diff --git a/Sources/ReaderMd/Models/Settings.swift b/Sources/ReaderMd/Models/Settings.swift
index d312dfe..f3fc44f 100644
--- a/Sources/ReaderMd/Models/Settings.swift
+++ b/Sources/ReaderMd/Models/Settings.swift
@@ -118,8 +118,10 @@ enum Settings {
         defaults.set(value, forKey: sidebarWidthKey)
     }
 
-    // Focus mode. All four default on: the mode's advertised behaviour is the
-    // full takeover, and each switch only subtracts from it.
+    // Focus mode. Fullscreen, the narrow canvas and the hidden toolbar default
+    // on: that takeover is the mode's advertised behaviour, and each switch only
+    // subtracts from it. Dimming is the exception — it rewrites the page itself
+    // rather than the chrome around it, so it is opt-in.
     static func loadFocusFullscreen() -> Bool {
         defaults.object(forKey: focusFullscreenKey) as? Bool ?? true
     }
@@ -128,7 +130,7 @@ enum Settings {
     }
 
     static func loadFocusDimSections() -> Bool {
-        defaults.object(forKey: focusDimSectionsKey) as? Bool ?? true
+        defaults.object(forKey: focusDimSectionsKey) as? Bool ?? false
     }
     static func saveFocusDimSections(_ value: Bool) {
         defaults.set(value, forKey: focusDimSectionsKey)
@@ -148,13 +150,17 @@ enum Settings {
         defaults.set(value, forKey: focusHideToolbarKey)
     }
 
-    // Both default to what focus mode shipped with, so an existing install sees
-    // no change until it touches a setting. `object(forKey:) as?` rather than
-    // `integer`/`double`: those return 0 for an absent key, and 0 is neither a
-    // valid heading level nor a legible opacity.
+    // The region defaults to `.h3`: every heading is too fine a grain for most
+    // documents, where an `h4` is a step inside the section being read rather
+    // than a section of its own. Only an ABSENT key takes that default, and the
+    // focus keys move independently — an install that switched dimming on but
+    // never opened the picker keeps its toggle and does widen to `.h3`.
+    // `object(forKey:) as?` rather than `integer`/`double`: those return 0 for
+    // an absent key, and 0 is neither a valid heading level nor a legible
+    // opacity.
     static func loadFocusRegionDepth() -> FocusRegionDepth {
         guard let raw = defaults.object(forKey: focusRegionDepthKey) as? Int,
-              let depth = FocusRegionDepth(rawValue: raw) else { return .any }
+              let depth = FocusRegionDepth(rawValue: raw) else { return .h3 }
         return depth
     }
     static func saveFocusRegionDepth(_ value: FocusRegionDepth) {
diff --git a/Sources/ReaderMd/Resources/docs/CHANGELOG.md b/Sources/ReaderMd/Resources/docs/CHANGELOG.md
index f5e33fe..52410af 100644
--- a/Sources/ReaderMd/Resources/docs/CHANGELOG.md
+++ b/Sources/ReaderMd/Resources/docs/CHANGELOG.md
@@ -6,6 +6,18 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Changed
+
+- **Focus mode no longer dims other sections out of the box.** ⌥⌘F still hides
+  the chrome, goes fullscreen and narrows the canvas; dimming — the one piece
+  that changes the page itself rather than the chrome around it — is now
+  opt-in, under **Dim other sections** in Settings → Focus Mode.
+- **A dim region now ends at H3 rather than at every heading.** With dimming
+  on, an `h4` inside the section you're reading no longer splits it. *Any
+  heading* is still there in **Region ends at**, alongside H2 and H1 — and the
+  setting is now a floor rather than a hard rule, so a document with no headings
+  that coarse falls back to the next level down instead of dimming nothing.
+
 ## [1.20.1] - 2026-09-07
 
