diff --git a/rtengine/dcp.cc b/rtengine/dcp.cc
index c60e80587..1c99b682c 100644
--- a/rtengine/dcp.cc
+++ b/rtengine/dcp.cc
@@ -1265,7 +1265,7 @@ void DCPProfile::step2ApplyTile(float* rc, float* gc, float* bc, int width, int
                     float cnewb = FCLIP(newb);
 
                     float h, s, v;
-                    Color::rgb2hsvdcp(cnewr, cnewg, cnewb, h, s, v);
+                    Color::rgb2hsvtc(cnewr, cnewg, cnewb, h, s, v);
 
                     hsdApply(look_info, look_table, h, s, v);
                     s = CLIP01(s);
@@ -1648,7 +1648,7 @@ std::vector<DCPProfile::HsbModify> DCPProfile::makeHueSatMap(const ColorTemp& wh
     return res;
 }
 
-void DCPProfile::hsdApply(const HsdTableInfo& table_info, const std::vector<HsbModify>& table_base, float& h, float& s, float& v) const
+inline void DCPProfile::hsdApply(const HsdTableInfo& table_info, const std::vector<HsbModify>& table_base, float& h, float& s, float& v) const
 {
     // Apply the HueSatMap. Ported from Adobes reference implementation.
     float hue_shift;
diff --git a/rtengine/imagedata.cc b/rtengine/imagedata.cc
index 8507d7611..dcaf6e53c 100644
--- a/rtengine/imagedata.cc
+++ b/rtengine/imagedata.cc
@@ -306,6 +306,38 @@ FrameData::FrameData(rtexif::TagDirectory* frameRootDir_, rtexif::TagDirectory*
         }
 
         if (lens == "Unknown") {
+            const auto lens_from_make_and_model =
+                [this, exif]() -> bool
+                {
+                    if (!exif) {
+                        return false;
+                    }
+
+                    const rtexif::Tag* const lens_model = exif->getTag(0xA434);
+
+                    if (lens_model) {
+                        const rtexif::Tag* const lens_make = exif->getTag(0xA433);
+                        const std::string make =
+                            lens_make
+                                ? lens_make->valueToString()
+                                : std::string();
+                        const std::string model = lens_model->valueToString();
+
+                        if (!model.empty()) {
+                            lens = make;
+
+                            if (!lens.empty()) {
+                                lens += ' ';
+                            }
+
+                            lens += model;
+
+                            return true;
+                        }
+                    }
+
+                    return false;
+                };
 
             if (mnote) {
 
@@ -412,11 +444,15 @@ FrameData::FrameData(rtexif::TagDirectory* frameRootDir_, rtexif::TagDirectory*
                     rtexif::Tag *lt = mnote->getTag("LensType");
 
                     if (lt) {
-                        std::string ldata = lt->valueToString();
+                        if (lt->toInt()) {
+                            std::string ldata = lt->valueToString ();
 
-                        if (ldata.size() > 1) {
-                            found = true;
-                            lens = "Canon " + ldata;
+                            if (ldata.size() > 1) {
+                                found = true;
+                                lens = "Canon " + ldata;
+                            }
+                        } else {
+                            found = lens_from_make_and_model();
                         }
                     }
 
@@ -446,7 +482,13 @@ FrameData::FrameData(rtexif::TagDirectory* frameRootDir_, rtexif::TagDirectory*
                     }
 
                     if (mnote->getTag("LensType")) {
-                        lens = mnote->getTag("LensType")->valueToString();
+                        lens = mnote->getTag ("LensType")->valueToString();
+                        if (!mnote->getTag("LensType")->toInt()) {
+                            // try to find something better than "M-42 or No Lens"
+                            lens_from_make_and_model();
+                        }
+                    } else {
+                        lens_from_make_and_model();
                     }
 
