diff --git a/src/ev_calculator.py b/src/ev_calculator.py
index 8d68204..4efb71b 100644
--- a/src/ev_calculator.py
+++ b/src/ev_calculator.py
@@ -1,9 +1,24 @@
 from math import log
 
+'''
+This class does all the required math. It is based on the Wikipedia page
+https://en.wikipedia.org/wiki/Exposure_value
+(N^2)/t = L*S/K
+where:
+    N is the relative aperture (f-number)
+    t is the exposure time ("shutter speed") in seconds
+    L is the average scene luminance
+    S is the ISO arithmetic speed
+    K is the reflected-light meter calibration constant
+'''
 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/Exposure_value#EV_as_a_measure_of_luminance_and_illuminance)
         # says E = 2.5 * (2^EV), so the inverse is EV = ln(2*E/5)/ln(2)
         return log(2*lux/5, 2)
 
+
diff --git a/src/widgets/aperture_priority_page.ui b/src/widgets/aperture_priority_page.ui
index f02f1d3..aa2335f 100644
--- a/src/widgets/aperture_priority_page.ui
+++ b/src/widgets/aperture_priority_page.ui
@@ -29,7 +29,7 @@
     </child>
     <child>
       <object class="GtkLabel" id="time-priority-aperture-label">
-        <property name="label" translatable="no">f/60</property>
+        <property name="label" translatable="no">--s</property>
         <style>
           <class name="lumos-big-result"/>
         </style>
diff --git a/src/widgets/time_priority_page.ui b/src/widgets/time_priority_page.ui
index ea1a269..10917c9 100644
--- a/src/widgets/time_priority_page.ui
+++ b/src/widgets/time_priority_page.ui
@@ -29,7 +29,7 @@
     </child>
     <child>
       <object class="GtkLabel" id="time-priority-aperture-label">
-        <property name="label" translatable="no">f/11</property>
+        <property name="label" translatable="no">f/--</property>
         <style>
           <class name="lumos-big-result"/>
         </style>
diff --git a/src/window.py b/src/window.py
index 6a83ac1..7f8e070 100644
--- a/src/window.py
+++ b/src/window.py
@@ -38,8 +38,6 @@ class LumosWindow(Adw.ApplicationWindow):
 
     def onSensorRead(self, value: float, unit: str):
         # Called when the light value changed
-        print("Read {} {}".format(value, unit))
-
         if self.lux_label:
             self.lux_label.set_label("{:.0f} {}".format(value, unit))
 
diff --git a/src/window.ui b/src/window.ui
index 6fb14e6..85e858e 100644
--- a/src/window.ui
+++ b/src/window.ui
@@ -9,6 +9,26 @@
       <object class="AdwToolbarView">
         <child type="top">
           <object class="AdwHeaderBar" id="header_bar">
+            <child type="start">
+              <!-- ISO Selector in header bar (left) -->
+              <object class="GtkDropDown" id="iso-dropdown">
+                <property name="model">
+                  <object class="GtkStringList">
+                    <items>
+                      <item translatable="no">ISO 50</item>
+                      <item translatable="no">ISO 100</item>
+                      <item translatable="no">ISO 200</item>
+                      <item translatable="no">ISO 400</item>
+                      <item translatable="no">ISO 800</item>
+                      <item translatable="no">ISO 1600</item>
+                      <item translatable="no">ISO 3200</item>
+                      <item translatable="no">ISO 6400</item>
+                    </items>
+                  </object>
+                </property>
+                <property name="selected">1</property> <!-- ISO 100 -->
+              </object>
+            </child>
             <child type="end">
               <object class="GtkMenuButton">
                 <property name="primary">True</property>
@@ -31,7 +51,7 @@
                   <!-- Aperture priority page -->
                   <object class="AdwViewStackPage" id="aperture-priority-page">
                     <property name="name">aperture-priority-page</property>
-                    <property name="title" translatable="true">Aperture priority</property>
+                    <property name="title" translatable="true">Aperture</property>
                     <property name="icon-name">camera-shutter-symbolic</property>
                     <property name="use-underline">true</property>
                     <property name="child">
@@ -45,7 +65,7 @@
                 <child>
                   <object class="AdwViewStackPage" id="time-priority-page">
                     <property name="name">time-priority-page</property>help
-                    <property name="title" translatable="true">Time priority</property>
+                    <property name="title" translatable="true">Time</property>
                     <property name="icon-name">camera-timer-symbolic</property>
                     <property name="use-underline">true</property>
                     <property name="child">
@@ -118,9 +138,12 @@
             <child>
               <object class="AdwViewSwitcher" id="switcher_title">
                 <property name="stack">stack</property>
-
+                <property name="margin-bottom">8</property>
+                <property name="margin-start">2</property>
+                <property name="margin-end">2</property>
               </object>
             </child>
+
           </object>
         </property>
       </object>