How large is parallax error?

[WayneConrad]

Is there a formula or rule-of-thumb to estimate the effect of parallax error?

For example: If my 3x9 scope is parallax free at 100 yards and I am shooting at 25 yards with the scope at 3 power, how much would the POI change if I moved my head so that the crosshairs were all the way to one side of the tube?

What I'm trying to figure out is what sort of scope to put on my CZ 453. I will be using it for hunting and for position shooting (but not competitively). No benchrest. I will be choosing between rimfire and centerfire scopes, and between fixed objective and adjustable objective. Since a fixed objective centerfire scope is probably set for 100 yards, I need how much error that could potentially cause at 25 and 50 yards.

 

[LAK]

Unless someone here has the answers you are looking for, I would call the folks at Burris or Leupold and ask them. Dial their 1 800 and ask to speak to someone who can give technical advice.

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[WayneConrad]

The fellow at Burris said, "Oh, a lot."

I found, in this here page, a formula:

max error = 0.5 * objective diameter * (range to target - range to focus) / range to focus

So, for example, if my scope were parallax free at 100 yards and I were shooting at 200 yards, max error would be 0.5 * 40mm * (200yd - 100yd) / 100yd, or 20mm.

I don't know if this formula gives correct answers if the target is closer than the scope's parallax-free range. If it does, then when shooting at 25 yards with a 100 yard scope and a 40mm objective, max error is 0.5 * 40mm * (25yd - 100yd) / 100yd, or -30mm. I guess I'd assume that you can ignore the minus sign in that answer.

 

[JohnBT]

I know from experience that parallax will really foul up a pleasant afternoon of paper punching. I guess my question is - Is that 20mm of total error or 20mm to the right AND the possibility of another 20mm to the left? There's a big difference between almost an inch and almost two inches.

John

 

[WayneConrad]

I believe that it's the maximum that the POI will change from moving your eye off center. So if I were to make a shot with my eye centered, a second with my eye moved left until the reticule center was barely visible, and a third with my eye moved right until the reticule center was barely visible, and keeping the cross hairs on the bullseye, my 3 shots would be spaced 20mm apart, or 40mm from end to end. Or so I think.