Why should I buy a 50mm prime lens?

Challenge #5 in the Opportunities series. Hosted by juanlucas.
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I made a mistake... haven't bought a 50mm prime lens yet. Show me why I should buy it!! MANDATORY to indicate brand/model of lens used for taking the picture (of course, only 50mm prime lens for this challenge).
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Eyes
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You should buy a 50mm prime because: - Almost without exception they're really fast, so you can shoot lowlight. This one was taken with just a TV on. -They can achieve low depth of field, which can be used to isolate the focused parts of the picture, in order to give emphasis to them, or to create an optical effect for whatever artistic reasons. I believe this is the case here (emphasis on the eye, isolated from the rest of the face) - Some of the are really cheap and have incredible price/performance ratio (this one costs around 100 euros) - They can achieve very high image quality, especially regarding sharpness. This is not the case here (this is iso 1600 at f 1.8 at very low light), but generally speaking it is.
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Submitted: Friday, 1st July, 2011 07:12 (GMT)
Taken: Sunday, 2nd May, 2010
Focal length: 50 mm
Shutter speed: 1/60 sec
Aperture: F1.8
ISO: 1600
Notes: Taken using the Canon 50mm /1.8 at 1.8
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