                     // Try to get the FocalLength from the LensInfo structure, where length below 10mm will be correctly set
@@ -490,9 +532,7 @@ FrameData::FrameData(rtexif::TagDirectory* frameRootDir_, rtexif::TagDirectory*
                 }
             } else if (exif->getTag("DNGLensInfo")) {
                 lens = exif->getTag("DNGLensInfo")->valueToString();
-            } else if (exif->getTag("LensModel")) {
-                lens = exif->getTag("LensModel")->valueToString();
-            } else if (exif->getTag("LensInfo")) {
+            } else if (!lens_from_make_and_model() && exif->getTag ("LensInfo")) {
                 lens = exif->getTag("LensInfo")->valueToString();
             }
         }
diff --git a/rtexif/canonattribs.cc b/rtexif/canonattribs.cc
index 7b30d9ae6..2dcbdd96f 100644
--- a/rtexif/canonattribs.cc
+++ b/rtexif/canonattribs.cc
@@ -32,7 +32,7 @@ namespace rtexif
 class CAOnOffInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int n = t->toInt();
 
@@ -51,7 +51,7 @@ class CAIntSerNumInterpreter : public Interpreter
 {
 public:
     CAIntSerNumInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         return "";
     }
@@ -63,7 +63,7 @@ class CAApertureInterpreter : public Interpreter
 {
 public:
     CAApertureInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = pow (2.0, t->toDouble() / 64.0);
@@ -92,7 +92,7 @@ CAMacroModeInterpreter caMacroModeInterpreter;
 class CASelfTimerInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int sec = t->toInt (0, SHORT);
 
@@ -385,7 +385,7 @@ CAExposureModeInterpreter caExposureModeInterpreter;
 class CAFlashBitsInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream s;
         unsigned bits = t->toInt (0, SHORT);
@@ -533,7 +533,7 @@ CARAWQualityInterpreter caRAWQualityInterpreter;
 class CAFocalInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         Tag *unitTag = t->getParent()->getRoot()->findTag ("FocalUnits");
         double v = unitTag ? unitTag->toDouble() : 1.;
@@ -957,7 +957,7 @@ public:
         };
     }
 
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int lensID = t->toInt();
 
@@ -1109,7 +1109,7 @@ CAFocalTypeInterpreter caFocalTypeInterpreter;
 class CAFocalPlaneInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int val = t->toInt();
 
@@ -1127,7 +1127,7 @@ CAFocalPlaneInterpreter caFocalPlaneInterpreter;
 class CAExposureTimeInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double d = pow (2, - t->toInt() / 32.0);
@@ -1139,7 +1139,7 @@ CAExposureTimeInterpreter caExposureTimeInterpreter;
 
 class CAEVInterpreter : public Interpreter
 {
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%.1f", t->toDouble() / 32.0  );
@@ -1151,7 +1151,7 @@ CAEVInterpreter caEVInterpreter;
 class CABaseISOInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         int a = t->toInt();
@@ -1288,7 +1288,7 @@ CASlowShutterInterpreter caSlowShutterInterpreter;
 class CAFlashGuideNumberInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int n = t->toInt();
 
@@ -1349,7 +1349,7 @@ CAControModeInterpreter caControModeInterpreter;
 class CAFocusDistanceInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%.2f", t->toDouble() / 100 );
@@ -1361,7 +1361,7 @@ CAFocusDistanceInterpreter caFocusDistanceInterpreter;
 class CAMeasuredEVInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%.1f", t->toDouble() / 8 - 6 );
@@ -1496,7 +1496,7 @@ CAToningEffectInterpreter caToningEffectInterpreter;
 class CAFileNumberInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         unsigned long val = t->toInt (0, LONG);
         char buffer[32];
diff --git a/rtexif/nikonattribs.cc b/rtexif/nikonattribs.cc
index 70246d6b2..a1fa28c34 100644
--- a/rtexif/nikonattribs.cc
+++ b/rtexif/nikonattribs.cc
@@ -34,7 +34,7 @@ class NAISOInterpreter : public Interpreter
 {
 public:
     NAISOInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%d", t->toInt (2));
@@ -47,7 +47,7 @@ class NAISOInfoISOInterpreter : public Interpreter
 {
 public:
     NAISOInfoISOInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         int a = t->toInt();
@@ -83,7 +83,7 @@ class NAISOExpansionInterpreter : public Interpreter
 {
 public:
     NAISOExpansionInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt();
 
