Good afternoon, my name is ***** ***** I will assist with your query today.
In the majority of case aperture priority would normally be the best mode.
You may also wish to refer to the exposure setting on page 108 here, this lets you re-calibrate it the exposure.
With regard to over sensitivity of the button this isnt a user definable setting, as such I would think this is a hardware fault, and can only be fixed by replacing the button.
Please let me know if you require any further assistance, or need anything clarified.
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