Image quality to have the best face recognition performance

Dear,

It might be a noob but I'm wondering which kind of image quality is bad for face feature extractor?

Searching around in Google brought me some concepts about image blur level, contrast, illumination, sharpness,...too many things and I'm not sure which one is bad for face extractor, which one can bring the best performance.
What is your idea about it? Do you have any experiments with image enhancement methods for face recognition? Which one brought the best performance?

Thanks,
Can