 ### Changed
diff --git a/Sources/ReaderMd/Resources/docs/SHORTCUTS.md b/Sources/ReaderMd/Resources/docs/SHORTCUTS.md
index cf94de6..bb95ecb 100644
--- a/Sources/ReaderMd/Resources/docs/SHORTCUTS.md
+++ b/Sources/ReaderMd/Resources/docs/SHORTCUTS.md
@@ -70,7 +70,7 @@ document you're reading.
 | ⌘0 | Actual Size |
 | ⇧⌘\\ | Cycle Canvas Width (Narrow / Wide / Full) |
 | ⇧⌘D | Toggle Diff (in a git repository) |
-| ⌥⌘F | Focus Mode (hides the chrome, dims other sections) |
+| ⌥⌘F | Focus Mode (hides the chrome, goes fullscreen, narrows the canvas) |
 
 ## Settings
 
diff --git a/Sources/ReaderMd/Resources/web/bridge.js b/Sources/ReaderMd/Resources/web/bridge.js
index f7ac83d..61ef178 100644
--- a/Sources/ReaderMd/Resources/web/bridge.js
+++ b/Sources/ReaderMd/Resources/web/bridge.js
@@ -649,20 +649,23 @@ function postTOC() {
 }
 
 let focusDim = false;
-// The deepest heading level that ends a region; 4 = every heading, the default.
-let focusDepth = 4;
+// The deepest heading level that ends a region; 3 = H3 or above. Mirrors
+// `Settings.loadFocusRegionDepth`'s default — Swift pushes the real value on
+// `ready`, so this only has to agree with it.
+let focusDepth = 3;
 
 // Classes the top-level blocks OUTSIDE the active heading's region. Deliberately
 // no <section> wrappers: marked emits a flat h2/p/p/h2 sibling list, and mark
 // anchoring, find, footnotes and diff hunks all read that flat structure.
 //
-// The region ends at the next heading at or above `focusDepth` — every heading
-// by default. Depth is an ABSOLUTE level, fixed by the setting, never one
-// relative to the active heading. The relative rule ("the next heading of the
-// same or higher level") would make a 20px scroll across a nested heading swing
-// the lit region between a paragraph and its whole parent section; an absolute
-// level changes the region only when a boundary heading is crossed, and a
-// heading deeper than the setting is not a boundary at all.
+// The region ends at the next heading at or above `focusDepth` — H3 or above by
+// default, and a floor rather than a fixed rule (see below). Depth is an
+// ABSOLUTE level, fixed by the setting, never one relative to the active
+// heading. The relative rule ("the next heading of the same or higher level")
+// would make a 20px scroll across a nested heading swing the lit region between
+// a paragraph and its whole parent section; an absolute level changes the region
+// only when a boundary heading is crossed, and a heading deeper than the depth
+// in force is not a boundary at all.
 function applyFocusDim() {
   const blocks = [...contentEl.children];
   for (const b of blocks) b.classList.remove('focus-dim');
@@ -672,13 +675,16 @@ function applyFocusDim() {
   // side-by-side).
   if (!focusDim || diffMode || findQuery) return;
 
-  const headings = [];
-  blocks.forEach((b, i) => {
-    if (/^H[1-4]$/.test(b.tagName) && +b.tagName[1] <= focusDepth) headings.push(i);
-  });
-  // One region means dimming has nothing to say. Also the answer when every
-  // heading in the document is deeper than the chosen depth: no boundaries, so
-  // no regions to tell apart.
+  // `focusDepth` is a floor, not a fixed rule: a document with fewer than two
+  // headings that coarse (an h1 title over h4 steps, say) would have no
+  // boundaries at all, so widen a level at a time until it has two.
+  let headings = [];
+  for (let depth = focusDepth; depth <= 4 && headings.length < 2; depth++) {
+    headings = blocks.flatMap((b, i) =>
+      /^H[1-4]$/.test(b.tagName) && +b.tagName[1] <= depth ? [i] : []);
+  }
+  // Still one region at every depth: a document with fewer than two headings,
+  // where dimming has nothing to say.
   if (headings.length < 2) return;
 
