Back button focus, like manual mode is one of the things that can separate photographers from those who just want to grab a camera snap some pictures. Modern DSLR and Mirrorless cameras come from the factory set to activate Autofocus each time the shutter is pressed. Back button focus changes this to a button located under your right thumb on the back of the camera.
First of all this frees the shutter button up to exactly what it was designed to do, release the shutter.,which makes the camera more responsive. You can focus only point you choose and wait for the action to happen instead relying on Autofocus to snap in AFTER you press the shutter button. Secondly it allows you to lock focus on a point and not worry about it changing every time you try to take a picture.
Now I can't explain in this post exactly how to change all camera brands and models to back button focus but there are tons of resources available if you do a simple internet search. Once changed there is a bit of a learning curve so I would suggest just going out to take some pictures to get used to it before it becomes important. Given a little practice it will quickly be second nature.
Keep in mind you will no longer be able to just hand your camera to someone else and let them take a picture. Unless they are familiar with using back button focus they won't really understand how to make it work. However, for you it's an important step towards taking control of your camera and becoming more than just a picture taker. Back button focus allows you to unlock your photographic creativity.
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