diff --git a/src/ev_calculator.py b/src/ev_calculator.py
index 6b41c3d..ebd126a 100644
--- a/src/ev_calculator.py
+++ b/src/ev_calculator.py
@@ -11,10 +11,12 @@ where:
     S is the ISO arithmetic speed
     K is the reflected-light meter calibration constant
 '''
+
+# K is the reflected-light meter calibration constant (unit: cd s/m2 ISO)
+# (https://en.wikipedia.org/wiki/Light_meter#Calibration_constants)
+K = 12.5
+
 class EVCalculator:
-    # K is the reflected-light meter calibration constant (unit: cd s/m2 ISO)
-    # (https://en.wikipedia.org/wiki/Light_meter#Calibration_constants)
-    K = 12.5    # TODO: Add to settings?
 
     def luxToEV(lux: float) -> float:
         # Wikipedia (https://en.wikipedia.org/wiki/Light_meter#Exposure_equations)
@@ -22,8 +24,9 @@ class EVCalculator:
         return log(2*lux/5, 2)
 
     def calcShutterSpeed(isoSpeed: int, lux: float, aperture: float):
-        return ((aperture^2)*K)/(lux*isoSpeed)
+        return (aperture*aperture*K)/(lux*isoSpeed)
 
     def calcAperture(isoSpeed: int, lux: float, shutterSpeed: float):
+        # shutterSpeed is in seconds
         return sqrt(lux*isoSpeed*shutterSpeed/K)
 
diff --git a/src/widgets/aperture_priority_page.py b/src/widgets/aperture_priority_page.py
index 843ed1b..7ad3e2a 100644
--- a/src/widgets/aperture_priority_page.py
+++ b/src/widgets/aperture_priority_page.py
@@ -19,13 +19,37 @@
 
 from gi.repository import Adw
 from gi.repository import Gtk
+from .ev_calculator import EVCalculator
 
 @Gtk.Template(resource_path='/eu/ichibi/Lumos/widgets/aperture_priority_page.ui')
 class AperturePriorityPage(Gtk.Box):
     __gtype_name__ = 'AperturePriorityPage'
 
+    # Values of aperture dropdown entries defined in the .ui file
+    aperture_priority_speed_dropdown_values = [
+        1/32,
+        1/22,
+        1/16,
+        1/11,
+        1/8,
+        1/5.6,
+        1/4,
+        1/2.8,
+        1/2,
+        1/1.4
+    ]
+
+    # Widgets
+    aperture_priority_aperture_dropdown = Gtk.Template.Child()
+    aperture_priority_time_label = Gtk.Template.Child()
+
     def onValuesChanged(self, isoSpeed: int,  sensorValue: float, sensorUnit: str):
         # Check the unit is absolute ("lux")
         if sensorUnit != "lux":
             return
 
+        apertureValue = self.aperture_priority_speed_dropdown_values[self.aperture_priority_aperture_dropdown.get_selected()]
+        shutterSpeed = EVCalculator.calcShutterSpeed(isoSpeed, sensorValue, apertureValue)
+        # TODO: Round shutter speed value to nearest existing value and set label color to red if outside 1 stop range
+        self.aperture_priority_time_label.set_label("f/ {:.2f}".format(shutterSpeed))
+
diff --git a/src/widgets/aperture_priority_page.ui b/src/widgets/aperture_priority_page.ui
index aa2335f..0c4e959 100644
--- a/src/widgets/aperture_priority_page.ui
+++ b/src/widgets/aperture_priority_page.ui
@@ -28,7 +28,7 @@
       </object>
     </child>
     <child>
-      <object class="GtkLabel" id="time-priority-aperture-label">
+      <object class="GtkLabel" id="aperture_priority_time_label">
         <property name="label" translatable="no">--s</property>
         <style>
           <class name="lumos-big-result"/>
@@ -43,7 +43,7 @@
       </object>
     </child>
     <child>
-      <object class="GtkDropDown" id="aperture-priority-speed-dropdown">
+      <object class="GtkDropDown" id="aperture_priority_aperture_dropdown">
         <property name="model">
           <object class="GtkStringList">
             <items>
diff --git a/src/widgets/time_priority_page.py b/src/widgets/time_priority_page.py
index 9529bc8..e32144a 100644
--- a/src/widgets/time_priority_page.py
+++ b/src/widgets/time_priority_page.py
@@ -19,13 +19,45 @@
 
 from gi.repository import Adw
 from gi.repository import Gtk
+from .ev_calculator import EVCalculator
 
 @Gtk.Template(resource_path='/eu/ichibi/Lumos/widgets/time_priority_page.ui')
 class TimePriorityPage(Gtk.Box):
     __gtype_name__ = 'TimePriorityPage'
 
-    def onValuesChanged(self, isoSpeed: int,  sensorValue: float, sensorUnit: str):
+    # Values of time dropdown entries defined in the .ui file
+    time_priority_speed_dropdown_values = [
+        1/4000,
+        1/2000,
+        1/1000,
+        1/500,
+        1/250,
+        1/125,
+        1/60,
+        1/30,
+        1/15,
+        1/8,
+        1/4,
+        1/2,
+        1,
+        2,
+        4,
+        8,
+        15,
+        30
+    ]
+
+    # Widgets
+    time_priority_speed_dropdown = Gtk.Template.Child()
+    time_priority_aperture_label = Gtk.Template.Child()
+
+    def onValuesChanged(self, isoSpeed: int, sensorValue: float, sensorUnit: str):
         # Check the unit is absolute ("lux")
         if sensorUnit != "lux":
             return
 
+        shutterSpeed = self.time_priority_speed_dropdown_values[self.time_priority_speed_dropdown.get_selected()]
+        apertureValue = EVCalculator.calcAperture(isoSpeed, sensorValue, shutterSpeed)
+        # TODO: Round aperture value to nearest existing value and set label color to red if outside 1 stop range
+        self.time_priority_aperture_label.set_label("f/ {:.2f}".format(apertureValue))
+
diff --git a/src/widgets/time_priority_page.ui b/src/widgets/time_priority_page.ui
index 10917c9..e1f9437 100644
--- a/src/widgets/time_priority_page.ui
+++ b/src/widgets/time_priority_page.ui
@@ -28,7 +28,7 @@
       </object>
     </child>
     <child>
-      <object class="GtkLabel" id="time-priority-aperture-label">
+      <object class="GtkLabel" id="time_priority_aperture_label">
         <property name="label" translatable="no">f/--</property>
         <style>
           <class name="lumos-big-result"/>
@@ -43,7 +43,7 @@
       </object>
     </child>
     <child>
-      <object class="GtkDropDown" id="time-priority-speed-dropdown">
+      <object class="GtkDropDown" id="time_priority_speed_dropdown">
         <property name="model">
           <object class="GtkStringList">
             <items>