diff --git a/src/openflexure_microscope_server/scan_planners.py b/src/openflexure_microscope_server/scan_planners.py
index b30cacc7..065b3258 100644
--- a/src/openflexure_microscope_server/scan_planners.py
+++ b/src/openflexure_microscope_server/scan_planners.py
@@ -33,7 +33,7 @@ def enforce_xy_tuple(value: XYPos) -> XYPos:
     :raises ValueError: if the input cannot be coerced to a tuple of length 2.
     """
     if not isinstance(value, (list, tuple)):
-        raise ValueError("2 value tuple expected")
+        raise TypeError("2 value tuple expected")
     if not len(value) == 2:
         raise ValueError("2 value tuple expected")
     if isinstance(value, list):
@@ -49,7 +49,7 @@ def enforce_xyz_tuple(value: XYZPos) -> XYZPos:
     :raises ValueError: if the input cannot be coerced to a tuple of length 3.
     """
     if not isinstance(value, (list, tuple)):
-        raise ValueError("3 value tuple expected")
+        raise TypeError("3 value tuple expected")
     if not len(value) == 3:
         raise ValueError("3 value tuple expected")
     if isinstance(value, list):
diff --git a/src/openflexure_microscope_server/server/__init__.py b/src/openflexure_microscope_server/server/__init__.py
index dc864e11..d5058fe8 100644
--- a/src/openflexure_microscope_server/server/__init__.py
+++ b/src/openflexure_microscope_server/server/__init__.py
@@ -61,7 +61,7 @@ def _get_scans_dir(config: dict) -> Optional[str]:
         try:
             return config["things"]["/smart_scan/"]["kwargs"]["scans_folder"]
         except KeyError as e:
-            msg = "Configuration error smart scan should have scans_folder kwarg set"
+            msg = "Configuration error: smart scan should have scans_folder kwarg set"
             raise RuntimeError(msg) from e
     return None
 
diff --git a/src/openflexure_microscope_server/things/autofocus.py b/src/openflexure_microscope_server/things/autofocus.py
index 0e11d456..a6c0e92a 100644
--- a/src/openflexure_microscope_server/things/autofocus.py
+++ b/src/openflexure_microscope_server/things/autofocus.py
@@ -26,6 +26,10 @@ from .camera import CameraDependency as CameraClient
 from .stage import StageDependency as Stage
 
 
+class NotStreamingError(RuntimeError):
+    """No images captured from stream. The camera is almost certainly not streaming."""
+
+
 class StackParams:
     """A class for holding for stack parameters, and returning computed ones."""
 
@@ -310,8 +314,9 @@ class JPEGSharpnessMonitor:
         """Return the z position of the sharpest image on a given move."""
         _, jpeg_heights, jpeg_sizes = self.move_data(data_index)
         if len(jpeg_sizes) == 0:
-            raise ValueError(
-                "No images were captured during the move of the stage.  Perhaps the camera is not streaming images?"
+            raise NotStreamingError(
+                "No images were captured during the move of the stage. "
+                "Perhaps the camera is not streaming images?"
             )
         return jpeg_heights[np.argmax(jpeg_sizes)]