diff --git a/src/r_main.c b/src/r_main.c
index 9223a0a3..00ed7491 100644
--- a/src/r_main.c
+++ b/src/r_main.c
@@ -48,6 +48,7 @@ int validcount = 1;         // increment every time a check is made
 lighttable_t *fixedcolormap;
 int      centerx, centery;
 fixed_t  centerxfrac, centeryfrac;
+fixed_t  focallength;
 fixed_t  projection;
 fixed_t  viewx, viewy, viewz;
 angle_t  viewangle;
@@ -259,7 +260,7 @@ static fixed_t centerxfrac_nonwide;
 static void R_InitTextureMapping(void)
 {
   register int i,x;
-  fixed_t focallength, slopefrac;
+  fixed_t slopefrac;
   angle_t fov;
 
   // Use tangent table to generate viewangletox:
@@ -273,8 +274,9 @@ static void R_InitTextureMapping(void)
   {
     const double slope = (tan(custom_fov * M_PI / 360.0) *
                           centerxfrac / centerxfrac_nonwide);
-    fov = atan(slope) * FINEANGLES / M_PI;
-    slopefrac = finetangent[FINEANGLES / 4 + fov / 2];
+    const double angle = atan(slope) + M_PI / FINEANGLES; // finetangent offset.
+    fov = angle * FINEANGLES / M_PI;
+    slopefrac = tan(angle) * FRACUNIT;
     focallength = FixedDiv(centerxfrac, slopefrac);
     projection = centerxfrac / slope;
   }
@@ -285,10 +287,12 @@ static void R_InitTextureMapping(void)
     focallength = FixedDiv(centerxfrac_nonwide, slopefrac);
     projection = centerxfrac_nonwide;
 
-    if (widescreen != RATIO_ORIG)
+    if (centerxfrac != centerxfrac_nonwide)
     {
-      fov = atan((double)centerxfrac / centerxfrac_nonwide) * FINEANGLES / M_PI;
-      slopefrac = finetangent[FINEANGLES / 4 + fov / 2];
+      const double slope = (double)centerxfrac / centerxfrac_nonwide;
+      const double angle = atan(slope) + M_PI / FINEANGLES; // finetangent offset.
+      fov = angle * FINEANGLES / M_PI;
+      slopefrac = tan(angle) * FRACUNIT;
     }
   }
 
diff --git a/src/r_main.h b/src/r_main.h
index 6dfb6b20..1c4ab2c0 100644
--- a/src/r_main.h
+++ b/src/r_main.h
@@ -37,6 +37,7 @@ extern int      centerx;
 extern int      centery;
 extern fixed_t  centerxfrac;
 extern fixed_t  centeryfrac;
+extern fixed_t  focallength;
 extern fixed_t  projection;
 extern int      validcount;
 extern int      linecount;
diff --git a/src/r_segs.c b/src/r_segs.c
index ca1a6b94..99d3c9aa 100644
--- a/src/r_segs.c
+++ b/src/r_segs.c
@@ -157,7 +157,7 @@ void R_RenderMaskedSegRange(drawseg_t *ds, int x1, int x2)
       {
         if (!fixedcolormap)      // calculate lighting
           {                             // killough 11/98:
-            unsigned index = FixedDiv(spryscale, video.xscale) >> LIGHTSCALESHIFT;
+            unsigned index = FixedDiv(spryscale * 160, focallength) >> LIGHTSCALESHIFT;
 
             if (index >=  MAXLIGHTSCALE )
               index = MAXLIGHTSCALE-1;
@@ -378,7 +378,7 @@ static void R_RenderSegLoop (void)
           texturecolumn >>= FRACBITS;
 
           // calculate lighting
-          index = FixedDiv(rw_scale, video.xscale) >> LIGHTSCALESHIFT;  // killough 11/98
+          index = FixedDiv(rw_scale * 160, focallength) >> LIGHTSCALESHIFT;
 
           if (index >=  MAXLIGHTSCALE )
             index = MAXLIGHTSCALE-1;
diff --git a/src/r_things.c b/src/r_things.c
index 7ba025d1..7a8b9a32 100644
--- a/src/r_things.c
+++ b/src/r_things.c
@@ -664,7 +664,7 @@ void R_ProjectSprite (mobj_t* thing)
     vis->colormap[0] = vis->colormap[1] = fullcolormap;       // full bright  // killough 3/20/98
   else
     {      // diminished light
-      int index = FixedDiv(xscale, video.xscale) >> LIGHTSCALESHIFT;  // killough 11/98
+      int index = FixedDiv(xscale * 160, focallength) >> LIGHTSCALESHIFT;
       if (index >= MAXLIGHTSCALE)
         index = MAXLIGHTSCALE-1;
       vis->colormap[0] = spritelights[index];
diff --git a/src/r_voxel.c b/src/r_voxel.c
index ac3d1ec3..bed1736f 100644
--- a/src/r_voxel.c
+++ b/src/r_voxel.c
@@ -724,7 +724,7 @@ boolean VX_ProjectVoxel (mobj_t * thing)
 	else
 	{
 		// diminished light
-		int index = FixedDiv(xscale, video.xscale) >> LIGHTSCALESHIFT;  // killough 11/98
+		int index = FixedDiv(xscale * 160, focallength) >> LIGHTSCALESHIFT;
 
 		if (index < 0)               index = 0;
 		if (index > MAXLIGHTSCALE-1) index = MAXLIGHTSCALE-1;