diff --git a/README.md b/README.md
index a7fc1eff2bea2a281fbe3b3b8ff2d521af41a056..75ffb0fc6935bfc793c928bfe55b1caad758ab5b 100644
--- a/README.md
+++ b/README.md
@@ -56,6 +56,8 @@ TODO
 
 <img src='img/assembly.jpg' height=400px>
 
+The brightness of the LEDs around the camera is set by a resistor value.  Unfortunately the circuit used by this SuperEyes microscope is quite sensitive to this value (higher resisistance is brighter, so it's like the the LEDs and resistor are in parallel, rather than series as is usual with current limiting resistors... not sure why).  I've found 330Kohm to be a good value for this.
+
 
 ### Alignment