Lightroom Classic · Lesson 21 Masking: Brush
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Lightroom Classic — Lesson 21
When AI Gives Up,
You Paint.
The Brush mask is the most flexible masking tool in Lightroom — you paint exactly what you want affected, with complete control over hardness, opacity, and build-up.
🖌️ When Select Subject and Select Sky fall short, the Brush gives you total freehand control over any area of any image.
Location
Finding the Brush
1
Open the Masking panel — Shift+W or click the mask icon
The Masking panel opens in the right panel group in Develop. You'll see AI masks at the top, then Brush, Linear Gradient, Radial Gradient, and Range Masks below.
K
Press K — or click "Brush" in the Masking panel
K is the shortcut. Your cursor becomes a brush circle with two rings: inner ring = hard core, outer ring = feather edge. Brush controls appear in the panel below.
👁
Press O to toggle the red mask overlay
The red overlay shows exactly what area is masked. Toggle it while painting to check coverage. Press Shift+O to cycle overlay colors (red → green → white → black).
⌨️ K activates the brush instantly in Develop. The same shortcut from the old Adjustment Brush — muscle memory transfers.
Controls
The Four Brush Controls
📏
Size
Diameter of the brush. Use [ ] bracket keys to resize without touching the slider. Left bracket shrinks, right bracket grows.
🌫️
Feather
Softness of the brush edge. High feather (80–100) = soft fade — great for skin and sky. Low feather (0–20) = hard edge — for crisp geometric boundaries.
💨
Flow
Rate of build-up per stroke. Low flow (25–50) lets you build up gradually with multiple passes — like airbrushing. High flow (100) reaches full density immediately.
🔒
Density
The maximum cap — no matter how many strokes you paint, the mask can never exceed this value. Density 100 = full strength possible. Density 50 = always half strength.
✅ Good starting point: Size 50, Feather 70, Flow 50, Density 100. Adjust from there based on the subject.
Key Distinction
Flow vs. Density
These two controls are related but different. Knowing the distinction gives you precise control over mask strength.
💨
Flow — Build-Up Rate
Each overlapping stroke adds more mask opacity. Flow 50 = each stroke deposits 50%. Paint over the same spot multiple times to build up. Like spraying paint in layers.
🔒
Density — Hard Ceiling
No matter how many strokes, the mask stops at the Density value. Density 50 = always half strength everywhere, even with 100 overlapping strokes. A master volume knob.
💡 Best all-round recipe: Flow 50, Density 100. Gradual build-up with the option to go all the way to full strength.
Core Technique
Painting a Mask
1
Set your controls, then click and drag to paint
Hold the mouse button and drag across the area you want to affect. Paint in smooth strokes. You can paint multiple passes over the same area to build up coverage.
2
Hold Shift + click to paint a straight line
Click to set a start point, then hold Shift and click elsewhere — Lightroom paints a perfectly straight stroke between the two points. Great for horizons, fences, architectural edges.
3
Press O to check coverage — toggle overlay on/off
The red overlay shows the masked area. Toggle while painting. If the red blends with your subject, press Shift+O to change overlay color.
🎯 Adjustment sliders appear after your first stroke. Raise or lower Exposure, Texture, Clarity, etc. to apply the effect to your painted area.
Erase Mode
Alt / Option to Erase
Hold Alt (Windows) or Option (Mac) while painting to instantly switch to erase mode. Release to return to paint mode — no clicking needed.
🖌️
Paint Mode (+)
Default. Adds area to the mask with every stroke. The cursor shows a + symbol. Paint broadly at first — precision comes after.
🧹
Erase Mode (–)
Hold Alt/Option. Removes mask from any area you paint. The cursor shows a – symbol. Shrink the brush with [ first for precise edge work.
⚡ The workflow: paint broadly → hold Alt/Option → erase back with precision. Fast and forgiving.
Smart Feature
Auto Mask
A checkbox in the brush controls. When ON, Lightroom analyzes the color/brightness under the cursor's crosshair and only paints matching pixels — the brush snaps to edges automatically.
✅
Auto Mask ON — Use for:
🦁 Animal fur & feathers
💇 Human hair strands
🌿 Foliage & leaves
👁️ Painting around eyes
🏠 Architectural edges
⬜
Auto Mask OFF — Use for:
☁️ Open sky / backgrounds
🔆 Broad dodging & burning
🌫️ Smooth gradual transitions
🎭 Vignette-style darkening
🖼️ Any area without a clear edge
🔑 Key: the crosshair at the cursor center is the sample point — keep it inside the object you want to paint. Don't let the crosshair drift across the edge.
