Thanks for the replies. Interesting. The "diopter" at the back of the scope is very hard to turn on mine... I understand that part. Glad to hear that someone else thought the 100, 200, 300, etc markings were not correct.
That’s by design. You set the diopter ONCE to focus the reticle clearly with YOUR vision. Only if someone else uses the scope would the diopter adjustment need to change settings. And as Fireman JG says, do this looking at a plain background such as a wall or the sky. And don’t stare too long. Look briefly and adjust diopter. Repeat as necessary to get clear reticle focus.
The parallax setting as mentioned, should be dialed to make the sight picture clear (nothing to do with the reticle). That will change as you view things at different distances. On most scopes without this adjustment, it’s fixed at 100 yards since that’s a common zero range.
To see what parallax does, using your dominant eye, hold your index finger on both hands up in front of you spaced apart but aligned side to side exactly behind one another. Now shift your head slightly and you’ll see a separation of the two fingers. Now, bring the back finger right up behind the front one and do the same thing. You’ll see that both fingers seem to stay in alignment. That’s because they are both in the same distance plane and moving your head a little bit doesn’t introduce the parallax error you saw in the first part.
To summarize, the parallax adjustment brings the sight picture to the same optical plane as the reticle so that slightly shifting your head off center doesn’t affect what you perceive as POA.
With a fixed parallax scope, you have to make sure you’re looking directly through the center of the eyepiece at various distances, otherwise you will attempt to place reticle on target by moving rifle and your POI will be off.
And yeah, the yardage markings are notoriously inaccurate on parallax knobs.