Manual focus through an optical viewfinder can be difficult because many modern viewfinders are relatively small and not optimized for critical manual focusing. A few things can help:

• Use the camera’s focus-confirmation indicator if it works with your lens.
• Check and adjust the viewfinder diopter so the finder display looks perfectly sharp to your eye.
• If your camera offers live view, use it and magnify the image for much more precise focus.
• Stop down slightly if depth of field is extremely thin, since very fast apertures make small focus errors obvious.
• Use a brighter focusing screen or a manual-focus-friendly screen if your camera supports interchangeable screens.
• For static subjects, take a test shot and review at high magnification.

If you regularly focus manually, live view magnification is usually the most reliable method.