I'm trying to achieve a scratched surface effect using a bump map, however it turns out the surface details are not preserved at a distance:

The first picture shows the metal part at high resoltion, the second picture is a down-scaled version of it and the third picture shows the same part rendered at a distance.

This surprises me actually because Cycles obviously does supersampling for the primary rays, however it seems that it downsamples the textures on the fly or pre-calculates mipmaps instead of sampling the textures at full resolution.