I made a new mask for the 120 size negatives. These smallest photos in our collection (well not strictly true, we do have a few hundred thousand 35mm, but not in the historic section) are the hardest for me to handle. They tend to be the last ones I digitise in any box because I avoid them as long as I can. In each box there are dozens of paper bags, each one containing a single frame of film. Typically it is Ilford FP4 (fine grain, high sharpness).
I have investigated the commercial masks sold with scanners like the Epson V700. And there's a generic 120 negative holder from DigitaLIZER and another from Plustek. All of these devices are basically a plastic frame for holding the 120 size negative. Then you pop it on your scanner, turn on the back light and scan away.
I've already made my thoughts on scanners vs camera well known. You get good results with a scanner, but I use a camera for the ultimate in picture quality.
The problem I have with these holders is that the negative is not pressed. There will be micro-wrinkles, warps and bends. In short, the negative is not perfectly flat.
My technique is to mask down the light source to about 200x150mm. I then place the 120 neg on the illuminated glass, cover with a another piece of glass, centre and finally mask with my 120 size cut out. This 3 tier sandwich ensures the negative is flattened perfectly. I focus in as close as I can. The 60mm Macro lens allows me to get close, really close. The end of the lens is just 50mm from the negative. I work at f4.5 and adjust the exposure for maximum contrast. I know serious photographers will be thinking about depth of field, but this isn't a concern for me. My subject is flat, really flat, so there is no depth of field. I've worked this lens from one end to the other and f4.5 is my sweet spot for this type of work.
And Newton rings are not a problem. The first layer of glass is from a gutted scanner - anti-Newton Ring scanner glass. The top layer is 5mm plain glass. For some lucky reason, this combination doesn't seem to give me rings. I absolutely see rings through the eye piece, but not in the captured image.
Oh, I forgot to say, I work in the dark. All the lights are off and the curtains drawn. It isn't a traditional dark room, but the light is severely restricted. I even angle my monitor away from the camera to avoid reflections. Virtually all of the light entering the lens is from the light box, passing through the negative.
I take a series of shots until I am absolutely convinced I have the best exposure. This is then developed into our digital negative.
And there you have it. There's no magic in this, it's just good technique and good equipment. You have to have the right tool for the job, as my late father was fond of saying.