita and Affinity all support this. GIMP does it through *Layer > Mask > Add Layer Mask* plus grouping, which is clumsier.

Luminosity masks: the selection with no edge

A hard-edged selection always risks a visible seam. Sometimes you want to affect *the bright parts* rather than a region.

In Photoshop or Photopea, open the Channels panel and Ctrl/Cmd-click the RGB thumbnail. You get a selection whose strength at every pixel equals that pixel's brightness. Now add a Curves layer: the adjustment automatically applies fully to highlights, partly to midtones, not at all to shadows, with no boundary anywhere. Invert it to target the shadows instead.

This is how you recover a blown sky without a halo around the roofline, and it costs two clicks. GIMP does the same by adding a layer mask with the *Grayscale copy of layer* option — one of the places where the free tool is not worse at all.

The honest limits

A mask cannot invent edge detail that the photograph does not contain. If the subject is motion-blurred against the background, no mask separates them cleanly, because there is no boundary in the pixels to find — the edge pixels are genuinely a mixture of both.

Masks also cost memory. A dozen high-resolution masks in an 8-bit file is fine; the same in 16-bit on a four-gigabyte laptop is not. And a mask painted at 200 per cent zoom often looks crude at 100 per cent. Check at fit-to-screen before you export, then at 100 per cent along the edge.

Today: take any photo, add a Curves layer, drag the middle of the line down until it is clearly too dark, invert the mask, and paint the darkening back in only where you want it. That is the whole craft in one gesture.