I've been wondering about this for a while.... I use Omega D-II with a 6x6 neg carrier to do 645 films. (I don't own 645 carriers) I don't use any masks or anything to cover the blank area. My reasoning is, since I use easel and never print film borders, it wouldn't make any difference. I've never experienced any obvious problems either. I also reasoned, this is more like having HUGE shadow area on both side of 6x6.... Is there a reason why many folks here recommend making mask to do such thing? What am I missing?