Consider a compound lens system comprising two thin lenses in contact, having focal lengths f1 and f2 respectively. What is the equivalent focal length F of the combined optical system?

💡 Solution & Explanation

The optical power of thin lenses in contact is additive, meaning the net power is the sum of individual powers, and since power is the reciprocal of focal length, the equivalent focal length is given by the product of the individual focal lengths divided by their sum. Incorrect reciprocal rules confuse lens combinations with parallel resistor formulas, whereas optical powers add algebraically. Lens combinations are routinely used in optical design to eliminate chromatic and spherical aberrations.