Since my upgrade to 4x5, I have been rereading a few of my books to gleam some technical skills on work flow when shooting. One of the things that popped up again was a contrast viewing glass to help visualize a scene. I have never needed such a filter when visualizing a scene shooting 35mm or medium format, but setting up a 4x5 and the extra cost of everything, Id like to get it in one or two shots, and I thought maybe it was time to try out this filter. So there is the wratten #90 filter made just for this, as well as premade filters such as the tiffen viewing filters in 3 grades for film types. Seeing as this is just all gels, Id like to make my own with 35mm snap locking plastic slides and a rosco swatch book. I am thinking of just cutting it to size, and drilling a small hole to attach it, maybe sandwich the gel between a slightly thicker piece of clear plastic. What would be the equivalent filter for the wratten 90? I looked through the set, and it seems the closest is #3406, Roscosun 85N.6 Its the darkest amber type rosco has. Also, I have read that perhaps a dark blue filter my also work, something like the #80 or 83 primary blue/medium blue filters. Does anyone have experience with viewing filters and can provide some insight?