Index: trunk/psModules/src/objects/pmPSF.c
===================================================================
--- trunk/psModules/src/objects/pmPSF.c	(revision 24206)
+++ trunk/psModules/src/objects/pmPSF.c	(revision 26893)
@@ -42,4 +42,7 @@
 #include "pmErrorCodes.h"
 
+
+#define MAX_AXIS_RATIO 20.0             // Maximum axis ratio for PSF model
+
 /*****************************************************************************/
 /* FUNCTION IMPLEMENTATION - PUBLIC                                          */
@@ -405,2 +408,23 @@
     return psf;
 }
+
+
+float pmPSFtoFWHM(const pmPSF *psf, float x, float y)
+{
+    PS_ASSERT_PTR_NON_NULL(psf, NAN);
+
+    pmModel *model = pmModelFromPSFforXY(psf, x, y, 1.0); // Model of source
+    if (!model) {
+        psError(PS_ERR_UNKNOWN, false, "Unable to determine PSF model at %f,%f\n", x, y);
+        return NAN;
+    }
+    psF32 *params = model->params->data.F32; // Model parameters
+    psEllipseAxes axes = pmPSF_ModelToAxes(params, MAX_AXIS_RATIO); // Ellipse axes
+
+    // Curiously, the minor axis can be larger than the major axis, so need to check.
+    float fwhm = 2.355 * PS_MAX(axes.minor, axes.major); // FWHM, converted from sigma
+
+    psFree(model);
+
+    return fwhm;
+}