diff --git a/backend/__tests__/integration/slideshowPhotoSource.test.js b/backend/__tests__/integration/slideshowPhotoSource.test.js
new file mode 100644
index 00000000..f141c8be
--- /dev/null
+++ b/backend/__tests__/integration/slideshowPhotoSource.test.js
@@ -0,0 +1,150 @@
+/**
+ * Slideshow photo source (#1015).
+ *
+ * The bug: with `lightbox_preview_enabled` off (the default), /photos emitted
+ * `preview_url: null`, so the slideshow's `preview_url || hero_url || url`
+ * chain fell through to `hero_url` — a 1920x1080 `fit: 'cover'` centre crop
+ * meant for gallery header banners. With the "Black Bars (No crop)" fit the
+ * show then letterboxed an already-cropped frame: portrait photos lost their
+ * top and bottom and the setting looked broken.
+ *
+ * The contract pinned here: `slideshow_url` points at the aspect-preserved
+ * preview tier and is emitted for image photos REGARDLESS of the lightbox
+ * toggle, so the slideshow never has a reason to reach for `hero_url`.
+ * `preview_url` itself must stay gated — the lightbox opt-in is unchanged.
+ */
+
+const request = require('supertest');
+const express = require('express');
+const cookieParser = require('cookie-parser');
+const jwt = require('jsonwebtoken');
+
+const { bootCrmDb, seedMinimal } = require('./helpers/crmDb');
+
+process.env.JWT_SECRET = process.env.JWT_SECRET || 'slideshow-src-test-secret';
+
+const SLUG = 'slideshow-source-event';
+
+describe('Slideshow photo source (#1015)', () => {
+  let db;
+  let cleanup;
+  let app;
+  let eventId;
+  let imagePhotoId;
+  let videoPhotoId;
+
+  const galleryToken = () => jwt.sign(
+    { eventId, eventSlug: SLUG, type: 'gallery' },
+    process.env.JWT_SECRET,
+    { expiresIn: '1h', issuer: 'picpeak-auth' }
+  );
+
+  const setLightboxPreview = async (on) => {
+    await db('app_settings').where({ setting_key: 'lightbox_preview_enabled' }).del();
+    await db('app_settings').insert({
+      setting_key: 'lightbox_preview_enabled',
+      setting_value: JSON.stringify(on),
+      setting_type: 'general',
+      updated_at: new Date().toISOString(),
+    });
+  };
+
+  const fetchPhotos = async () => {
+    const res = await request(app)
+      .get(`/api/gallery/${SLUG}/photos`)
+      .set('Authorization', `Bearer ${galleryToken()}`)
+      .expect(200);
+    return res.body.photos;
+  };
+
+  beforeAll(async () => {
+    ({ db, cleanup } = await bootCrmDb());
+    await seedMinimal(db);
+
+    const inserted = await db('events').insert({
+      slug: SLUG,
+      event_type: 'wedding',
+      event_name: 'Slideshow Source Test',
+      event_date: '2026-08-01',
+      host_email: 'host@example.com',
+      admin_email: 'admin@example.com',
+      password_hash: 'x',
+      share_link: `/gallery/${SLUG}/share`,
+      share_token: 'slideshow-source-share',
+      expires_at: new Date(Date.now() + 7 * 24 * 3600 * 1000).toISOString(),
+      is_active: 1,
+      is_archived: 0,
+      is_draft: 0,
+      created_at: new Date().toISOString(),
+    }).returning('id');
+    eventId = inserted[0]?.id ?? inserted[0];
+
+    const img = await db('photos').insert({
+      event_id: eventId,
+      filename: 'portrait.jpg',
+      path: 'events/slideshow-source/portrait.jpg',
+      type: 'individual',
+      mime_type: 'image/jpeg',
+      uploaded_at: new Date().toISOString(),
+    }).returning('id');
+    imagePhotoId = img[0]?.id ?? img[0];
+
+    const vid = await db('photos').insert({
+      event_id: eventId,
+      filename: 'clip.mp4',
+      path: 'events/slideshow-source/clip.mp4',
+      type: 'individual',
+      media_type: 'video',
+      mime_type: 'video/mp4',
+      uploaded_at: new Date().toISOString(),
+    }).returning('id');
+    videoPhotoId = vid[0]?.id ?? vid[0];
+
+    app = express();
+    app.use(express.json());
+    app.use(cookieParser());
+    app.use('/api/gallery', require('../../src/routes/gallery'));
+  }, 120000);
+
+  afterAll(async () => {
+    if (cleanup) await cleanup();
+  });
+
+  it('emits slideshow_url for image photos even when lightbox previews are OFF', async () => {
+    await setLightboxPreview(false);
+    const photos = await fetchPhotos();
+    const image = photos.find((p) => p.id === imagePhotoId);
+
+    expect(image.slideshow_url).toBe(`/api/gallery/${SLUG}/preview/${imagePhotoId}`);
+    // The regression: this is what used to be null, pushing the show to hero.
