This study proposes modification to f-scan using a liquid-crystal spatial light modulator (SLM). By programmatically controlling the phase of the incident light beam, continuous variation of the focal point along an arbitrary direction is realized, and thus achieve f-scan. A subfocus phenomenon occurs because of the discontinuous phase-modulation depth in the liquid-crystal SLM. Therefore, the intensity of the incident light is corrected based on theoretical analysis. Subsequently, experimental validation is performed using ZnSe materials. Theoretical and experimental studies suggest that the arrangement of this method is simple, and that no beam-collimation issue is caused by sample movement, unlike the Z-scan technique. Furthermore, this method overcomes the stringent precision requirements of coaxial optical systems and mitigates the constraints associated with minimal variations in the focal length of the f-scan technique. Consequently, this approach simplifies the experiment while enabling effective measurements of third-order optical nonlinearity in materials.