I am using an Aristo V54 D2-Hi Cold Light head on my 45MX with a RH Designs Stop Clock Vario. I have been doing 35mm and 6x6 without issue for some time using under lens Ilford Multigrade filters.

But last night, I did my first 16x20 from a 4x5 neg in a glass holder and got noticeable falloff on the sides and corners, as much as half a stop. I had the lamp lowered all the way down to where it was touching the top of the glass and used my 150mm 5.6 Rodagon versus my 135, so I thought I was golden in terms of adequate coverage and even illumination, lens was at F/16.

I have a spare cold light head and was thinking of taking the diffuser out of it and doubling up my existing one to spread the light out more...

Is this the solution or am I facing another issue I am not identifying yet?