@@ -142,7 +142,7 @@ class NALensTypeInterpreter : public Interpreter
 {
 public:
     NALensTypeInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt();
         std::ostringstream str;
@@ -191,7 +191,7 @@ class NAShootingModeInterpreter : public Interpreter
 {
 public:
     NAShootingModeInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt();
         std::ostringstream str;
@@ -234,14 +234,26 @@ public:
         afpchoices[0x9] = "Far Left";
         afpchoices[0xa] = "Far Right";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
+        const auto get_from_choices =
+            [](const std::map<int, std::string>& choices, int index) -> std::string
+            {
+                const std::map<int, std::string>::const_iterator choice = choices.find(index);
+
+                if (choice != choices.end()) {
+                    return choice->second;
+                }
+
+                return {};
+            };
+
         int am = t->toInt (0, BYTE);
         int afp = t->toInt (1, BYTE);
         int aff = t->toInt (2, SHORT);
         std::ostringstream str;
-        str << "AFAreaMode = " << amchoices[am] << std::endl;
-        str << "AFAreaMode = " << afpchoices[afp] << std::endl;
+        str << "AFAreaMode = " << get_from_choices(amchoices, am) << std::endl;
+        str << "AFAreaMode = " << get_from_choices(afpchoices, afp) << std::endl;
 
         std::ostringstream af;
 
@@ -314,7 +326,7 @@ class NALensDataInterpreter : public Interpreter
     static const std::map<std::string, std::string> lenses;
 
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
 
         static const unsigned char xlat[2][256] = {
diff --git a/rtexif/olympusattribs.cc b/rtexif/olympusattribs.cc
index ccf28d0ca..e42763e35 100644
--- a/rtexif/olympusattribs.cc
+++ b/rtexif/olympusattribs.cc
@@ -33,7 +33,7 @@ class OLOnOffInterpreter : public Interpreter
 {
 public:
     OLOnOffInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         if (t->toInt() == 0) {
             return "Off";
@@ -48,7 +48,7 @@ class OLYesNoInterpreter : public Interpreter
 {
 public:
     OLYesNoInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         if (t->toInt() == 0) {
             return "No";
@@ -63,7 +63,7 @@ class OLApertureInterpreter : public Interpreter
 {
 public:
     OLApertureInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         str.precision (2);
@@ -194,7 +194,7 @@ public:
         lenses["03 02 00"] = "Leica D Summilux 25mm f/1.4 Asph.";
         lenses["05 01 10"] = "Tamron 14-150mm f/3.5-5.8 Di III";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream lid;
         lid.setf (std::ios_base::hex, std::ios_base::basefield);
@@ -203,7 +203,7 @@ public:
         lid << std::setw (2) << std::setfill ('0') << t->toInt (2) << ' '; //model
         lid << std::setw (2) << std::setfill ('0') << t->toInt (3); // submodel
 
-        std::map<std::string, std::string>::iterator r = lenses.find (lid.str());
+        std::map<std::string, std::string>::const_iterator r = lenses.find (lid.str());
 
