In this day and age, auto-focus is so quick and accurate that there is little reason to manual focus in most situations. So much so, in fact, that lenses and cameras nowadays hardly even have the features necessary to accurately manual focus even if you wanted to. This has lead to techniques like Zone focusing falling by the wayside.
So, if modern technology has virtually left manual focusing behind, what is zone focusing and why is it still relevant today? Well, to oversimplify it, zone focusing is using the distance and depth of field scales on your lens, in combination with the aperture, to quickly and easily nail focus on a subject. The problem with many (most) modern lenses, is that they no longer have these features on the lens – so how is zone focusing still relevant?
If you are like me, and like using old vintage manual focus lenses, either on your SLR or mirrorless camera, zone focusing is a technique that can help you nail focus quickly, and fairly accurately without relying on the LCD to try and tell if you are in focus or not. Specifically with mirrorless cameras, since there is no optical viewfinder, you must rely on an LCD to try and tell what is in focus when using old vintage lenses. Some cameras have handy 100% zoom features in live view that can help, but even with those, it can be difficult to nail focus quickly. That is where zone focusing comes in.
Using the markings on the lens, with the approximate distance to your subject in mind, you can quickly and accurately nail sharp focus in no time. The video below is a great overview of zone focusing using Fuji‘s system (but is applicable to to any lens with the correct markings).
This has helped me a great deal since moving to using almost exclusively vintage manual focus lenses on my mirrorless EOS-M camera bodies. Using this system, in conjunction with zooming to 100% in live view, has really helped improve the speed with which I can nail focus on these tiny mirrorless bodies.
I highly recommend practicing it and giving it a try if you are into using old vintage lenses.