The answer to this is the same as that for questions relating to Back Focusing on Page 31 and 34, with the addition of the following: When setting the back focus of a Colour camera for low light conditions you should place an ND1 (Neutral Density) filter in front of the lens. When setting the back focus of a Mono camera for low light conditions you should place an ND3 (Neutral Density) filter in front of the lens. When setting the back focus of a Mono camera fitted with I/R lighting for low light conditions you should place an IRP (Infra- Red Pass) filter in front of the lens. Should you not have any of the above filters you may have to attend site during the hours of darkness.