+    expect(image.preview_url).toBeNull();
+  });
+
+  it('leaves preview_url gated so the lightbox opt-in is unchanged', async () => {
+    await setLightboxPreview(true);
+    const photos = await fetchPhotos();
+    const image = photos.find((p) => p.id === imagePhotoId);
+
+    expect(image.preview_url).toBe(`/api/gallery/${SLUG}/preview/${imagePhotoId}`);
+    expect(image.slideshow_url).toBe(image.preview_url);
+  });
+
+  it('never points the slideshow at the cover-cropped hero tier', async () => {
+    await setLightboxPreview(false);
+    const photos = await fetchPhotos();
+    const image = photos.find((p) => p.id === imagePhotoId);
+
+    // hero_url still ships (the gallery header uses it) — it just must not be
+    // what the slideshow resolves to.
+    expect(image.hero_url).toBe(`/api/gallery/${SLUG}/hero/${imagePhotoId}`);
+    expect(image.slideshow_url).not.toBe(image.hero_url);
+  });
+
+  it('emits slideshow_url: null for videos, which have no preview tier', async () => {
+    await setLightboxPreview(false);
+    const photos = await fetchPhotos();
+    const video = photos.find((p) => p.id === videoPhotoId);
+
+    expect(video.slideshow_url).toBeNull();
+  });
+});
diff --git a/backend/src/routes/gallery.js b/backend/src/routes/gallery.js
index 196cb746..420808eb 100644
--- a/backend/src/routes/gallery.js
+++ b/backend/src/routes/gallery.js
@@ -726,6 +726,17 @@ router.get('/:slug/photos', verifyGalleryAccess, resolveGuest, async (req, res)
             && (!photo.mime_type || !photo.mime_type.startsWith('video/'))
             ? `/api/gallery/${req.params.slug}/preview/${photo.id}${wmQuery}`
             : null,
+          // Slideshow source (#1015). Same preview tier, but emitted
+          // unconditionally: the slideshow has no `url` fallback worth
+          // taking (originals are projector-sized) and must never land on
+          // `hero_url`, which is cover-cropped to 16:9 — that made the
+          // "no crop" fit letterbox an already-cropped frame. The preview
+          // route generates lazily and redirects to the original on any
+          // failure, so this is safe even where no preview exists yet.
+          slideshow_url: photo.media_type !== 'video'
+            && (!photo.mime_type || !photo.mime_type.startsWith('video/'))
+            ? `/api/gallery/${req.params.slug}/preview/${photo.id}${wmQuery}`
+            : null,
           secure_url_template: `/api/secure-images/${req.params.slug}/secure/${photo.id}/{{token}}`,
           download_url_template: `/api/secure-images/${req.params.slug}/secure-download/${photo.id}/{{token}}`,
           type: photo.type,
diff --git a/frontend/src/pages/gallery/SlideshowPage.tsx b/frontend/src/pages/gallery/SlideshowPage.tsx
index 540491b3..0d097055 100644
--- a/frontend/src/pages/gallery/SlideshowPage.tsx
+++ b/frontend/src/pages/gallery/SlideshowPage.tsx
@@ -23,8 +23,15 @@ const STATE_POLL_MS = 3000;
 // Prefer the aspect-preserved preview (≤1920px) over the full original; fall
 // back to the standard url. Always absolutised so it works whether the API is
 // same-origin or an explicit absolute base.
+//
+// Deliberately never `hero_url` (#1015): that tier is cover-cropped to 16:9
+// for gallery header banners, so with fit='contain' the show letterboxed an
+// already-cropped frame — portrait photos lost their top and bottom and the
+// "Black Bars (No crop)" setting looked broken. `slideshow_url` is the same
+// aspect-preserved preview as `preview_url` but is always emitted, so the
+// crop can't come back when lightbox previews are off (the default).
 function photoSrc(photo: Photo): string {
-  return buildResourceUrl(photo.preview_url || photo.hero_url || photo.url);
+  return buildResourceUrl(photo.slideshow_url || photo.preview_url || photo.url);
 }
 
 // CSS `filter` applied directly to the image for filters that are pure tone
diff --git a/frontend/src/types/index.ts b/frontend/src/types/index.ts
index eefbb05b..5fac65a7 100644
--- a/frontend/src/types/index.ts
+++ b/frontend/src/types/index.ts
@@ -116,6 +116,11 @@ export interface Photo {
   // ≤1920px JPEG; the lightbox prefers it over `url` for image photos
   // and falls back to `url` when null (off, video, or not yet generated).
   preview_url?: string | null;
+  // Aspect-preserved ≤1920px source for the fullscreen slideshow (#1015).
+  // Always set for image photos, unlike `preview_url` — the slideshow must
+  // never fall back to `hero_url`, which is a 16:9 centre crop and makes
+  // the "Black Bars (No crop)" fit letterbox an already-cropped frame.
+  slideshow_url?: string | null;
   secure_url_template?: string;
   download_url_template?: string;
   requires_token?: boolean;