         if (r != lenses.end()) {
             return r->second;
@@ -465,7 +465,7 @@ class OLNoiseFilterInterpreter : public Interpreter
 {
 public:
     OLNoiseFilterInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0);
         int b = t->toInt (2);
@@ -490,7 +490,7 @@ class OLFlashModeInterpreter : public Interpreter
 {
 public:
     OLFlashModeInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         int a = t->toInt ();
@@ -509,7 +509,7 @@ class OLNoiseReductionInterpreter : public Interpreter
 {
 public:
     OLNoiseReductionInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         int a = t->toInt ();
diff --git a/rtexif/pentaxattribs.cc b/rtexif/pentaxattribs.cc
index 971f07ce1..7e861d64f 100644
--- a/rtexif/pentaxattribs.cc
+++ b/rtexif/pentaxattribs.cc
@@ -415,7 +415,7 @@ class PAFNumberInterpreter: public Interpreter
 {
 public:
     PAFNumberInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = t->toDouble() / 10;
@@ -610,7 +610,7 @@ public:
         choices[256 * 255 +   0] = "Video (Auto Aperture)";
         choices[256 * 255 +   4] = "Video (4)";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int c = 256 * t->toInt (0, BYTE) + t->toInt (1, BYTE);
         const ChoicesIterator r = choices.find (c);
@@ -669,12 +669,12 @@ public:
         choices3[224] = "HDR Auto";
         choices3[255] = "Video";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const ChoicesIterator r  = choices.find (t->toInt (0, BYTE));
-        std::map<int, std::string>::iterator r1 = choices1.find (t->toInt (1, BYTE));
-        std::map<int, std::string>::iterator r2 = choices2.find (t->toInt (2, BYTE));
-        std::map<int, std::string>::iterator r3 = choices3.find (t->toInt (3, BYTE));
+        std::map<int, std::string>::const_iterator r1 = choices1.find (t->toInt (1, BYTE));
+        std::map<int, std::string>::const_iterator r2 = choices2.find (t->toInt (2, BYTE));
+        std::map<int, std::string>::const_iterator r3 = choices3.find (t->toInt (3, BYTE));
         std::ostringstream s;
         s << ((r != choices.end()) ? r->second : "");
         s << ((r1 != choices1.end()) ? r1->second : "") << " ";
@@ -993,7 +993,7 @@ public:
         choices.insert (p_t (256 * 22 + 4, "04 Toy Lens Wide 6.3mm f/7.1"));
         choices.insert (p_t (256 * 22 + 5, "05 Toy Lens Telephoto 18mm f/8"));
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         double *liArray = nullptr;
         double maxApertureAtFocal = 0.;
@@ -1061,7 +1061,7 @@ class PASRResultInterpreter: public Interpreter
 {
 public:
     PASRResultInterpreter() { }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         int b = t->toInt (0, BYTE);
@@ -1146,7 +1146,7 @@ public:
         choices[ 2 << 8 | 4 ] = "Auto";
     }
 
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int idx = 0;
 
@@ -1173,7 +1173,7 @@ public:
         choices[2] = "Standard";
         choices[3] = "Fast";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const ChoicesIterator r  = choices.find (t->toInt (0, BYTE));
         std::ostringstream s;
@@ -1211,7 +1211,7 @@ public:
         choices[2] = "Medium";
         choices[3] = "High";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const ChoicesIterator r  = choices.find (t->toInt (0, BYTE));
         std::ostringstream s;
@@ -1243,11 +1243,11 @@ public:
         choices2[8]   = "2 EV";
         choices2[12]  = "3 EV";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const ChoicesIterator r  = choices.find  (t->toInt (0, BYTE));
-        std::map<int, std::string>::iterator r1 = choices1.find (t->toInt (1, BYTE));
-        std::map<int, std::string>::iterator r2 = choices2.find (t->toInt (2, BYTE));
+        std::map<int, std::string>::const_iterator r1 = choices1.find (t->toInt (1, BYTE));
+        std::map<int, std::string>::const_iterator r2 = choices2.find (t->toInt (2, BYTE));
         std::ostringstream s;
         s << ((r != choices.end() ) ?  r->second : "") << std::endl;
         s << ((r1 != choices1.end()) ? r1->second : "") << std::endl;
@@ -1290,7 +1290,7 @@ class PALensModelQInterpreter: public Interpreter
 {
 public:
     PALensModelQInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[31];
         buffer[0] = 0;  //
@@ -1308,7 +1308,7 @@ class PALensInfoQInterpreter: public Interpreter
 {
 public:
     PALensInfoQInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[21];
         buffer[0] = 0;
@@ -1326,7 +1326,7 @@ class PAFlashExposureCompInterpreter: public Interpreter
 {
 public:
     PAFlashExposureCompInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a;
 
