DSLR manual focusing on Nikon D5100
Today's viewfinders/focusing screens are designed for auto focus, with no split-prism or Fresnel ring. KatzEye makes a replacement screen that will get you back to the old days. I have no first-hand knowledge of how well they perform. The claim is you lose a bit of brightness, which might be a problem with anything but a larger pentaprism finder that your D5100 doesn't have.
I just went to the website link and the company has screens for many cameras, including the D5100. I will have to give it some thought and a bit more research but thanks for sending me to the right direction! Maybe it will be satisfactory for me.