   let active = headings[0];
diff --git a/Tests/ReaderMdTests/FocusModeTests.swift b/Tests/ReaderMdTests/FocusModeTests.swift
index 7b89a83..4a90f8e 100644
--- a/Tests/ReaderMdTests/FocusModeTests.swift
+++ b/Tests/ReaderMdTests/FocusModeTests.swift
@@ -11,10 +11,10 @@ final class FocusModeTests: XCTestCase {
     private var savedTOC = false
     private var savedWidth = ContentWidth.wide
     private var savedFocusFullscreen = true
-    private var savedFocusDimSections = true
+    private var savedFocusDimSections = false
     private var savedFocusNarrowCanvas = true
     private var savedFocusHideToolbar = true
-    private var savedFocusRegionDepth = FocusRegionDepth.any
+    private var savedFocusRegionDepth = FocusRegionDepth.h3
     private var savedFocusDimOpacity = 0.38
 
     override func setUp() async throws {
@@ -29,14 +29,15 @@ final class FocusModeTests: XCTestCase {
         savedFocusDimOpacity = Settings.loadFocusDimOpacity()
 
         // Tests below flip individual focus switches, and those persist to
-        // UserDefaults like any other setter. Reset to the shipped defaults
-        // (all four on) so each test starts from a known baseline instead of
-        // whatever an earlier test in this suite left behind.
+        // UserDefaults like any other setter. Pin the shipped defaults so each
+        // test starts from a known baseline instead of whatever an earlier test
+        // left behind — except dimming, which ships off and is pinned on here so
+        // the dimming tests have something to assert.
         Settings.saveFocusFullscreen(true)
         Settings.saveFocusDimSections(true)
         Settings.saveFocusNarrowCanvas(true)
         Settings.saveFocusHideToolbar(true)
-        Settings.saveFocusRegionDepth(.any)
+        Settings.saveFocusRegionDepth(.h3)
         Settings.saveFocusDimOpacity(0.38)
     }
 
@@ -133,16 +134,30 @@ final class FocusModeTests: XCTestCase {
 
     // MARK: - Configurable dimming
 
-    /// Both new preferences default to today's behaviour, so an existing install
-    /// sees no change until it touches a setting.
-    func testDimmingPreferencesDefaultToTodaysBehaviour() {
+    /// Dimming is opt-in, and when it is switched on the region is a section
+    /// down to `h3` rather than every heading.
+    func testDimmingDefaultsToOffAtH3() {
+        Settings.defaults.removeObject(forKey: "reader.md.focus.dimSections")
         Settings.defaults.removeObject(forKey: "reader.md.focus.regionDepth")
         Settings.defaults.removeObject(forKey: "reader.md.focus.dimOpacity")
 
-        XCTAssertEqual(Settings.loadFocusRegionDepth(), .any)
+        XCTAssertFalse(Settings.loadFocusDimSections())
+        XCTAssertEqual(Settings.loadFocusRegionDepth(), .h3)
         XCTAssertEqual(Settings.loadFocusDimOpacity(), 0.38, accuracy: 0.0001)
     }
 
+    /// The other three switches are the mode's advertised takeover, so they stay
+    /// on out of the box.
+    func testTheOtherThreeSwitchesDefaultOn() {
+        Settings.defaults.removeObject(forKey: "reader.md.focus.fullscreen")
+        Settings.defaults.removeObject(forKey: "reader.md.focus.narrowCanvas")
+        Settings.defaults.removeObject(forKey: "reader.md.focus.hideToolbar")
+
+        XCTAssertTrue(Settings.loadFocusFullscreen())
+        XCTAssertTrue(Settings.loadFocusNarrowCanvas())
+        XCTAssertTrue(Settings.loadFocusHideToolbar())
+    }
+
     /// An absent key must not read as 0: `defaults.integer` would make depth 0
     /// (no valid heading level) and `defaults.double` would make the document
     /// invisible.
@@ -180,10 +195,10 @@ final class FocusModeTests: XCTestCase {
 
     /// A depth stored by a future build (or corrupted) must not become a depth of
     /// 0, which would produce an empty boundary list and silently kill dimming.
-    func testUnknownStoredDepthFallsBackToAny() {
+    func testUnknownStoredDepthFallsBackToTheDefault() {
         Settings.defaults.set(99, forKey: "reader.md.focus.regionDepth")
 
