diff --git a/crates/engine/src/media.rs b/crates/engine/src/media.rs
index 5d0c5d821..91b7bdb8f 100644
--- a/crates/engine/src/media.rs
+++ b/crates/engine/src/media.rs
@@ -881,6 +881,32 @@ impl crate::Session {
         Some(job)
     }
 
+    /// A render of one window of the loupe's frame, for a view zoomed past what one whole-frame
+    /// render can hold: the frame is `full_w × full_h` (the size the loupe would draw it at) and
+    /// the result is `window`'s pixels of it, at that scale, from the source level that size needs.
+    /// Nothing is cached here (a window is only worth keeping while it is on screen).
+    pub fn region_job(
+        &mut self,
+        id: PhotoId,
+        full_w: usize,
+        full_h: usize,
+        window: lightcraft_pipeline::PixelWindow,
+        apply_crop: bool,
+    ) -> Option<RenderJob> {
+        let mut job = self.render_job(id, full_w, full_h, false, apply_crop)?;
+        job.request.window = Some(window);
+        job.key = Hasher128::new()
+            .u64(job.key)
+            .str("window")
+            .u64(window.x as u64)
+            .u64(window.y as u64)
+            .u64(window.w as u64)
+            .u64(window.h as u64)
+            .finish()
+            .0 as u64;
+        Some(job)
+    }
+
     /// The embedded preview of an unedited raw (path, loader), when the app installed a loader.
     fn embedded_of(&self, p: &Photo) -> Option<(String, PreviewLoader)> {
         match (&p.source, &self.media.preview_loader) {
@@ -1218,6 +1244,31 @@ mod tests {
         assert_ne!(a.key, b.key);
     }
 
+    // Issue #323: a window of a 1:1 view is its own job: the window's pixels, at the zoom scale,
+    // from the original, with a key of its own
+    #[test]
+    fn region_jobs_render_a_window_of_the_zoomed_frame() {
+        use lightcraft_pipeline::PixelWindow;
+        let mut s = crate::Session::with_demo();
+        let p = s.catalog.photos().next().unwrap().clone();
+        let (w, h) = (p.width as usize, p.height as usize);
+        assert!(w.max(h) > 2560, "demo photos are camera-sized");
+        let win = PixelWindow { x: w / 3, y: h / 3, w: 300, h: 200 };
+        let job = s.region_job(p.id, w, h, win, true).unwrap();
+        assert_eq!(job.level, SourceLevel::Full, "a 1:1 window reads the original");
+        let img = job.run().rendered.unwrap().image;
+        assert_eq!((img.width, img.height), (300, 200));
+        // other windows, and the whole-frame job, never share a key
+        let other = s.region_job(p.id, w, h, PixelWindow { x: win.x + 64, ..win }, true).unwrap();
+        let whole = s.render_job(p.id, w, h, false, true).unwrap();
+        let again = s.region_job(p.id, w, h, win, true).unwrap();
+        assert_ne!(other.key, again.key);
+        assert_ne!(whole.key, again.key);
+        assert_eq!(s.region_job(p.id, w, h, win, true).unwrap().key, again.key, "the same window keeps its key");
+        // a window never writes the photo's view preview or the thumbnail cache
+        assert!(again.view_cache.is_none() && again.cache.is_none());
+    }
+
     #[test]
     fn variant_jobs_have_their_own_keys_and_hit_the_cache() {
         let mut s = crate::Session::with_demo();
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 98e9367e4..a4e009e4f 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -19,6 +19,7 @@ pub mod menubar;
 pub mod menus;
 pub mod merge;
 pub mod panels;
+pub mod region;
 pub mod render;
 pub mod shortcuts;
 pub mod softpaint;
diff --git a/crates/ui-egui/src/region.rs b/crates/ui-egui/src/region.rs
new file mode 100644
index 000000000..75cc062c9
--- /dev/null
+++ b/crates/ui-egui/src/region.rs
@@ -0,0 +1,113 @@
+//! Sharp zoomed views (issue #323). The loupe renders the whole frame in one texture, which can't
+//! be as large as a 1:1 or 8:1 view of a big photo. Past that size it also renders just the window
+//! of the frame that is on screen, at the zoom scale, and draws it over the whole-frame render.
+//!
+//! This module is the geometry: when a window is needed and which one. Windows are snapped to a
+//! grid and carry a margin, so panning by a few pixels re-uses the render and the spatial stages
+//! (clarity, dehaze…) have the context they read around the visible edge.
+
+use lightcraft_engine::pipeline::PixelWindow;
+
+/// Windows start and end on multiples of this many pixels of the zoomed frame.
+pub const SNAP: usize = 256;
+/// Context rendered beyond the visible pixels (a multiple of [`SNAP`] after snapping).
+pub const MARGIN: usize = 256;
+/// The most a window may span on either axis: a bound on its texture whatever the window size.
+pub const MAX_SPAN: usize = 8192;
+
+/// Whether a view drawn `drawn_long` pixels along its long edge needs a window render on top of
+/// the whole-frame render, which has only `rendered_long` of them.
+pub fn needed(drawn_long: f32, rendered_long: usize) -> bool {
+    drawn_long.is_finite() && drawn_long > rendered_long as f32 * 1.02 + 1.0
+}
+
+/// The window of a `full_w × full_h` frame to render so that `visible` — `(x0, y0, x1, y1)` in the
+/// frame's pixels, as much of the frame as is on screen — is covered: expanded by [`MARGIN`],
+/// snapped outwards to the [`SNAP`] grid and clamped into the frame. `None`: nothing is visible.
