diff --git a/src/openflexure_microscope_server/things/autofocus.py b/src/openflexure_microscope_server/things/autofocus.py
index 3ec21a74..de2fc1d1 100644
--- a/src/openflexure_microscope_server/things/autofocus.py
+++ b/src/openflexure_microscope_server/things/autofocus.py
@@ -141,8 +141,14 @@ class CaptureInfo:
 
     @property
     def filename(self) -> str:
-        """The filename for this image generated from the position."""
-        return f"{self.position['x']}_{self.position['y']}_{self.position['z']}.jpeg"
+        """The filename for this image generated from the position.
+
+        The file name is in the format ``img_{x}_{y}_{z}`` where x, y, and z are the
+        positions from the microscope stage.
+        """
+        return (
+            f"img_{self.position['x']}_{self.position['y']}_{self.position['z']}.jpeg"
+        )
 
 
 def _get_capture_by_id(captures: list[CaptureInfo], buffer_id: int) -> CaptureInfo: