Re #8436. Corrected ConvFit page.

Code/Mantid/Framework/CurveFitting/test/LorentzianTest.h

@@ -5,6 +5,8 @@
 
 #include "MantidCurveFitting/Lorentzian.h"
 #include "MantidCurveFitting/Jacobian.h"
+#include "MantidAPI/FunctionDomain1D.h"
+#include "MantidAPI/FunctionValues.h"
 
 #include <boost/make_shared.hpp>
 
@@ -50,6 +52,22 @@ public:
     TS_ASSERT( categories[0] == "Peak" );
   }
 
+  void test_FWHM()
+  {
+      double hwhm = 0.5;
+      Mantid::CurveFitting::Lorentzian lor;
+      lor.initialize();
+      lor.setParameter("Amplitude",1.0);
+      lor.setParameter("PeakCentre",0.0);
+      lor.setParameter("FWHM",hwhm*2);
+
+      Mantid::API::FunctionDomain1DVector x(0,hwhm,2);
+      Mantid::API::FunctionValues y(x);
+      lor.function(x,y);
+
+      TS_ASSERT_DELTA( y[1]/y[0], 0.5, 1e-15 );
+  }
+
 private:
   
   class TestableLorentzian : public Mantid::CurveFitting::Lorentzian

Code/Mantid/MantidQt/CustomInterfaces/src/ConvFit.cpp

@@ -844,11 +844,11 @@ namespace IDA
     // Always want FWHM to display as positive.
     if ( val > peakCentre )
     {
-      m_cfDblMng->setValue(m_cfProp["Lorentzian 1.FWHM"], val-peakCentre);
+      m_cfDblMng->setValue(m_cfProp["Lorentzian 1.FWHM"], val*2.0-peakCentre);
     }
     else
     {
-      m_cfDblMng->setValue(m_cfProp["Lorentzian 1.FWHM"], peakCentre-val);
+      m_cfDblMng->setValue(m_cfProp["Lorentzian 1.FWHM"], peakCentre-val*2.0);
     }
   }
 
@@ -868,8 +868,8 @@ namespace IDA
   void ConvFit::hwhmUpdateRS(double val)
   {
     const double peakCentre = m_cfDblMng->value(m_cfProp["Lorentzian 1.PeakCentre"]);
-    m_cfHwhmRange->setMinimum(peakCentre-val);
-    m_cfHwhmRange->setMaximum(peakCentre+val);
+    m_cfHwhmRange->setMinimum(peakCentre-val/2);
+    m_cfHwhmRange->setMaximum(peakCentre+val/2);
   }
 
   void ConvFit::checkBoxUpdate(QtProperty* prop, bool checked)