diff --git a/lib/pd.py b/lib/pd.py
index 7e78593..608e10a 100755
--- a/lib/pd.py
+++ b/lib/pd.py
@@ -56,7 +56,7 @@ def __init__(self, exposure, max_reads=2048):
             Maximum number of reads of monitoring photodiode.  Default: 2048.
         """
 
-        buffertime = 1.0
+        buffertime = exptime*5./800.+2
 
 
         # for exposures over 0.5 sec, nominal PD readout at 60Hz,