further condense logic

Author: rjmoerland

optiland/paraxial.py

@@ -64,11 +64,11 @@ def _focal_length(self, y, u) -> be.array:
         """Determine the focal length and sign, reasoned from left to right (f2)"""
         focus = be.abs(y[0] / u[-1])[0]
 
-        if y[-1] == 0.0:  # At a real focus
-            return focus
-
-        # y[-1] < 0, u[-1] > 0 or v.v.: positive focus, -y[0]/u[-1] > 0
-        if be.sign(y[-1] * u[-1]) < 0 or self._has_internal_focus_point(y):
+        if (
+            y[-1] == 0.0  # Real internal focus
+            or be.sign(y[-1] * u[-1]) < 0  # Real external focus
+            or self._has_internal_focus_point(y)  # Real internal focus
+        ):
             return focus
         else:
             return -focus