poi/ power adjustments

[eml9]

I have had a few scopes where power setting did affect poi and some scopes that did not ? Anyone no why ?

[7mmfreak]

Were they SFP?

[Jon]

SFP and FFP affect scopes differently.

[eml9]

Yes they were ... Did some research on it have a better understanding now ... So a ffp scope should never be affected by different power settings unless something is wrong with the scope ..correct?

[rossman40]

What you run into is with a variable power scope the two lens sets in the erector tube move to change the magnification.



On a SFP scope, since the lenses are in front of the reticle, any variance in the focal plane of a individual lens or (a term used by scope makers) columnation of lenses, will shift the image in relation to the reticle. On a FFP reticle scope the lenses that move are behind the reticle so there is no shift.

In the old days that is why target scopes were always fixed magnification. They didn't even use  turrets, external adjustment, so no erector,

 Manufacturing has improved and the internals of a high end scope looks like this,



To check a scope one smith I knew had this huge vise mounted on what he said was a forging anvil, basicly a chunk of steel that weighed over 2000lb. He had a mount on  another chunk that he would clamp in the vise and put the scope on it and then set-up a grid target out side the shop. then he would run the scope thru the mag range and watch for shift.

Sometimes recoil will make a lens shift just the slightest. This has been called reticle "float". When the Army was spec'ing scopes, even though they were fixed power, they were spec'ed with FFP reticles to cut down on the chance of float. But you have to look at that when you look at the overall probability of error or dispersion (group size).

[eml9]

Thanks for the explanation rossman .. I'm getting it now ..

Eric