March's MD Disk helps to reduce the amount of light coming into a scope by up to 50%, depending on the conditions. This light reduction increases the scopes depth of field and, perhaps more importantly for competition shooters, reduces the amount of seen mirage. Using a MD Disk at 60 or 80 power gives you a competitive advantage when the conditions turn bad and lets you see the target better than your competitor. March Scopes 42mm MD Disk DB740-0