Ditch Your Point and Shoot Five Reasons You Need an SLR

Like David vs. Goliath, only if David had gotten thoroughly whupped

The downside to interchangeable lenses, apart from the obvious additional cost, is a dirty sensor. You see, whenever you change an SLR lens, you essentially open up the camera and expose the interior to whatever conditions currently exist. If those conditions include blowing dust, loose dirt, or precipitation, chances are that some of it will enter your camera and lodge itself on your sensor, where it negatively impacts future photos. You can prevent a dirty sensor by protecting your camera when you change lenses, and you can clean a sensor after the fact, but dirty sensors are simply far less...

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http://www.maximumpc.com/article/features/ditch_your_point_and_shoot_five_reasons_you_need_slr