Counterflow diffusion flame is a favorable platform for fundamental investigation of soot kinetics. A diffuse back-illumination imaging technique for measuring soot volume fractions in these flames was rigorously demonstrated here. It was noticed that the technique is extremely sensitive to slight asymmetry of the flame. Misleading conclusions could be drawn due to the surprisingly large distortion of the measured SVF profile caused by flame tilting, even when the tilting is so slight as to be undetectable through the flame images. To address this issue, the effect of the flame tilting on soot measurements were quantitatively analyzed and a novel procedure was proposed to identify and correct the measurement distortions.