@@ -1359,7 +1359,7 @@ class PAFocalLengthInterpreter: public Interpreter
 {
 public:
     PAFocalLengthInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         double a = double (t->toInt (0, LONG));
 
@@ -1388,7 +1388,7 @@ class PALensDataFocalLengthInterpreter: public Interpreter
 {
 public:
     PALensDataFocalLengthInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         float b = float (10 * int (a >> 2)) * pow (4.f, float (int (a & 0x03) - 2));
@@ -1419,7 +1419,7 @@ class PAISOfInterpreter: public Interpreter
 {
 public:
     PAISOfInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         char buffer[32];
@@ -1439,7 +1439,7 @@ class PAMaxApertureInterpreter: public Interpreter
 {
 public:
     PAMaxApertureInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         a &= 0x7F;
@@ -1476,7 +1476,7 @@ class PAAEXvInterpreter: public Interpreter
 {
 public:
     PAAEXvInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         char buffer[32];
@@ -1496,7 +1496,7 @@ class PAAEBXvInterpreter: public Interpreter
 {
 public:
     PAAEBXvInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, SBYTE);
         char buffer[32];
@@ -1516,7 +1516,7 @@ class PAApertureInterpreter: public Interpreter
 {
 public:
     PAApertureInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         char buffer[32];
@@ -1536,7 +1536,7 @@ class PAExposureTimeInterpreter: public Interpreter
 {
 public:
     PAExposureTimeInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt (0, BYTE);
         char buffer[32];
@@ -1556,7 +1556,7 @@ class PANominalMinApertureInterpreter: public Interpreter
 {
 public:
     PANominalMinApertureInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         int a = t->toInt (0, BYTE);
@@ -1576,7 +1576,7 @@ class PANominalMaxApertureInterpreter: public Interpreter
 {
 public:
     PANominalMaxApertureInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         int a = t->toInt (0, BYTE);
@@ -1694,7 +1694,7 @@ class PAExternalFlashGNInterpreter: public Interpreter
 {
 public:
     PAExternalFlashGNInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         int b = t->toInt (0, BYTE) & 0x1F;
@@ -1708,7 +1708,7 @@ class PAEVStepsInterpreter: public Interpreter
 {
 public:
     PAEVStepsInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
 
@@ -1727,7 +1727,7 @@ class PAEDialinInterpreter: public Interpreter
 {
 public:
     PAEDialinInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
 
@@ -1746,7 +1746,7 @@ class PAApertureRingUseInterpreter: public Interpreter
 {
 public:
     PAApertureRingUseInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
 
@@ -1776,7 +1776,7 @@ public:
         choices[9] = "Slow-sync, Red-eye reduction";
         choices[10] = "Trailing-curtain Sync";
     }
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const ChoicesIterator r = choices.find (t->toInt (0, BYTE) >> 4);
 
@@ -1795,7 +1795,7 @@ class PAMeteringMode2Interpreter: public Interpreter
 {
 public:
     PAMeteringMode2Interpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         int v = (t->toInt (0, BYTE) & 0xF);
@@ -1859,7 +1859,7 @@ class PAProgramLineInterpreter: public Interpreter
 {
 public:
     PAProgramLineInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::ostringstream str;
         int c = t->toInt (0, BYTE);
@@ -1899,7 +1899,7 @@ class PAAFModeInterpreter: public Interpreter
 {
 public:
     PAAFModeInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         switch (t->toInt (0, BYTE) & 0x3) {
             case 0:
@@ -1925,7 +1925,7 @@ class PAAFPointSelectedInterpreter: public Interpreter
 {
 public:
     PAAFPointSelectedInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int c = t->toInt (0, SHORT);
 
