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That's how I do it. I set the button for exposure lock only, since I usually wouldn't want to lock focus if I'm recomposing.One of two ways:
1) Shoot in all manual mode (with auto ISO off). Point at scence, set aperture, shutter, and ISO as needed (using the light meter in viewfinder as refernce. then recompose and shoot. By virture of the fact your settings are all manually set and won't change they are 'locked' until you change them.
2) There is a button to the right of the viewfinder labeled "AE-L/AF-L." That is for locking exposure, autofocus, or both, when you are in P, S, or A mode. By default I believe it will lock both exposure and focus, but there are settings in the menus. You press and hold it to lock exposure, recompose and shoot. Alternately, in your menus, you can set that button so that you press it once and release it...after which you exposure will be locked until you press it again. Whenever you exposure is locked via this button you will see the letters "AE-L" light up in your viewfinder to remind you.