The SWFA SS 2.5-10x32 Ultralight Rifle Scope(s)

are offered in a rimfire version with fixed parallax focus at 50 yards and centerfire rifle version(s) with 150 yard settings.

With QD return to zero mounts I could swap one scope among my rimfire and air rifles for 25 to 100 yard shooting and my AR's for 50-200 yard targets.

Assuming I can keep my cheek weld/eye relief positions close but not perfect, which would be the better choice?

In other words, would I likely have more parallax error shooting the 150 yard scope at 50, or shooting the 50 yard scope at one hundred fifty yards?

It seems like a solvable math problem but by the time I figure it out for myself
a) I could have worked more overtime and bought both
b) They'll all be sold out
c) All of the above
d) Hookers


https://www.swfa.com/catalogse...y=&q=swfa+ultralight

You can find an intro to the math here:

http://rimfirebenchrest.com/articles/parallax.html

Fixed parallax at 50 yards, target at 150:

.5 * 32mm * abs(150-50)/50 = 32mm

So the worst possible parallax error shooting the 50 yard fixed parallax scope at 150 yards is about 32mm (a bit over an inch).

Fixed parallax at 150 yards, target at 50:

.5 * 32mm * abs(50-150)/150 = 10.6mm

So the worst possible parallax error shooting the 150 yard fixed parallax scope at 50 yards is about 10.6mm (a bit under a half inch).

Note that for these calculations, "worst possible parallax error" means with your eye off center to the point that if you move even a tiny bit more you can no longer see the crosshairs. No one actually shoots with their eye that far off center, so you wouldn't see actual parallax error that large.

Also note that parallax error is magnification sensitive. If you consider the parallax error of moving your eye 1mm off center, shooting at 10x the parallax error will be twice what it would be at 5x, and five times what it would be at 2x. That doesn't change the worst possible parallax error - but it does mean that the actual parallax errors you will see will increase with magnification.

For example, if you can keep your eye within 1mm of center, the parallax error you will see shooting the 50 yard fixed parallax scope at 150 yards at 10x power is:

1mm * 10 * (150-50)/50 = 20mm

At 2x power it is:

1mm * 2 * (150-50)/50 = 2mm

This is why you rarely see low magnification scopes with adjustable parallax - they just aren't that sensitive to parallax error. You can get just as much parallax error (assuming the objective is the same size), but you have to move your eye WAY farther off-center.

Thank you!