I found the D90 manual online. Pg. 200 states that the AE-L/AF-L button can be assigned as follows:AE lock and AF lockAE lock onlyAF lock onlyAE lock and holdAF-ONFV lockSo, it appears that focus can be decoupled from the shutter. Gaining the AF-ON function (while losing AE-L) is certainly preferable to my usage.
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