Rifle Scopes Point of Aim shift when changing power on Scope

[frankythefly]

I was looking at various scope specs today, and one was a little strange. "Point of aim shift during power change (moa): 0.25"

I am not going to say where this came from as I am not trying to bash a company, I rather want someone to explain what it means?

 

[bcrich]

Re: Point of Aim shift when changing power on Scope

It wont change point of impact if you go with a first focal plane scope.

 

[KeithR41]

Re: Point of Aim shift when changing power on Scope

Or a quality SFP

 

[dcjs]

Re: Point of Aim shift when changing power on Scope

Actually that is the tightest outspoken tolerance for POI change in a SFP scope that I have seen, so don't worry about "bashing" the company.

It probably means that the manufacturer guarantees a POI shift of no more than 0.25" MOA throughout the power range. Remember that POI shift <span style="font-weight: bold">will</span> happen in SFP scopes until someone has figured out how to eliminate mechnical tolerances altogether, it's just a matter of whether the amount is acceptable for the intended purpose.

 

[sandwarrior]

Re: Point of Aim shift when changing power on Scope

frankthefly,

What happens when the power is increased is a lens within the scope is moved forward or backward. Just like if you took a magnifying glass and looked at something and as you back up a little bit the object becomes bigger. Eventually the magnifying glass loses focus. But, if you put that perspective in a scope you then have another adjustment to bring it back into focus.

The point of aim/impact changing is a product of the magnifying lens being non-concentric with the front objective and rear ocular. In manufacturing we try to get everything as perfect as we can. But, that isn't possible. So what happens is we go with the tightest tolerances possible or feasible according to financial constraints. In all honesty, a .25 MOA POI change is being honest and isn't that much at all. It's 1/4" @ 100 yds. and 2.5" @ 1000 yds. That is <span style="text-decoration: underline">'generally'</span> tighter than most people and rifles can shoot. So the difference isn't noticed so much. It's even tighter than most tactical shooting tolerances.

it doesn't matter if you have an FFP or a 2FP. Even though you are looking at the same crosshairs the motion is in the power adjustment and focus ring. And due to the tolerances, no matter how slight, there is going to be movement. No scope manufacturer really wants to advertise this as people would tend to say, "Why did you build me a crappy scope?" Well, they didn't. They built you the best scope their resources allow. Obviously, when you pay more money to the better scope builders that tolerance is decreased.

 

[dcjs]

Re: Point of Aim shift when changing power on Scope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sandwarrior</div><div class="ubbcode-body">...
it doesn't matter if you have an FFP or a 2FP.
...</div></div>
It does matter, because in a FFP scope the tolerances of the moving lenses in the erector system don't have any influence on POI. The same tolerances in a FFP scope will cause the target image <span style="font-style: italic">with</span> the reticle on top of it to shift slightly within the FOV, something that is very hard to see unless it is excessive and has no effect on POI whatsoever. In a SFP scope, the shift is between target image and reticle, thus the shift in POI.

 

[sandwarrior]

Re: Point of Aim shift when changing power on Scope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: David S.</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sandwarrior</div><div class="ubbcode-body">...
it doesn't matter if you have an FFP or a 2FP.
...</div></div>
It does matter, because in a FFP scope the tolerances of the moving lenses in the erector system don't have any influence on POI. The same tolerances in a FFP scope will cause the target image <span style="font-style: italic">with</span> the reticle on top of it to shift slightly within the FOV, something that is very hard to see unless it is excessive and has no effect on POI whatsoever. In a SFP scope, the shift is between target image and reticle, thus the shift in POI.</div></div>

I go with what you say. As you appear to have more expertise in the field.

My own experience differs though. Although as stated, not enough to matter to most.

 

[frankythefly]

Re: Point of Aim shift when changing power on Scope

This is a FFP scope, thanks for the information. But it sounds like its really nothing to worry about. Perhaps just a manufacture guideline. I just have never seen that before in a scope specs, so I had to ask.

 

[fostercd]

Re: Point of Aim shift when changing power on Scope

good explanation by sandwarrior and as stated, a lot of folks won't even notice.

 

[Munimula]

Re: Point of Aim shift when changing power on Scope

For those of you interested this can be illustrated by locking down the rifle in a vise or shooting rest and putting a bore laser in it. Then moving the scope through its magnification range.


As pointed out it is .25 MOA so you need a long distance or an accurate way to measure. 100 feet across the shop measured with .001 calipers should show what your looking for. difference between the laser if its not to large a dot and the crosshair location.

 

[sandwarrior]

Re: Point of Aim shift when changing power on Scope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Munimula</div><div class="ubbcode-body">For those of you interested this can be illustrated by locking down the rifle in a vise or shooting rest and putting a bore laser in it. Then moving the scope through its magnification range.


As pointed out it is .25 MOA so you need a long distance or an accurate way to measure. 100 feet across the shop measured with .001 calipers should show what your looking for. difference between the laser if its not to large a dot and the crosshair location.
</div></div>

How I found this out was when shooting. I would start and zero the shots on the center, after dialing up the power the group center moved to the left of the centerline. I dial back down and the group center goes back to the center of the line. The size of the group (.4 MOA group) was about 4X as big as the movement I noticed. But it definitely shifted and shifted back with power change.

 

[lsm62]

Re: Point of Aim shift when changing power on Scope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: David S.</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sandwarrior</div><div class="ubbcode-body">...
it doesn't matter if you have an FFP or a 2FP.
...</div></div>
It does matter, because in a FFP scope the tolerances of the moving lenses in the erector system don't have any influence on POI. The same tolerances in a FFP scope will cause the target image <span style="font-style: italic">with</span> the reticle on top of it to shift slightly within the FOV, something that is very hard to see unless it is excessive and has no effect on POI whatsoever. In a SFP scope, the shift is between target image and reticle, thus the shift in POI. </div></div>

i have noticed this in my ffp but as stated the point of impact did not change. it dose cause a little worry and hesitation though. when i first noticed it i sent the scope in and a new one was sent out. when the new one did the same thing i took it for a range test. zero holds fine and if your careful you can see the whole image move with the cross heirs. the movement is very slight and at first it seemed like i was moving the gun when changing power but after stabilizing the rifle you could tell it was the optic.