<math>f_2</math> should come out as a negative number. {{clear}} [[image:Negativelensfocallenght.png|thumb|center|450px|Simple method for determining the focal length of a negative lens.]]{{-}} Another method for measuring focal length of a negative lens: [[image:MeasuringNegativeLensFocalLenght.png|thumb|center|450px|Measuring the focal length of a negative lens using measuring microscope. Link->[http://commons.wikimedia.org/wiki/File:MeasuringMicroscope.jpg]]]