diff --git a/src/astro_image_display_api/interface_definition.py b/src/astro_image_display_api/interface_definition.py
index 8a2e26e..243906d 100644
--- a/src/astro_image_display_api/interface_definition.py
+++ b/src/astro_image_display_api/interface_definition.py
@@ -441,9 +441,10 @@ def set_viewport(
             The center of the viewport. If not given, the current center is used.
         fov : `astropy.units.Quantity` or float, optional
             The field of view (FOV) of the viewport. If not given, the current FOV
-            is used. If a float is given, it is interpreted a size in pixels. For images
-            that are not square, the FOV is interpreted as the size of the longer side
-            of the image.
+            is used. If a float is given, it is interpreted as a size in pixels. For viewers
+            that are not square, the FOV is interpreted as the size of the shorter side
+            of the viewer such that the FOV is guaranteed to be entirely visible
+            regardless of the aspect ratio of the viewer.
         image_label : str, optional
             The label of the image to set the viewport for. If not given and there is
             only one image loaded, the viewport for that image is set. If there are