-        XCTAssertEqual(Settings.loadFocusRegionDepth(), .any)
+        XCTAssertEqual(Settings.loadFocusRegionDepth(), .h3)
     }
 
     /// The preview widens what counts as "dimming is showing" — never what counts
diff --git a/docs/assets/screenshots/settings/01-window.png b/docs/assets/screenshots/settings/01-window.png
index 820adee..9ea36d0 100644
Binary files a/docs/assets/screenshots/settings/01-window.png and b/docs/assets/screenshots/settings/01-window.png differ
diff --git a/docs/features.md b/docs/features.md
index fcf9456..0f3ec11 100644
--- a/docs/features.md
+++ b/docs/features.md
@@ -43,7 +43,7 @@ See [Reading a document](features/reading.md).
 - **Text size** (⌘+ / ⌘− / ⌘0) and **canvas width** (⇧⌘\\, narrow / wide / full) — both persisted
 - **Reading feedback** — a progress bar under the toolbar, with the word count and reading time under the file name
 - **Resume where you stopped** — a long document reopens at the place you left it
-- **Focus mode** (⌥⌘F) — one toggle hides the chrome, goes fullscreen, and dims everything outside the section you're reading
+- **Focus mode** (⌥⌘F) — one toggle hides the chrome, goes fullscreen, and narrows the canvas; an optional setting also dims everything outside the section you're reading
 
 ## How a document is rendered
 
@@ -186,7 +186,7 @@ reaches only text fields.
 | ⌘0 | Actual Size |
 | ⇧⌘\\ | Cycle Canvas Width (Narrow / Wide / Full) |
 | ⇧⌘D | Toggle Diff (in a git repository) |
-| ⌥⌘F | Focus Mode (hides the chrome, dims other sections) |
+| ⌥⌘F | Focus Mode (hides the chrome, goes fullscreen, narrows the canvas) |
 
 ### Settings and help
 
diff --git a/docs/features/reading.md b/docs/features/reading.md
index fd753ab..781aca0 100644
--- a/docs/features/reading.md
+++ b/docs/features/reading.md
@@ -75,11 +75,12 @@ tables and long code blocks.
 ## Focus mode
 
 **⌥⌘F** takes everything away except the page. The sidebar and outline
-collapse, the toolbar goes, the window moves to fullscreen, the canvas
-narrows, and every section but the one you're reading dims. The toolbar's
-focus button and `>Focus Mode` in ⌘P do the same thing.
+collapse, the toolbar goes, the window moves to fullscreen, and the canvas
+narrows. Switch on **Dim other sections** in Settings and every section but
+the one you're reading dims too. The toolbar's focus button and `>Focus Mode`
+in ⌘P do the same thing.
 
-![Focus mode — chrome gone, everything but the current section dimmed](../assets/screenshots/reading/06-focus-mode.png)
+![Focus mode with Dim other sections switched on — chrome gone, everything but the current section dimmed](../assets/screenshots/reading/06-focus-mode.png)
 
 **⌥⌘F again, or ⎋, brings it all back** — and back means back: the sidebar,
 outline, and column width return to what they were, not to a default. ⎋
@@ -90,27 +91,29 @@ doesn't outlive the fullscreen it's running in. (If the window was
 already in fullscreen before ⌥⌘F, exiting focus mode leaves it there rather
 than dropping out.)
 
-The dimming follows the outline rather than the scroll position, so it holds
-still while you read a section and fades across when you reach the next
-heading. **Region ends at** in Settings decides how wide "a section" is: every
-heading by default, or only headings down to H3, H2 or H1 — at *H2 or above* an
-`h2` stays lit across all of its subheadings, and crossing one of them changes
-nothing. A document whose headings are all deeper than that setting has no
-regions to tell apart, so nothing dims. **Dimming** sets how far the rest fades,
-from 40% to 88%. Dimming steps aside entirely while you're searching, in diff
-mode, and in a document with fewer than two headings.
+Dimming is off until you turn it on, and once on it follows the outline rather
+than the scroll position, so it holds still while you read a section and fades
+across when you reach the next heading. **Region ends at** in Settings decides
+how wide "a section" is: headings down to H3 by default, or H2, H1, or every
+heading — at *H2 or above* an `h2` stays lit across all of its subheadings, and
+crossing one of them changes nothing. The setting is a floor rather than a hard
+rule: a document with no headings that coarse — an `h1` title over `h4` steps,
+say — falls back to the next level down rather than dimming nothing. **Dimming**
+sets how far the rest fades, from 40% to 88%. Dimming steps aside entirely
+while you're searching, in diff mode, and in a document with fewer than two
+headings.
 
