There is no equivalent focal length to the human eye.

It depends on what you are doing with the eye. For example, when you are focusing on a single thing, like a person talking to you, then a picture with a 50 mm (135) lens generally looks sorta similar in terms of proportions and perceived perspective. But if you are taking in the scenery, a 50 mm lens would seem to be too narrow and you need a moderate wide angle to convey a similar feel. Similarly, intently focusing on a distant object will make a 50 mm seem to broad, and a 105 mm will seem more like it.