Autofocus using a torch, that way you can see the object you're focussing on. Then switch the camera into manual.. I also use the distance scale if you have a rough idea of how far away things are... If you've got an iPhone there's a great little free app called "Simple DoF calculator" that does exactly that.. You put in the camera you use (so it knows the size of your sensor) the focal length, F stop and distance and it'll tell you the distance in front of and behind the subject your camera will focus as well as the hyperfocal distance.. In your circumstances I'd use the hyperfocal distance for setting the focus and make sure the subject was at least that far away.
Nice thing about that app is that you can sit at home and fiddle with it and see how hyperfocal length changes with focal length and f stop and decide how you'll set your camera up before you leave your nice warm house!
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A photo needs to start and finish in your imagination, if it passes through your camera in between, that's cool, if it doesn't, that's cool also.
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