You are not alone, almost everyone in beginning is surrounded by such queries and optional subject selection has been one of the major decisions one has to take during Civil Services Examination preparation.
First of all we would like to clarify that UPSC gives you liberty to pick any optional subject of your choice and for candidates like you selection of any subject is obvious as there is no subject available that match with Computer Science.
When you write Prelims or Main Examination, examiner is not aware of your educational background and the marks are awarded on the correct answers and contents you present.
In interview as well, when you have performed in written examination then only you reach interview stage hence there is no negative impact on minds of members of interview board as far as your intellect is concerned.
Coming back to your choice, you are thinking logically and the effort you are putting right now will prove to be vital as the process you are following for optional subject selection is correct.
If you feel, you can do justice with Geography as optional subject; you can consider it keeping as optional subject. The only thing we suggest is to look at the syllabus in detail and previous years’ question papers to understand the level and current trends.
The following article would help you in getting some more insights about optional subject selection.