Magnification vs parallax

[Don357]

Now before I get started please don't flame me for owning an inexpensive optic. I know I should have invested more money.

A while back I bought a scope for my AR. I forget the name but it is a 2x6x28 compact (tactical) scope. When I was bore sighting it using a muzzle type laser, I noticed that when I sighted it in on the laser dot at a certain magnification and then changed magnification level, the point of aim would move rather drastically. I learned to compensate by zeroing at a certain magnification and just leaving it there, and it holds zero quite well. BTW the ocular focus had no effect on zero. I had read where the ocular focus could, on some optics, effect the focal plane adjustment which is why I mentioned this.
I would appreciate any constructive comments on this situation.

 

[DustyDog]

That may be related to the distance at which you are zeroing on the laser. If your scope doesn't have a parallax adjustment, it has a set distance at which it should be parallax-free. Also, taking into account your scope's height above bore, the zero may change quickly with magnification changes if you are sighting on the laser too closely. Eyeglasses can also complicate things if you wear them. Best to draw final conclusions from actual test-firing at distance. Note that I also try to work with a set magnification on my "cheap optics" AND disturb the turrets only when absolutely necessary.

Just rank speculation off the top of my head... don't take my word for any of this : )

 

[Bowhntr6pt]

No expert here... What you're seeing isn't related to parallax. If simply dialing magnification moves the POA, that's bad internals.

If your reticle is moving when magnification changes, that's not parallax, that's cheap internals doing what cheap internals do.

Bag your scoped rifle at 100 yards, place the crosshairs on the target, now, WOTHOUT touching the rifle, look through the scope and move your head around slowing and in small increments, if the reticle dances around, that's parallax being exhibited. Most non-adjustable scopes are set at about 100 yards.

Once an adjustable scope is set for no Parallax at 100, move to 200, 300, and repeat before shooting as variations in your cheek weld can and usually will result in differing POI's per each shot, Parallax adjustable scopes allow for near-Parallax free performance at various distances.

Parallax exists when the reticle cell isn't perfectly aligned at the focal plane. If the focal point is in front or behind the reticle cell there will be Parallax to some degree. Dialing a Parallax adjustment moves the reticle cell forward/rearward in order it to be at the focal plane.

 

[Jhunter]

I think i read your post correctly and it sounds like your scope is broken. Your reticle should not be moving drastically throughout the magnification range. @Bowhntr6pt makes some great points. Blocking up the rifle with sand bags takes the human element out. When looking at a 1” dot with my scoped rifle at 100 yds I am able to get the reticle to move from one side to the other by moving my head slightly from side to side inside of the scope box. This is maybe measures .25” at 100.

 

[DustyDog]

Your reticle should not be moving drastically throughout the magnification range.

I'm just curious what happens when you zero on a spot that's 10 feet away, then start cranking your magnification up and down... never tried it. And I REALLY want to set up a rock-solid concrete bench that I can mount scopes DIRECTLY to for testing reticle transit, etc. at 100 yds. or so. Anything less... including a rifle on bags, in a sled, etc.... and you're still likely to get movement when manipulating the scope : )

 

[Jhunter]

I'm just curious what happens when you zero on a spot that's 10 feet away, then start cranking your magnification up and down... never tried it. And I REALLY want to set up a rock-solid concrete bench that I can mount scopes DIRECTLY to for testing reticle transit, etc. at 100 yds. or so. Anything less... including a rifle on bags, in a sled, etc.... and you're still likely to get movement when manipulating the scope : )

I do shoot at 7 yards when setting up all my long range rigs. This gives me a very precise mechanical offset. I work off a 100 yard zero. In mils, I measure my point of impact vs point of aim at 7 yards. I enter all of these factors in my kestrel and adjust mechanical offset height over bore until everything lines up. I usually get close with a set of calipers but never perfect with just the calipers. I have seen a setup at Nightforce similiar to this. They have it set up on a tall target to test tracking. It’s mounted to a solid surface https://www.badgerordnance.com/dead-level-reticle-leveling-device-200-50.html this is also what I use to level scopes inside rings. I usually hang a string with a plumb bob and line up my vertical stadia line with the string. A google search for a “tall target test” will point you in the right direction for scope test.

 

[DustyDog]

Bingo!

That Badger piece looks great for leveling, though for reticle transit, parallax vs. focus checking, etc., I'd want something bolted to an absolutely rigid bench. I haven't gone that far yet, even though my "cheap scopes" need to be tested, and any high-dollar ones certainly should be.

I'm pretty proud of my "prone bench", though (pic below). The main bench is 19(?) years old, and stays out in the weather. I have a shorter matching bench that stacks on top, and its legs locate into the square cutouts above the main bench legs. The short 4 x 4 on the side allows my compound miter saw to stick out past the edge, making it easier to get clamps on the lumber. I also have some support extensions that engage under the saw to support longer lumber when necessary.

And then, I recently added the "prone" extension, which makes it about 9-1/2' long. The extension locates itself, via the 4 x 4, a small block on the other side, and two blocks under the end of the main bench by just sliding it into place, and is then retained by four pins : )

Just came in from testing my .223 Savage "Sidewinder" on the bench (pic below), with its offset scope. It won't fit in the case with the scope mounted, so I used Lever Lock rings, but it seems to return to dead-nuts, no problem. It is so much more comfortable to shoot than a centered scope. Pretty sure the biggest hold-up with the rifle is the crappy factory stock. The idea is to achieve offset convergence @ 300 yds., and it was on track to do that today, with 1/3 convergence @ 100 yds. with two extremes of ammo (WPA steel-case 55 gr., and Hornady 75 gr. BTHP). (scope is 1-3/8" to the left of bore centerline)

And that with no touching of those "cheap" Barska turrets necessary. OK, it's actually their Point Black .223 (AC11392), which is hard to beat for the money. 1/8 MOA per click (supposedly : ), 3-color reticle illumination, side parallax, and I like the 3G reticle! (also pic below)

 

[Jhunter]

I would definitely look at a stock that fits. The offset scope is problematic. With the scope being the x and the muzzle being the y the two work best if they are in line vertically. If my bipod is canted the target reflects to the degree.

 

[DustyDog]

Yep... I've considered a Boyds At-One stock... and I would go with the one cut for the bull barrel, which would give me the option to switch to a heavy barrel down the road and still be able to use the stock. With the factory stock, you can put a pinky on top of the butt and easily rock the barrel into contact with the channel. BUT, maybe I should just bail to a better rifle anyway. (mine is an older one that doesn't even have the Accu-Trigger, though it's not bad as-is)

As for the offset, I can deal with groups centered 1 inch to the right @ 100 yds, and 1-1/2" to the left @ 600 yds. in a $200 (+/-) rifle : )

EDIT: AHA! And don't forget the extra clearance for bolt-throw AND the fact that the scope's height-above-bore can be reduced, thanks to the offset. Plus, my head is lower and perfectly vertical, and my cheekbone falls into a natural position on the stock, whereas it didn't with the scope mounted on-center : )

 

[DustyDog]

I wonder what happened to poor old Don...