The Diopter I am aware of, and have adjusted and may have to again, is on the side of the view finder, (on my Nikon) and most DSLR's a lot of cameras but not all have one so need to check your manual, but I think some point and shoot cameras may have a software adjustment.
This helps to compensate for the variation in peoples eyes, so depending on the eye you use with the view finder for focusing, if you haven't adjusted give it a go. The adjustments are in small + & - increments.
As you are looking through the view finder and chosen your focus point, single centre point auto or manual focus the subject and then move the adjustment up or down until you can get the sharpest image possible try a few shots to check and adjust if necessary. This however may not resolve the situation for yourself, there corrective lenses for the view finder but you would need to try these before purchase. If you wear glasses then how you approach this is how you feel comfortable and what works for you
I know I have probably not covered this very comprehensively but is a start so over the next week for those who have not known about or tried it give it a go and let us know and or add to this as required.
With regards Back Button Focusing there is a thread currently running with a link to a site which give a bit more info on the subject , I will try to how to add the link unless some one beets me to it rather than extend this further.
So good luck and look forward to responses and as usual all CC accepted good or bad.
hope this ends up in the right spot.