In 1984, Kozlowski, Mainster and Avila derived a formula for the power of the lens needed in the negative lens field expander. Their method is based on the principle of telescopic magnification. In this technical note, we use the principle of spectacle magnification and obtain the power of the same lens. Both methods are presented for comparison. Practitioners may choose either method in determining the power of the negative lens field expander for their low vision patients, but the spectacle magnification method has the advantages of simplicity and familiarity to ophthalmic practitioners.