gma, The Kodak infra red film doesn't have the anti-halation backing, so the blooming you're seeing is similar to what would be expected in the Maco Aura in 35mm. (The other common infrared films - Maco 750, Konica 750 (and Ilford SFX) do have an antihalation coating, and so don't show the blooming)

Ole, thanks, I'd expected the effect to be marginal in 5x4, so wasn't going to try it in that size. I'll order a couple of rolls of the 120 next time I order from someone who has it in stock, and will report back. (I'm not planning on re-rolling it! It will have whatever backing paper Maco deliver it on)