If you are a landscape photographer (or sports photographer for that matter), I would highly encourage you to give back-button focusing a try.
As you know, by default, all cameras are shipped with the focus tied to the shutter button. This is fine until you try focusing under dawn or dusk light, or more importantly, night! Let’s first take a look at how autofocus works. Essentially, the camera’s autofocus system is looking for contrast in the scene. At the times of day I just mentioned, contrast is very low.
Say we are shooting a full moon such as in these images from Monument Valley, Utah. I wanted to make sure the moon was razor sharp. It’s easy to point the camera at the moon, use center dot focusing, then recompose the scene. The problem comes when it is time to trip the shutter and capture your image. If that center dot focus is re-aligned on to a low contrast portion of the scene, your camera’s autofocus will wig in-and-out hunting for focus.
The way around this is to assign a back button the task of auto-focusing. Each camera is different but all pro models allow for it. Yep – time to search for your Instruction Manual.
I can help you if you are a Sony user, especially if you own the following Sony cameras: α7RII or α7SII.
Here are the steps:
1. Choose Menu > Custom Settings > Custom Key Settings > AEL Button > AF On.
2. Choose Menu > Custom Settings > Custom Key Settings > AF/MF Button > AF/MF Ctrl Toggle.
3. Choose Menu > Custom Settings > AF w/shutter > Off.
Back-button focusing works well if you are using a cable release also. According to the camera engineers I talked to, the user will see no difference in autofocus speed regardless of what button you assign you focus.
Give it a try and see if it doesn’t help and please share your experiences with all of us!