For cosmologically interesting f ( R ) gravity models , we derive the complete set of the linearized field equations in the Newtonian gauge , under environments of the solar system , galaxies and clusters respectively .
Based on these equations , we confirmed previous \gamma = 1 / 2 solution in the solar system .
However , f ( R ) gravity models can be strongly environment-dependent and the high density ( comparing to the cosmological mean ) solar system environment can excite a viable \gamma = 1 solution for some f ( R ) gravity models .
Although for f ( R ) \propto - 1 / R , it is not the case ; for f ( R ) \propto - \exp ( - R / \lambda _ { 2 } H _ { 0 } ^ { 2 } ) , such \gamma = 1 solution does exist .
This solution is virtually indistinguishable from that in general relativity ( GR ) and the value of the associated curvature approaches the GR limit , which is much higher than value in the \gamma = 1 / 2 solution .
We show that for some forms of f ( R ) gravity , this solution is physically stable in the solar system and can smoothly connect to the surface of the Sun .

The derived field equations can be applied directly to gravitational lensing of galaxies and clusters .
We find that , despite significant difference in the environments of galaxies and clusters comparing to that of the solar system , gravitational lensing of galaxies and clusters can be virtually identical to that in GR , for some forms of f ( R ) gravity .
Fortunately , galaxy rotation curve and intra-cluster gas pressure profile may contain valuable information to distinguish these f ( R ) gravity models from GR .