 ⌘F still works: rather than dropping you out of focus mode, it slides the
 toolbar back down with the find field ready, and the toolbar stays down until
 you leave focus mode.
 
 Settings (⌘,) has a switch for each of the four pieces — fullscreen, dimming,
-narrow canvas, hidden toolbar — all on by default, so you can keep only the
-parts you want. Hiding the toolbar behaves the same whether or not fullscreen
-is on, and the toolbar is never far away: nudge the pointer to the top edge of
-the screen and it slides back down for as long as you're up there — long enough
-to reach a control and click it. ⌘F brings it down too, and keeps it down while
-you search. Neither costs you focus mode.
+narrow canvas, hidden toolbar — so you can keep only the parts you want; all
+are on out of the box except dimming. Hiding the toolbar behaves the same
+whether or not fullscreen is on, and the toolbar is never far away: nudge the
+pointer to the top edge of the screen and it slides back down for as long as
+you're up there — long enough to reach a control and click it. ⌘F brings it
+down too, and keeps it down while you search. Neither costs you focus mode.
 
 Focus mode never persists. However you leave the app, it starts up outside it.
 
diff --git a/docs/features/reading.shots.json b/docs/features/reading.shots.json
index b7a9150..9e8835a 100644
--- a/docs/features/reading.shots.json
+++ b/docs/features/reading.shots.json
@@ -56,12 +56,12 @@
     {
       "id": "06-focus-mode",
       "open": "field-notes/guides/setup.md",
-      "prefs": { "reader.md.focus.fullscreen": false },
+      "prefs": { "reader.md.focus.fullscreen": false, "reader.md.focus.dimSections": true },
       "actions": [
         { "key": "f", "mods": ["command", "option"] },
         { "waitMs": 900 }
       ],
-      "caption": "Focus mode — chrome gone, everything but the current section dimmed"
+      "caption": "Focus mode with Dim other sections switched on — chrome gone, everything but the current section dimmed"
     }
   ]
 }
diff --git a/docs/features/settings.md b/docs/features/settings.md
index 9a26b46..ea0be34 100644
--- a/docs/features/settings.md
+++ b/docs/features/settings.md
@@ -60,14 +60,17 @@ same three ⇧⌘\\ cycles through. Both are described in more detail on
 
 Focus mode (⌥⌘F) is four things at once, and each is a switch here: **Enter
 fullscreen**, **Dim other sections**, **Narrow the canvas**, and **Hide the
-toolbar**. All four are on by default. Turning all four off leaves ⌥⌘F with
-nothing to do, and the window says so.
+toolbar**. Three of them are on by default; **Dim other sections** is off, since
+it changes the page itself rather than the chrome around it — switch it on if
+you want it. Turning all four off leaves ⌥⌘F with nothing to do, and the window
+says so.
 
 Two settings shape the dimming itself. **Region ends at** decides how much of
-the document counts as the section you're reading: *Any heading* is the default
-and lights one heading's worth at a time, while *H2 or above* keeps a whole `h2`
-section lit including its subheadings. **Dimming** sets how far everything else
-fades, from 40% to 88%.
+the document counts as the section you're reading: *H3 or above* is the default,
+so an `h4` step inside a section doesn't split it, while *H2 or above* keeps a
+whole `h2` section lit including its subheadings and *Any heading* lights one
+heading's worth at a time. **Dimming** sets how far everything else fades, from
+40% to 88%.
 
 Both only matter with **Dim other sections** on, and grey out without it. While
 this window is open the document behind it previews them, so dragging the slider