diff --git a/src/lib/OpenEXR/ImfTiledInputFile.cpp b/src/lib/OpenEXR/ImfTiledInputFile.cpp
index 8f1bc52d2..e48d307b1 100644
--- a/src/lib/OpenEXR/ImfTiledInputFile.cpp
+++ b/src/lib/OpenEXR/ImfTiledInputFile.cpp
@@ -312,8 +312,6 @@ TiledInputFile::Data::getTileBuffer (int number)
 // in the bytesPerLineTable and the lineOffsets table.
 // Attempt to read the last entry in the first level of the table. Either the seekg() or the read()
 // call will throw an exception if the file is much too small to contain the table.
-// For speed, does not compute the entire table size for Mipmap/Ripmap images
-// (Roughly 50% or 100% bigger respectively)
 //
 
 // assumes the input stream pointer is at (or before) the beginning of the chunk table
@@ -322,28 +320,38 @@ TiledInputFile::Data::getTileBuffer (int number)
 void
 TiledInputFile::Data::validateStreamSize()
 {
-    const Box2i &dataWindow = header.dataWindow();
-    Int64 tileWidth = header.tileDescription().xSize;
-    Int64 tileHeight = header.tileDescription().ySize;
+    const TileDescription& td = header.tileDescription();
+    Int64 chunkCount;
 
-    Int64 tilesX = (static_cast<Int64>(dataWindow.max.x+1-dataWindow.min.x) + tileWidth -1) / tileWidth;
+    if (td.mode==RIPMAP_LEVELS)
+    {
+        // use slow function to calculate exact size of ripmap
+        chunkCount = getTiledChunkOffsetTableSize(header);
+    }
+    else
+    {
+        // for ONE_LEVEL image, calculate exact number of tiles
+        // MIPMAP_LEVELS images will have roughly 1/3 more tiles than this
+        // but 'chunkCount' can be less than the real offset table size for a meaningful sanity check
+        //
+        const Box2i &dataWindow = header.dataWindow();
+        Int64 tileWidth = td.xSize;
+        Int64 tileHeight = td.ySize;
 
-    Int64 tilesY = (static_cast<Int64>(dataWindow.max.y+1-dataWindow.min.y) + tileHeight -1) / tileHeight;
+        Int64 tilesX = (static_cast<Int64>(dataWindow.max.x+1-dataWindow.min.x) + tileWidth -1) / tileWidth;
+        Int64 tilesY = (static_cast<Int64>(dataWindow.max.y+1-dataWindow.min.y) + tileHeight -1) / tileHeight;
 
+        chunkCount = tilesX*tilesY;
+    }
 
-    Int64 chunkCount = tilesX*tilesY;
-    if ( chunkCount > gLargeChunkTableSize)
+    if (chunkCount > gLargeChunkTableSize)
     {
 
-        if (chunkCount > gLargeChunkTableSize)
-        {
-            Int64 pos = _streamData->is->tellg();
-            _streamData->is->seekg(pos + (chunkCount-1)*sizeof(Int64));
-            Int64 temp;
-            OPENEXR_IMF_INTERNAL_NAMESPACE::Xdr::read <OPENEXR_IMF_INTERNAL_NAMESPACE::StreamIO> (*_streamData->is, temp);
-            _streamData->is->seekg(pos);
-
-        }
+       Int64 pos = _streamData->is->tellg();
+       _streamData->is->seekg(pos + (chunkCount-1)*sizeof(Int64));
+       Int64 temp;
+       OPENEXR_IMF_INTERNAL_NAMESPACE::Xdr::read <OPENEXR_IMF_INTERNAL_NAMESPACE::StreamIO> (*_streamData->is, temp);
+       _streamData->is->seekg(pos);
     }
 
 }