@@ -1949,7 +1949,7 @@ class PADriveMode2Interpreter: public Interpreter
 {
 public:
     PADriveMode2Interpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int c = t->toInt (0, BYTE);
 
diff --git a/rtexif/rtexif.cc b/rtexif/rtexif.cc
index 39d85ace9..f8ec3e5db 100644
--- a/rtexif/rtexif.cc
+++ b/rtexif/rtexif.cc
@@ -1767,7 +1767,7 @@ std::string Tag::nameToString (int i)
     return buffer;
 }
 
-std::string Tag::valueToString ()
+std::string Tag::valueToString () const
 {
 
     if (attrib && attrib->interpreter) {
@@ -3439,7 +3439,7 @@ short int int2_to_signed (short unsigned int i)
  * <focal>-<focal>mm f/<aperture>-<aperture>
  * NB: no space between separator '-'; no space between focal length and 'mm'
  */
-bool extractLensInfo (std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal)
+bool extractLensInfo (const std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal)
 {
     minFocal = 0.0;
     maxFocal = 0.0;
diff --git a/rtexif/rtexif.h b/rtexif/rtexif.h
index 242e02dea..67ce90552 100644
--- a/rtexif/rtexif.h
+++ b/rtexif/rtexif.h
@@ -58,7 +58,7 @@ const enum ByteOrder HOSTORDER = MOTOROLA;
 #endif
 enum MNKind {NOMK, IFD, HEADERIFD, NIKON3, OLYMPUS2, FUJI, TABLESUBDIR};
 
-bool extractLensInfo (std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal);
+bool extractLensInfo (const std::string &fullname, double &minFocal, double &maxFocal, double &maxApertureAtMinFocal, double &maxApertureAtMaxFocal);
 
 unsigned short sget2 (unsigned char *s, ByteOrder order);
 int sget4 (unsigned char *s, ByteOrder order);
@@ -293,7 +293,7 @@ public:
     int     getDistanceFrom (const TagDirectory *root);
 
     // additional getter/setter for more comfortable use
-    std::string valueToString         ();
+    std::string valueToString         () const;
     std::string nameToString          (int i = 0);
     void        valueFromString       (const std::string& value);
     void        userCommentFromString (const Glib::ustring& text);
@@ -372,7 +372,7 @@ class Interpreter
 public:
     Interpreter () {}
     virtual ~Interpreter() {};
-    virtual std::string toString (Tag* t)
+    virtual std::string toString (const Tag* t) const
     {
         char buffer[1024];
         t->toString (buffer);
@@ -493,7 +493,7 @@ protected:
     Choices choices;
 public:
     ChoiceInterpreter () {};
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         const typename std::map<T, std::string>::const_iterator r = choices.find(t->toInt());
 
@@ -512,11 +512,11 @@ class IntLensInterpreter : public Interpreter
 {
 protected:
     typedef std::multimap< T, std::string> container_t;
-    typedef typename std::multimap< T, std::string>::iterator it_t;
+    typedef typename std::multimap< T, std::string>::const_iterator it_t;
     typedef std::pair< T, std::string> p_t;
     container_t choices;
 
-    virtual std::string guess (const T lensID, double focalLength, double maxApertureAtFocal, double *lensInfoArray)
+    virtual std::string guess (const T lensID, double focalLength, double maxApertureAtFocal, double *lensInfoArray) const
     {
         it_t r;
         size_t nFound = choices.count ( lensID );
diff --git a/rtexif/sonyminoltaattribs.cc b/rtexif/sonyminoltaattribs.cc
index c8bbeb91d..0c6e433ff 100644
--- a/rtexif/sonyminoltaattribs.cc
+++ b/rtexif/sonyminoltaattribs.cc
@@ -1158,7 +1158,7 @@ public:
         };
     }
 
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int lensID = t->toInt();
         Tag *lensInfoTag = t->getParent()->getRoot()->findTag ("LensInfo");
@@ -1309,7 +1309,7 @@ public:
         choices.insert (p_t (51507, "Samyang AF 35mm f/1.4"));
     }
 
