Open FALLOUT.INI found in C:\Users\user\Documents\My Games\Fallout3

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1st Step:

Hold down CTRL+F and type in (fDefault) until the line below appears. If it doesn't, add the line below.

fDefaultWorldFOV=90

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2nd Step:

Once again, press CTRL+F and type in (fDefault) and keep pressing until the line below appears, if it doesn't then add the line below.

fDefault1stPersonFOV=85.0000

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DO NOT CHANGE THE 1ST PERSON FOV HIGHER THAN 80-85, WEAPONS WILL WORK FUNKY AND CLIP THROUGH YOUR BODY. (Trust me, you won't like it. 

Or maybe you're a weirdo, who knows?)