If you assign te AFL/AEL function to the red button in place of video and then go into the menu -> grinded wheel -> AF mode and set it to mode 3, it will AF on pressing the red button and lock exposure on half-pressing the shutter. You can also enable "AFL/AEL hold" and it will keep the focus at that point until you decide to change it.
Several people do that, I did it myself but now I switched back to mode 1 (half shutter press AFL, red button AEL) because I was dumb enough to forget to refocus...
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