I have recently purchased a Saunders Super Dichroic 4500 II for medium format and 4x5 B&W printing. In the light path, between the lamp and filters, and before the diffusion box, there is a perforated metal disc that appears to block a good portion of the light. Can anyone tell me whether this standard for this unit, and if so why? I am a bit puzzled and curious about its function. Might it have been inserted to cut down on the light for smaller negative formats? Should it be removed for larger format negatives? Thanks for any information.