5 micron or 9 micron or 20 micron ground glass?

The 5 micron ground glass is the made using the 5 micron grade aluminum oxide powder. This means the powder is finer than the 9 and 20 aluminum powder. The finer the grind, the sharper the image will be. However, the finer the glass the less diffusion so therefore all 35mm adapters need to use a condenser lens to spread the light more evenly on the surface of the glass, just like in the gt35pro adapters. If you are building your own adapter and don't have a condenser lens then you will see hotspotting or vignetting in your images. The 20 micron ground glass will produce a much softer image but has stronger diffusion so some adapter makers use 20 micron or 25 or 35 micron ground glass without condensers.
This will cut down the vignetting by a measurable amount. But will give you a very soft picture. Also light loss is high and in situations when the aperture is stopped down to below f5.6, the grain becomes visible. The 9 micron ground glass is a good balance between the 2 other grinds. It gives a more film like softer image but not as soft as the 20 micron. I personally feel that the 5 micron is the best as it gives the sharpest and brightest pictures. We are now in the HD era and we want to keep that high resolution. Softer film look can be done in post production.