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int lensID = t->toInt();
         Tag *lensInfoTag = t->getParent()->getRoot()->findTag ("LensInfo");
@@ -2103,7 +2103,7 @@ class SAExposureTimeInterpreter : public Interpreter
 {
 public:
     SAExposureTimeInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         double a = t->toDouble();
 
@@ -2163,7 +2163,7 @@ class SAFNumberInterpreter : public Interpreter
 {
 public:
     SAFNumberInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         double a = double (t->toDouble());
 
@@ -2223,7 +2223,7 @@ class SAISOSettingInterpreter : public Interpreter
 {
 public:
     SAISOSettingInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->toInt();
 
@@ -2264,7 +2264,7 @@ class SAExposureCompSetInterpreter : public Interpreter
 {
 public:
     SAExposureCompSetInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         double a = t->toDouble();
         char buffer[32];
@@ -2285,7 +2285,7 @@ class SAAFMicroAdjValueInterpreter : public Interpreter
 {
 public:
     SAAFMicroAdjValueInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%d", t->getValue()[0] - 20);
@@ -2302,7 +2302,7 @@ class SAAFMicroAdjModeInterpreter : public Interpreter
 {
 public:
     SAAFMicroAdjModeInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int a = t->getValue()[0] & 0x80;
 
@@ -2324,7 +2324,7 @@ class SAAFMicroAdjRegisteredLensesInterpreter : public Interpreter
 {
 public:
     SAAFMicroAdjRegisteredLensesInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%d", t->getValue()[0] & 0x7f);
@@ -2341,7 +2341,7 @@ class SAFocusStatusInterpreter : public Interpreter
 {
 public:
     SAFocusStatusInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         std::string retval;
         int a = t->toInt();
@@ -2380,7 +2380,7 @@ class SAColorTemperatureSettingInterpreter : public Interpreter
 {
 public:
     SAColorTemperatureSettingInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         sprintf (buffer, "%d", t->toInt());
diff --git a/rtexif/stdattribs.cc b/rtexif/stdattribs.cc
index aa8ed2b97..ec5534381 100644
--- a/rtexif/stdattribs.cc
+++ b/rtexif/stdattribs.cc
@@ -327,7 +327,7 @@ class FNumberInterpreter : public Interpreter
 {
 public:
     FNumberInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = t->toDouble();
@@ -346,7 +346,7 @@ class ApertureInterpreter : public Interpreter
 {
 public:
     ApertureInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = pow (2.0, t->toDouble() / 2.0);
@@ -365,7 +365,7 @@ class ExposureBiasInterpreter : public Interpreter
 {
 public:
     ExposureBiasInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = t->toDouble();
@@ -384,7 +384,7 @@ class ShutterSpeedInterpreter : public Interpreter
 {
 public:
     ShutterSpeedInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double d = pow (2.0, -t->toDouble());
@@ -404,7 +404,7 @@ class ExposureTimeInterpreter : public Interpreter
 {
 public:
     ExposureTimeInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double d = t->toDouble();
@@ -424,7 +424,7 @@ class FocalLengthInterpreter : public Interpreter
 {
 public:
     FocalLengthInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char buffer[32];
         double v = t->toDouble();
@@ -443,7 +443,7 @@ class UserCommentInterpreter : public Interpreter
 {
 public:
     UserCommentInterpreter () {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int count = t->getCount();
 
@@ -575,7 +575,7 @@ class CFAInterpreter : public Interpreter
 {
 public:
     CFAInterpreter() {}
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         char colors[] = "RGB";
         char buffer[1024];
@@ -632,7 +632,7 @@ UTF8BinInterpreter utf8BinInterpreter;
 class RawImageSegmentationInterpreter : public Interpreter
 {
 public:
-    std::string toString (Tag* t) override
+    std::string toString (const Tag* t) const override
     {
         int segmentNumber = t->toInt(0, SHORT);
         int segmentWidth = t->toInt(2, SHORT);