已知数列\(\{a _{n} \}\)的前\(n\)项和\(S _{n} =- \dfrac {1}{4} +λa _{n}\)且\(a _{1} = \dfrac {1}{4}\)，设\(f(x)=e ^{x} -e ^{2-x} +1\)，则\(f(\log _{2} a _{1} )+f(\log _{2} a _{2} )+…+f(\log _{2} a _{7} )\)的值等于______．