diff --git a/acinclude.m4 b/acinclude.m4
index 0df89a4d4..b9b529b87 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -63,7 +63,7 @@ AC_DEFUN(SANE_CHECK_PTAL,
 [
 	PTAL_TMP_HAVE_PTAL=no
 	AC_ARG_WITH(ptal,
-	  [  --with-ptal=DIR         specify the top-level PTAL directory 
+	  [  --with-ptal[=DIR]       specify the top-level PTAL directory 
                           [default=/usr/local]])
 	if test "$with_ptal" = "no" ; then
 		echo disabling PTAL
@@ -196,7 +196,7 @@ AC_DEFUN(SANE_CHECK_GPHOTO2,
 
 	GPHOTO2_TMP_HAVE_GPHOTO2=no
 	AC_ARG_WITH(gphoto2,
-	  [  --with-gphoto2=DIR      specify the top-level GPHOTO2 directory 
+	  [  --with-gphoto2[=DIR]    specify the top-level GPHOTO2 directory 
                           [default=/usr/local]])