Perkeleellinen, I use a glassless negative carrier for my Opemus 6a and enlarge from 135 negs only. I don't know about larger negatives, but I never had a problem with flatness. I use a 75W bulb, so the generated heat is not that much to cause problems either. A 150W bulb might cause problems, but I don't know. As an added bonus (as Ian said) you only need to clean dust from the negatives, not the carrier.