+pub fn window_for(full_w: usize, full_h: usize, visible: (f32, f32, f32, f32)) -> Option<PixelWindow> {
+    if full_w == 0 || full_h == 0 || [visible.0, visible.1, visible.2, visible.3].iter().any(|v| !v.is_finite()) {
+        return None;
+    }
+    let axis = |lo: f32, hi: f32, full: usize| -> Option<(usize, usize)> {
+        let (lo, hi) = (lo.max(0.0), hi.min(full as f32));
+        if hi <= lo {
+            return None;
+        }
+        let a = (lo as usize).saturating_sub(MARGIN) / SNAP * SNAP;
+        let b = ((hi.ceil() as usize).saturating_add(MARGIN)).div_ceil(SNAP).saturating_mul(SNAP).min(full);
+        // a window larger than MAX_SPAN keeps the visible part (centred on it), snapped
+        if b - a > MAX_SPAN {
+            let mid = ((lo + hi) / 2.0) as usize;
+            let a = mid.saturating_sub(MAX_SPAN / 2) / SNAP * SNAP;
+            return Some((a, (a + MAX_SPAN).min(full)));
+        }
+        Some((a, b))
+    };
+    let (x0, x1) = axis(visible.0, visible.2, full_w)?;
+    let (y0, y1) = axis(visible.1, visible.3, full_h)?;
+    Some(PixelWindow { x: x0, y: y0, w: x1 - x0, h: y1 - y0 })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // Given a view drawn at 6000 px and a render of 6000, no window is needed; at 6000 vs 2560 it is
+    #[test]
+    fn a_window_is_needed_only_when_the_whole_frame_render_is_stretched() {
+        assert!(!needed(6000.0, 6000));
+        assert!(!needed(6100.0, 6000), "a couple of percent is not worth a second render");
+        assert!(needed(6000.0, 2560));
+        assert!(needed(48000.0, 8192));
+        assert!(!needed(f32::NAN, 100) && !needed(f32::INFINITY, 100));
+    }
+
+    // Given a visible area in the middle of a big frame, the window covers it with a margin, on the grid
+    #[test]
+    fn the_window_covers_the_visible_area_with_a_margin_on_the_grid() {
+        let w = window_for(48_000, 32_000, (20_000.0, 10_000.0, 21_400.0, 10_900.0)).unwrap();
+        assert!(w.x <= 20_000 - MARGIN && w.x + w.w >= 21_400 + MARGIN, "{w:?}");
+        assert!(w.y <= 10_000 - MARGIN && w.y + w.h >= 10_900 + MARGIN, "{w:?}");
+        assert_eq!((w.x % SNAP, w.y % SNAP, w.w % SNAP, w.h % SNAP), (0, 0, 0, 0));
+    }
+
+    // Given a small pan, the same window is kept (no re-render); a big pan changes it
+    #[test]
+    fn small_pans_keep_the_window() {
+        let a = window_for(48_000, 32_000, (20_000.0, 10_000.0, 21_400.0, 10_900.0)).unwrap();
+        let b = window_for(48_000, 32_000, (20_040.0, 10_030.0, 21_440.0, 10_930.0)).unwrap();
+        assert_eq!(a, b);
+        let c = window_for(48_000, 32_000, (26_000.0, 10_000.0, 27_400.0, 10_900.0)).unwrap();
+        assert_ne!(a, c);
+    }
+
+    // Given a frame smaller than the margin, or a view at its edge, the window is clamped into it
+    #[test]
+    fn the_window_stays_inside_the_frame() {
+        let w = window_for(3000, 2000, (-500.0, -500.0, 900.0, 700.0)).unwrap();
+        assert_eq!((w.x, w.y), (0, 0));
+        let w = window_for(3000, 2000, (2500.0, 1500.0, 9000.0, 9000.0)).unwrap();
+        assert!(w.x + w.w <= 3000 && w.y + w.h <= 2000, "{w:?}");
+        assert_eq!((w.x + w.w, w.y + w.h), (3000, 2000));
+    }
+
+    // Given nothing visible, or hostile numbers, there is no window (never a panic)
+    #[test]
+    fn nothing_visible_or_hostile_input_is_no_window() {
+        assert_eq!(window_for(1000, 1000, (2000.0, 0.0, 3000.0, 100.0)), None);
+        assert_eq!(window_for(1000, 1000, (10.0, 10.0, 10.0, 90.0)), None);
+        assert_eq!(window_for(0, 1000, (0.0, 0.0, 10.0, 10.0)), None);
+        for v in [f32::NAN, f32::INFINITY, f32::NEG_INFINITY] {
+            assert_eq!(window_for(1000, 1000, (v, 0.0, 10.0, 10.0)), None);
+        }
+        let w = window_for(usize::MAX / 2, usize::MAX / 2, (0.0, 0.0, 1e30, 1e30)).unwrap();
+        assert!(w.w <= MAX_SPAN && w.h <= MAX_SPAN, "{w:?}");
+    }
+
+    // Given a view of an enormous area, the window is bounded
+    #[test]
+    fn a_window_is_bounded() {
+        let w = window_for(100_000, 100_000, (0.0, 0.0, 90_000.0, 90_000.0)).unwrap();
+        assert!(w.w <= MAX_SPAN && w.h <= MAX_SPAN);
+    }
+}