diff --git a/src/avisynth.cpp b/src/avisynth.cpp
index 64e99d4..3925e21 100644
--- a/src/avisynth.cpp
+++ b/src/avisynth.cpp
@@ -255,10 +255,9 @@ class AvisynthVideoSource : public IClip {
             Env->ThrowError("BestVideoSource: %s", e.what());
         }
 
-        const BSVideoProperties &VP = V->GetVideoProperties();
         AVSMap *Props = Env->getFramePropsRW(Dst);
 
-        SetSynthFrameProperties(Src, VP, RFF, V->GetFrameIsTFF(n, RFF),
+        SetSynthFrameProperties(n, Src, *V, RFF, V->GetFrameIsTFF(n, RFF),
             [Props, Env](const char *Name, int64_t V) { Env->propSetInt(Props, Name, V, 1); },
             [Props, Env](const char *Name, double V) { Env->propSetFloat(Props, Name, V, 1); },
             [Props, Env](const char *Name, const char *V, int Size, bool Utf8) { Env->propSetData(Props, Name, V, Size, 1); });
diff --git a/src/synthshared.cpp b/src/synthshared.cpp
index 852d92a..a4a28a6 100644
--- a/src/synthshared.cpp
+++ b/src/synthshared.cpp
@@ -21,7 +21,9 @@
 #include "synthshared.h"
 #include "VSHelper4.h"
 
-void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const BSVideoProperties &VP, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData) {
+void SetSynthFrameProperties(int n, const std::unique_ptr<BestVideoFrame> &Src, const BestVideoSource &VS, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData) {
+    const BSVideoProperties VP = VS.GetVideoProperties();
+
     // Set AR variables
     if (VP.SAR.Num > 0 && VP.SAR.Den > 0) {
         mapSetInt("_SARNum", VP.SAR.Num);
@@ -49,11 +51,19 @@ void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const B
         mapSetInt("_FieldBased", FieldBased);
         mapSetInt("RepeatField", Src->RepeatPict);
 
-        // FIXME, use PTS difference between frames instead?
         if (Src->Duration > 0) {
             int64_t DurNum = VP.TimeBase.Num;
             int64_t DurDen = VP.TimeBase.Den;
-            vsh::muldivRational(&DurNum, &DurDen, Src->Duration, 1);
+
+            int64_t Dur = Src->Duration;
+            if (n < VP.NumFrames - 1) {
+                Dur = VS.GetFrameInfo(n + 1).PTS - Src->PTS;
+            } else if (VP.NumFrames >= 2) {
+                // Assume the last frame has the same duration as the previous one, if the latter exists
+                Dur = Src->PTS - VS.GetFrameInfo(n - 1).PTS;
+            }
+
+            vsh::muldivRational(&DurNum, &DurDen, Dur, 1);
             mapSetInt("_DurationNum", DurNum);
             mapSetInt("_DurationDen", DurDen);
         }
diff --git a/src/synthshared.h b/src/synthshared.h
index 15ac94b..5fc0107 100644
--- a/src/synthshared.h
+++ b/src/synthshared.h
@@ -26,6 +26,6 @@
 #include <functional>
 #include <cstdint>
 
-void SetSynthFrameProperties(const std::unique_ptr<BestVideoFrame> &Src, const BSVideoProperties &VP, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData);
+void SetSynthFrameProperties(int n, const std::unique_ptr<BestVideoFrame> &Src, const BestVideoSource &VS, bool RFF, bool TFF, const std::function<void(const char *, int64_t)> &mapSetInt, const std::function<void(const char *, double)> &mapSetFloat, const std::function<void(const char *, const char *, int, bool)> &mapSetData);
 
 #endif
\ No newline at end of file
diff --git a/src/vapoursynth.cpp b/src/vapoursynth.cpp
index 6d3fafb..9b8b606 100644
--- a/src/vapoursynth.cpp
+++ b/src/vapoursynth.cpp
@@ -108,12 +108,11 @@ static const VSFrame *VS_CC BestVideoSourceGetFrame(int n, int ActivationReason,
             return nullptr;
         }
 
-        const BSVideoProperties &VP = D->V->GetVideoProperties();
         VSMap *Props = vsapi->getFramePropertiesRW(Dst);
         if (AlphaDst)
             vsapi->mapConsumeFrame(Props, "_Alpha", AlphaDst, maAppend);
 
-        SetSynthFrameProperties(Src, VP, D->RFF, D->V->GetFrameIsTFF(n, D->RFF),
+        SetSynthFrameProperties(n, Src, *D->V, D->RFF, D->V->GetFrameIsTFF(n, D->RFF),
             [Props, vsapi](const char *Name, int64_t V) { vsapi->mapSetInt(Props, Name, V, maAppend); },
             [Props, vsapi](const char *Name, double V) { vsapi->mapSetFloat(Props, Name, V, maAppend); },
             [Props, vsapi](const char *Name, const char *V, int Size, bool Utf8) { vsapi->mapSetData(Props, Name, V, Size, Utf8 ? dtUtf8 : dtBinary, maAppend); });