A KTa<sub>1-x</sub>Nb<sub>x</sub>O<sub>3</sub> (KTN) varifocal lens achieves a fast response of microseconds, a large aperture of 3 mm, a random change of focusing length and a high transmission exceeding 99%. However, the variable lens power is smaller than that of other varifocal lenses. We successfully revealed the best way to obtain a variable lens power of 5 m<sup>-1</sup> with a KTN varifocal lens. We simulated the relationships between the length of a KTN lens and both wave aberration and variable lens power. The wave aberration increased with increases in variable lens power. We employed the ratio of the wave aberration to the variable lens power to evaluate the scanning resolution, which improved as the ratio decreased. The best length for a KTN lens was 5.3 mm for an octagonal structure whose aperture size, thickness, and electrode angle were 3 mm, 4 mm and 26.5 degrees, respectively. Our experimental results agreed with the simulation. A wave aberration of less than λ/ 10 (λ=1064 nm) and a variable lens power of 0-2.5 m<sup>-1</sup> were obtained. We concluded that combining a pair of optimized structures was the best way to obtain a variable lens power of 0-5 m<sup>-1</sup>, which is comparable to that of varifocal lenses using other principles.