The soft-focus effect of the Thambar is achieved by the deliberate preservation of a spherical flaw, which is normally corrected using complex optical calculations. Spherical aberration is caused by the lens’ perimeter rays – which is why it is minimised by stopping down. The Thambar’s screw-on centre spot filter consequently offers the opposite effect to stopping down, as it blocks the axial rays and only allows the perimeter rays to pass.