The human visual system exhibits a characteristic known as the Helmholtz-Kohlrausch (H-K) effect: even if the hue and the lightness retain the same values, the actual lightness (perceived lightness) changes with changes in the color saturation. Quantification of this effect is expected to be useful for the future development and evaluation of high-quality displays. We have been studying the H-K effect in natural images projected by LED projectors, which play important roles in practical uses. To verify the effectiveness of the determinations of the H-K effect for natural images, we have performed a subjective-evaluation experiment by method of adjustment for natural images and compared the experimental values with values calculated from extended form of Nayatani's equation to apply to natural images. In general, we found a high correlation between the two, although there was a low correlation for some images. Therefore, we obtained a correction function derived from the subjective evaluation experiment value of 108 color (hue: 12 × saturation: 3 × lightness: 3) patterns and have applied it to estimate the equation H-K effect.