diff --git a/src/pictureshow/backends.py b/src/pictureshow/backends.py
index 07bea7d..d8e5597 100644
--- a/src/pictureshow/backends.py
+++ b/src/pictureshow/backends.py
@@ -1,11 +1,18 @@
 import os
 
 from PIL import UnidentifiedImageError
+from reportlab.lib import pagesizes
 from reportlab.lib.utils import ImageReader
 from reportlab.pdfgen.canvas import Canvas
 
 
 class ReportlabBackend:
+    page_sizes = {
+        name: size
+        for name, size in vars(pagesizes).items()
+        if name.isupper()  # exclude deprecated names and function names
+    }
+
     read_errors = (
         OSError,  # file does not exist or is a dir
         UnidentifiedImageError,  # file not recognized as picture
diff --git a/src/pictureshow/cli.py b/src/pictureshow/cli.py
index d985475..5158f53 100644
--- a/src/pictureshow/cli.py
+++ b/src/pictureshow/cli.py
@@ -8,7 +8,7 @@
 from platformdirs import user_cache_path
 
 from . import __version__
-from .core import PAGE_SIZES, PictureShow
+from .core import PictureShow
 
 CACHE_PATH = user_cache_path('pictureshow', ensure_exists=True)
 ERROR_LOG = CACHE_PATH / 'errors.log'
@@ -85,11 +85,11 @@ def _setup_parser():
     page_group.add_argument(
         '-p',
         '--page-size',
-        choices=PAGE_SIZES,
+        choices=PictureShow.page_sizes,
         default='A4',
         metavar='SIZE',
         help='specify page size; default is %(default)s '
-             f'(available sizes: {", ".join(PAGE_SIZES)})',
+             f'(available sizes: {", ".join(PictureShow.page_sizes)})',
     )
     page_group.add_argument(
         '-L',
diff --git a/src/pictureshow/core.py b/src/pictureshow/core.py
index 99311d2..7fd36fe 100644
--- a/src/pictureshow/core.py
+++ b/src/pictureshow/core.py
@@ -5,18 +5,9 @@
 from pathlib import Path
 from typing import NamedTuple
 
-from reportlab.lib import pagesizes
-
 from .backends import ReportlabBackend
 from .exceptions import LayoutError, MarginError, PageSizeError, RGBColorError
 
-PAGE_SIZES = {
-    name: size
-    for name, size in vars(pagesizes).items()
-    # use isupper() to exclude deprecated names and function names
-    if name.isupper()
-}
-
 DELIMITER = re.compile('[x,]')
 
 
@@ -37,11 +28,15 @@ def size(self):
 
 _Result = namedtuple('_Result', 'num_ok errors num_pages')
 
+_backend_class = ReportlabBackend
+
 
 class PictureShow:
+    page_sizes = _backend_class.page_sizes.copy()
+
     def __init__(self, *pic_files):
         self._pic_files = pic_files
-        self._backend = ReportlabBackend()
+        self._backend = _backend_class()
 
     def save_pdf(
             self,
@@ -135,15 +130,15 @@ def _validate_target_path(path, force_overwrite):
 
         return path
 
-    @staticmethod
-    def _validate_page_size(page_size, landscape):
+    @classmethod
+    def _validate_page_size(cls, page_size, landscape):
         if isinstance(page_size, str):
             try:
-                page_size = PAGE_SIZES[page_size.upper()]
+                page_size = cls.page_sizes[page_size.upper()]
             except KeyError as err:
                 raise PageSizeError(
                     f'unknown page size {page_size!r},'
-                    f' please use one of: {", ".join(PAGE_SIZES)}'
+                    f' please use one of: {", ".join(cls.page_sizes)}'
                 ) from err
 
         page_size_error = PageSizeError(