In most types of photography, the most effective means of focusing with modern equipment is through autofocus. But there are situations where autofocus is simply unavailable, such as using an older lens, shooting in very low light, for landscape, macro, and astrophotography. Therefore, mastering manual focus is also essential.
Here are some tips that can help you with manual focus.
1. Electronic Visual Focusing Aids In digital cameras with live view on the rear screen and/or an electronic viewfinder, you are likely able to enlarge the electronic image to better see whether details are in focus. Electronic overlays such as focus peaking (where high-contrast edges are highlighted) and digital split-image (where out-of-focus areas are offset in position) can also help tell you whether you have achieved focus.
2. Other Visual Focusing Aids True rangefinder cameras mechanically link the viewfinder with the lens' focus position, overlaying two offset images over one another that come together as you obtain focus at the viewed-upon distance. SLR cameras can employ matte surfaces and microprism arrays on the focusing screen that make out-of-focus blur appear more obvious. And/or they can include a split-prism on the focusing screen that laterally offsets portions of the image when they are out of focus at the distance being viewed. These SLR focusing aids were more common in film cameras, especially in the pre-autofocus era, but many DSLR can also replace their focusing screens for one that includes aids.
3. Use Your Autofocus System Some cameras allow you to use the autofocus system to at least confirm for you when the autofocus sensors think that focus has been achieved. This still counts as manual focus because the photographer is still turning the focus ring to change focus, and does not require the camera or lens to automatically move or change focus.
4. Zone Focusing Some lenses include a distance scale which can help you estimate the distance that the lens is focusing to. A narrower aperture can help you enlarge the depth of field to hedge your bet to make sure that the target is within the in-focus region.
At the end of the day, just keep in mind that skill is acquired through repeated practice and practice makes perfect.