diff --git a/src/spectrogram.c b/src/spectrogram.c
index 7e294f2..c50f63e 100644
--- a/src/spectrogram.c
+++ b/src/spectrogram.c
@@ -496,9 +496,15 @@ render_heat_border (cairo_surface_t * surface, double magfloor, const RECT *r)
 ** allowing the caller to interpolate onto the input array.
 */
 static double
-magindex_to_specindex(int speclen, int maglen, int magindex, double min_freq, double max_freq, int samplerate)
+magindex_to_specindex(int speclen, int maglen, int magindex, double min_freq, double max_freq, int samplerate, bool log_freq)
 {
-	double freq = min_freq + (max_freq - min_freq) * magindex / (maglen - 1) ;
+	double freq; /* The frequency that this output value represents */
+
+	if (!log_freq)
+		freq = min_freq + (max_freq - min_freq) * magindex / (maglen - 1) ;
+	else
+		freq = min_freq * pow (max_freq / min_freq, (double) magindex / (maglen - 1)) ;
+
 	return (freq * speclen / (samplerate / 2)) ;
 }
 
@@ -527,9 +533,11 @@ interp_spec (float * mag, int maglen, const double *spec, int speclen, const REN
 	for (k = 0 ; k < maglen ; k++)
 	{	/* Average the pixels in the range it comes from */
 		double this = magindex_to_specindex(speclen, maglen, k,
-			        render->min_freq, render->max_freq, samplerate) ;
+			        render->min_freq, render->max_freq, samplerate,
+				render->log_freq) ;
 		double next = magindex_to_specindex(speclen, maglen, k+1,
-			        render->min_freq, render->max_freq, samplerate) ;
+			        render->min_freq, render->max_freq, samplerate,
+				render->log_freq) ;
 
 		/* Range check: can happen if --max-freq > samplerate / 2 */
 		if (this > speclen)
@@ -756,11 +764,6 @@ render_sndfile (RENDER * render)
 	SNDFILE *infile ;
 	SF_INFO info ;
 
-	if (render->log_freq)
-	{	printf ("Error : --log-freq option not working yet.\n\n") ;
-		exit (1) ;
-		} ;
-
 	memset (&info, 0, sizeof (info)) ;
 
 	infile = sf_open (render->sndfilepath, SFM_READ, &info) ;
@@ -769,7 +772,10 @@ render_sndfile (RENDER * render)
 		exit (1) ;
 		} ;
 
-	if (render->max_freq == 0.0) render->max_freq = (double) info.samplerate / 2 ;
+	if (render->max_freq == 0.0)
+		render->max_freq = (double) info.samplerate / 2 ;
+	if (render->min_freq == 0.0 && render->log_freq)
+		render->min_freq = 20.0;
 
 	/* Do this sanity check here, as soon as max_freq has its default value */
 	if (render->min_freq >= render->max_freq)
@@ -806,7 +812,7 @@ usage_exit (const char * argv0, int error)
 		"        --no-border            : Drop the border, scales, heat map and title\n"
 		"        --min-freq=<number>    : Set the minimum frequency in the output\n"
 		"        --max-freq=<number>    : Set the maximum frequency in the output\n"
-		/*-"        --log-freq             : Use a logarithmic frquency scale\n" -*/
+		"        --log-freq             : Use a logarithmic frquency scale\n"
 		"        --gray-scale           : Output gray pixels instead of a heat map\n"
 		"        --kaiser               : Use a Kaiser window function (the default)\n"
 		"        --rectangular          : Use a rectangular window function\n"