diff --git a/client/tests/test_step_canvas_transport.gd b/client/tests/test_step_canvas_transport.gd
index e53a525f6..c1e25b500 100644
--- a/client/tests/test_step_canvas_transport.gd
+++ b/client/tests/test_step_canvas_transport.gd
@@ -76,9 +76,21 @@ func test_spacing_for_rung_matches_d243_metre_values() -> void:
 # =============================================================================
 
 
-func test_display_ratio_deep_rungs_are_one_to_one() -> void:
+## The four deep rungs share ONE display ratio, whatever its current value —
+## asserting the relationship rather than the literal, so tuning
+## DISPLAY_RATIO_DEEP (pair session 2026-07-26: 1.0 -> 2.0, the cost/looks
+## experiment) doesn't require editing a test that isn't about the number.
+## The value itself is a presentation tunable (D-255(a)); what must hold is
+## that the deep rungs agree with each other and stay a texel-exact integer.
+func test_display_ratio_deep_rungs_all_share_the_deep_ratio() -> void:
+	var deep: float = StepCanvasTransport.DISPLAY_RATIO_DEEP
 	for rung in ["District", "Quarter", "Block", "Chunk"]:
-		assert_float(StepCanvasTransport.display_ratio_for_rung(rung)).is_equal_approx(1.0, 0.001)
+		assert_float(StepCanvasTransport.display_ratio_for_rung(rung)).is_equal_approx(deep, 0.001)
+	assert_float(deep).override_failure_message(
+		"the deep display ratio must be a whole number of screen px per gridunit —"
+		+ " a fractional ratio reintroduces the sub-pixel blur D-255 texel-exactness prevents"
+	).is_equal_approx(floorf(deep), 0.0001)
+	assert_float(deep).is_greater(0.0)
 
 
 func test_display_ratio_shallow_rungs_use_the_five_x_five_fallback() -> void:
@@ -98,10 +110,16 @@ func test_is_orbital_rung_true_only_for_global_and_region() -> void:
 # =============================================================================
 
 
-func test_viewport_fit_extent_at_deep_ratio_matches_viewport_pixels() -> void:
-	# 1x1 ratio -> extent in gridunits == viewport px, 1:1.
-	var extent: Vector2i = StepCanvasTransport.viewport_fit_extent(Vector2(800.0, 600.0), "Chunk")
-	assert_that(extent).is_equal(Vector2i(800, 600))
+## A deep-rung request asks for viewport_px / DISPLAY_RATIO_DEEP gridunits —
+## derived from the constant, not the literal, so the ratio stays tunable
+## (see the deep-ratio test above). This IS the cost lever: doubling the
+## ratio quarters the requested cell count.
+func test_viewport_fit_extent_at_deep_ratio_divides_by_the_deep_ratio() -> void:
+	var deep: float = StepCanvasTransport.DISPLAY_RATIO_DEEP
+	var viewport := Vector2(800.0, 600.0)
+	var extent: Vector2i = StepCanvasTransport.viewport_fit_extent(viewport, "Chunk")
+	var expected := Vector2i(int(ceil(viewport.x / deep)), int(ceil(viewport.y / deep)))
+	assert_that(extent).is_equal(expected)
 
 
 func test_viewport_fit_extent_at_shallow_ratio_divides_by_the_display_ratio() -> void:
diff --git a/client/ui/implant/apps/atlas/step_canvas/step_canvas_transport.gd b/client/ui/implant/apps/atlas/step_canvas/step_canvas_transport.gd
index 623d1cc8c..84e9aab8b 100644
--- a/client/ui/implant/apps/atlas/step_canvas/step_canvas_transport.gd
+++ b/client/ui/implant/apps/atlas/step_canvas/step_canvas_transport.gd
@@ -57,7 +57,17 @@ const RUNG_SPACING_M: Dictionary = {
 ## client-side, viewport-dependent parameter, kept architecturally separate
 ## from gridunit spacing") — it never touches a cache key or a wire request,
 ## only how many screen px one already-fetched gridunit occupies.
-const DISPLAY_RATIO_DEEP: float = 1.0
+## PAIR SESSION 2026-07-26 EXPERIMENT (Jeroen): DISPLAY_RATIO_DEEP raised
+## 1.0 -> 2.0. One gridunit now occupies a 2x2 screen-px block, so a
+## viewport-fit request asks for ~4x FEWER gridunits (a 1920x1080 window:
+## ~2.07M cells at 1x1 -> ~518K at 2x2) and the server-side derive cost falls
+## with it — the whole point of the experiment. Texel-exact either way (an
+## integer ratio, no fractional-pixel blur); the tradeoff is purely visual
+## chunkiness. LOOKS ARE THE GATE: revert to 1.0 if Jeroen judges the deep
+## rungs too coarse. Non-integer ratios are NOT an option here — they
+## reintroduce exactly the fractional-pixel blur D-255's texel-exactness
+## exists to prevent.
+const DISPLAY_RATIO_DEEP: float = 2.0
 const DISPLAY_RATIO_SHALLOW: float = 5.0
 
 ## Global/Region read at the shallow ratio (orbital-scale canvases — extent