You will never be able to foucus properly on both the front sight ring and the target. If you are still quite young (in your early 20's usually) you may have enough accmodation so that it apperas that both are in focus, but this can lead to eye strain after a while. You should be aiming (no pun intended) to keep the front sight ring in sharp focus, allowing the aiming mark to blur slightly (or a lot as you get older). That is why it is important for you to speak to an optician who knows about target shooting. Either that or take your rifle sights with you when you go to see your optician and discuss your needs. You really need a lens that will enable you to comfortably focus at about 6 feet. The other thing that is important is to tell your optician that it is very important for you to have even very minor Astigmatism corrected in your shooting lens.
I have always suffered from an Astigmatism inmy aiming eye, the problem being that it becomes worse as I use the eye for aiming. At the start of a shoot the target appears round by the end of about 30 shots the target (and fore sight ring) starts to look like an apostophe! Very difficult to get properly corrected.

Alan