Decision Guide
Auto Mask: ON vs. OFF
ON
Auto Mask ON — near distinct edges with color contrast
Hair against a wall, feathers against sky, a jacket edge, roofline against blue sky. High contrast between subject and background = Auto Mask works well. Turn it on just for the edge strokes, then turn it off for the open area fill.
OFF
Auto Mask OFF — open areas and smooth gradients
Broad skin areas, open sky, background zones. Auto Mask on these areas creates uneven, patchy coverage because it fires inconsistently on low-contrast areas. Keep it off for smooth, even results.
⚡
Pro workflow: start OFF, switch ON near edges
Paint the broad area with Auto Mask off (faster, smoother). When you approach an edge you need to respect, check Auto Mask on. After clearing the edge, turn it off again. Toggle as needed throughout the painting session.
Practical Use
Dodging & Burning
The oldest darkroom technique — selectively brightening (dodge) and darkening (burn) specific areas.
☀️
Dodge — Brighten
Paint mask → raise Exposure.

Eyes: small brush, Auto Mask ON, Exposure +0.4–0.6
Skin highlights: large soft brush, flow 30
Catchlights: micro brush for specular highlights
🌑
Burn — Darken
Paint mask → lower Exposure.

Corners: large soft brush, Auto Mask OFF, –0.4
Background: paint around subject, drop exposure
Hair shadow side: darken for depth
🎨 Subtle is more convincing. Start at ±0.3 exposure and build up. Over-dodging and over-burning look unnatural.
Practical Use
Painting Sharpness & Texture
Add Texture or Clarity to just one element — feathers, fur, fabric — while leaving skin and backgrounds completely untouched.
🦅
Add Detail Here
🦋 Bird feathers → Texture +40–60
🐾 Animal fur → Texture +30
🧵 Fabric weave → Clarity +25
🌲 Bark / stone → Texture +50
🌸
Leave These Smooth
👤 Skin — no global Texture
🌫️ Blurred backgrounds
🌅 Smooth sky gradients
💧 Water / glass surfaces
💡 Texture = fine detail (feather barbs, pores). Clarity = broader local contrast (punchy, dimensional look). Use both together for maximum impact on detailed subjects.
Advanced Workflow
AI + Brush = Best Results
1
Start with Select Subject (or Select Sky)
Let the AI do the broad work. It gets 85–90% right in seconds. Don't try to be perfect manually from scratch when AI can get you most of the way there instantly.
2
Select the AI mask → click Subtract → choose Brush
In the Masks panel, with the AI mask selected, click "Subtract." Choose "Brush" from the menu. Now your brush removes from the AI mask — you're refining it, not creating a second separate mask.
3
Paint to erase AI errors — or Add → Brush to fill gaps
Paint over areas the AI included incorrectly (removes them). Or click Add → Brush and paint areas the AI missed (adds them back). One mask, combined from multiple inputs.
🤝 AI does 85% of the work. You do the precise 15% that matters. Together, faster and more accurate than either tool alone.
Your Challenge
🖌️
Brush Masking Challenge
Find a portrait. Complete these tasks before Lesson 22.
👁️ Brighten eyes — small brush, Auto Mask ON, Exposure +0.4 to +0.6
💇 Add Texture to hair — medium brush, Auto Mask ON, Texture +40
🌑 Burn corners — large soft brush, Auto Mask OFF, Exposure –0.4
🔀 Try AI + brush — Select Subject first, then refine with brush erase
⌨️ Resize with [ ] bracket keys only — don't touch the Size slider
💬 Share a before/after — press \ (backslash) to compare in Develop
Up Next
3 Things to Remember
1
Size [ ] · Flow · Density
Bracket keys resize. Flow builds up gradually. Density caps the maximum. Start: Flow 50, Density 100.
2
Alt/Option = Erase
Paint broadly first. Hold Alt/Option to erase back the oversprayed edges. Fast, forgiving, efficient.
3
AI First, Then Refine
Select Subject → Subtract → Brush. Let the AI do the heavy lifting, you handle the precise 15%.
Lesson 22 — Lightroom Classic
Luminance Range Mask
Instead of painting a location by hand, you select by brightness — "only affect pixels in this tonal range." Perfect for skies, shadows, and specular highlights. No painting required.
Tonal Selection No Painting Required Sky / Shadow / Highlights
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