The Katz-Eye focusing screen helps you make manual focusing easier. They do that by having a microprism collar and a split-image rangefinder. We found it to be a great screen not just for macro but also for portraits with a medium fast telephoto lens, where the depth of field is minimal and the focus critical. The ones with an OptiBrite treatment, will give you the brightest viewfinder you ever had and it certainly gives the impression to make AF focusing a bit faster. I have one on each of my working cameras.
I have one on my D300. It helps me a lot with my several MF lenses. I also use it for macro when AF is not practical. I strongly recommend one for these uses. Note I have not added one to my D90 because it is may backup and I don't use it with MF lenses.
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