There are essentially three ways to vary the brightness; by increasing or decreasing the light intensity (using the on/off knob), by moving the condenserlens closer to or farther from the object using...There are essentially three ways to vary the brightness; by increasing or decreasing the light intensity (using the on/off knob), by moving the condenserlens closer to or farther from the object using the condenser adjustment knob, and/or by opening/closing the irisdiaphragm.