diff --git a/src/openflexure_microscope_server/scan_planners.py b/src/openflexure_microscope_server/scan_planners.py
index a2a93d82..6d430aa4 100644
--- a/src/openflexure_microscope_server/scan_planners.py
+++ b/src/openflexure_microscope_server/scan_planners.py
@@ -212,6 +212,8 @@ class ScanPlanner:
     def select_nearby_focus_site(self, xy_pos: XYPos) -> Optional[XYZPos]:
         """
         Return the xyz position of the nearby site with the lowest z position.
+        Lowest position is best, as starting too high causes smart stacking to
+        autofocus and restart. Starting too low just requires extra movements in +z.
         Nearby is defined as within 1.6 times the distance to the closest neighbour.
 
         Returns None if there if no focussed locations are present
diff --git a/src/openflexure_microscope_server/things/autofocus.py b/src/openflexure_microscope_server/things/autofocus.py
index 0680cebb..a2cd318d 100644
--- a/src/openflexure_microscope_server/things/autofocus.py
+++ b/src/openflexure_microscope_server/things/autofocus.py
@@ -313,6 +313,10 @@ class AutofocusThing(Thing):
         images_to_test = self.stack_images_to_test
 
         stack_z_range = stack_dz * (images_to_test - 1)
+        # Starting too low by "overshoot" makes smart stacking faster.
+        # Starting a stack too high requires it to move to the start,
+        # autofocus and then re-stack. Starting slightly too low only
+        # requires extra +z movements and captures.
         overshoot = stack_dz * 5
 
         # Ensure the stack settings are appropriate
@@ -533,6 +537,8 @@ class AutofocusThing(Thing):
             )
         if images_to_test % 2 == 0:
             raise RuntimeError("Images to test should be odd")
+        if images_to_test <= 0 or images_to_capture <= 0:
+            raise RuntimeError("Stack parameters need to be at least 1")
 
     def check_stack_result(self, sharpnesses: list[int]) -> str:
         """Test a list of sharpnesses, to decide whether the sharpest image from a stack is within them