https://purengom.com/2009/10/09/%ec%9c%a0%ec%be%8c-%ec%83%81%ec%be%8c-%eb%aa%85%ec%be%8c%ed%95%9c-dslr-%ea%b8%b0%ec%b4%88-3-%eb%85%b8%ec%b6%9c/