diff --git a/src/dials_rest/routers/find_spots.py b/src/dials_rest/routers/find_spots.py
index 542273a..3f97b7b 100644
--- a/src/dials_rest/routers/find_spots.py
+++ b/src/dials_rest/routers/find_spots.py
@@ -121,7 +121,7 @@ async def find_spots(
         else:
             experiments = ExperimentListFactory.from_filenames([params.filename])
         if params.scan_range and len(experiments) > 1:
-            # This means we've imported a sequence of still image: select
+            # This means we've imported a sequence of still images: select
             # only the experiment, i.e. image, we're interested in
             start, end = params.scan_range
             experiments = experiments[start - 1 : end]
@@ -129,7 +129,31 @@ async def find_spots(
         logger.exception(e)
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
-            detail=f"File not found: {params.filename}",
+            detail=str(e),
+        )
+    except ValueError as e:
+        logger.exception(e)
+        msg = str(e)
+        if "does not match any files" in msg:
+            logger.exception(e)
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail=msg,
+            )
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=msg,
+        )
+    except Exception as e:
+        logger.exception(e)
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=str(e),
+        )
+    if not experiments:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=f"Could not find matching image format for {params.filename}",
         )
 
     phil_params = find_spots_phil_scope.fetch(source=phil.parse("")).extract()
diff --git a/src/dials_rest/routers/image.py b/src/dials_rest/routers/image.py
index a8e129c..8c91d34 100644
--- a/src/dials_rest/routers/image.py
+++ b/src/dials_rest/routers/image.py
@@ -135,7 +135,26 @@ async def image_as_bitmap(
         logger.exception(e)
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
-            detail=f"File not found: {params.filename}",
+            detail=str(e),
+        )
+    except ValueError as e:
+        logger.exception(e)
+        msg = str(e)
+        if "does not match any files" in msg:
+            logger.exception(e)
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail=msg,
+            )
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=msg,
+        )
+    except Exception as e:
+        logger.exception(e)
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=str(e),
         )
 
     phil_params = export_bitmaps.phil_scope.extract()
diff --git a/tests/routers/test_find_spots.py b/tests/routers/test_find_spots.py
index 0c32b76..26a9a98 100644
--- a/tests/routers/test_find_spots.py
+++ b/tests/routers/test_find_spots.py
@@ -2,12 +2,47 @@
 from unittest import mock
 
 import pytest
+from fastapi import status
 
 
 def test_find_spots_file_not_found_responds_404(client, authentication_headers):
     data = {"filename": "/made/up/path.cbf"}
     response = client.post("find_spots", json=data, headers=authentication_headers)
-    assert response.status_code == 404
+    assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_find_spots_template_no_matching_file_responds_404(
+    client, authentication_headers
+):
+    data = {"filename": "/made/up/path_#####.cbf"}
+    response = client.post("find_spots", json=data, headers=authentication_headers)
+    assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_find_spots_fails_to_understand_h5_data_file_responds_422(
+    client, authentication_headers, dials_data
+):
+    data = {
+        "filename": os.fspath(
+            # this is the data file rather than NeXus file, so dials should
+            # fail to understand
+            dials_data("vmxi_thaumatin", pathlib=True)
+            / "image_15799_data_000001.h5"
+        ),
+    }
+    response = client.post("find_spots", json=data, headers=authentication_headers)
+    assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY
+
+
+def test_find_spots_fails_to_understand_text_file_responds_422(
+    client, authentication_headers, tmp_path
+):
+    filename = tmp_path / "image_00001.cbf"
+    filename.touch()
+    for filename in {filename, tmp_path / "image_#####.cbf"}:
+        data = {"filename": os.fspath(filename)}
+        response = client.post("find_spots", json=data, headers=authentication_headers)
+        assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY
 
 
 @pytest.mark.parametrize("filename", ["centroid_0001.cbf", "centroid_####.cbf"])
diff --git a/tests/routers/test_image.py b/tests/routers/test_image.py
index 781eb20..501f74a 100644
--- a/tests/routers/test_image.py
+++ b/tests/routers/test_image.py
@@ -2,18 +2,55 @@
 from io import BytesIO
 
 import pytest
+from fastapi import status
 from PIL import Image
 
 
 def test_export_bitmap_without_jwt_responds_403(client):
     response = client.post("export_bitmap")
-    assert response.status_code == 403
+    assert response.status_code == status.HTTP_403_FORBIDDEN
 
 
 def test_export_bitmap_file_not_found_responds_404(client, authentication_headers):
     data = {"filename": "/made/up/path.cbf"}
     response = client.post("export_bitmap", json=data, headers=authentication_headers)
-    assert response.status_code == 404
+    assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_export_bitmap_template_no_matching_file_responds_404(
+    client, authentication_headers
+):
+    data = {"filename": "/made/up/path_#####.cbf"}
+    response = client.post("export_bitmap", json=data, headers=authentication_headers)
+    assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_export_bitmap_fails_to_understand_h5_data_file_responds_422(
+    client, authentication_headers, dials_data
+):
+    data = {
+        "filename": os.fspath(
+            # this is the data file rather than NeXus file, so dials should
+            # fail to understand
+            dials_data("vmxi_thaumatin", pathlib=True)
+            / "image_15799_data_000001.h5"
+        ),
+    }
+    response = client.post("export_bitmap", json=data, headers=authentication_headers)
+    assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY
+
+
+def test_export_bitmap_fails_to_understand_text_file_responds_422(
+    client, authentication_headers, tmp_path
+):
+    filename = tmp_path / "image_00001.cbf"
+    filename.touch()
+    for filename in {filename, tmp_path / "image_#####.cbf"}:
+        data = {"filename": os.fspath(filename)}
+        response = client.post(
+            "export_bitmap", json=data, headers=authentication_headers
+        )
+        assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY
 
 
 @pytest.mark.parametrize("filename", ["centroid_0001.cbf", "centroid_####.cbf"])