I shoot manual Nikkor Ai lenses a lot, so that's a big one for me. I learned to adjust it with precision by using a tripod and an AF lens, letting it focus on something having an obvious focus point with good bright details, and then turn off the camera and set the diopter where that focus point looks best in the viewfinder.
Setting the diopter adjustment by looking at the heads up display in the viewfinder had me manually focusing beyond the subject for a long time, until I figured out how to set the diopter using the method above. Now I get good results with manual lenses almost every shot.
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