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understanding parallax when chossing scope zoom

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understanding parallax when chossing scope zoom

Postby dave on Thu Jan 11, 2007 1:15 pm

If I buy a 4.5-14x50 rifle scope with adjustable parallax, and set the power to 9x and the parallax to 100 yards, does the scope perform exactly as a 3-9x50 scope with a non adjustable parallax set at 100 yards (or is this over simplified?) Should the point of impact be the same for the two scopes at 25, 50, 100, and 200 yards? How much will the point of impact differ at each of these yards? If I set the scope to 14x and leave the parallax at 100 yards, what will be the difference in point of impact at 25, 50, 100, and 200 yards?
dave
 

Postby opticsplanet.com on Thu Jan 11, 2007 1:35 pm

Hi

You're asking several questions at once.

A scope that is parallax focused at 100 yards, will be free of parallax at 100 yards, regardless of the magnification. Even on a scope that is not parallax set for 100 yards, there is no parallax error if your eye is perfectly centered in the eyepiece. All a parallax adjustment does is eliminate the possibility of parallax. On an adjustable objective, though, the markings on the AO should not be taken too literally. You will need to focus and visually check for parallax, regardless of what the AO says. Use the markings as a ballpark gudeline.

Point of impact changes invovle much more than simply parallax. Other factors such as mount rigidity, barrel bedding, barrel heating, scope quality and condition, ammunition consistency and so on can also contribute to point of aim shifts and anyone of these can potentially be a far more serious issue than parallax.

Your choice of an AO model should reflect your application. The amount of error due to parallax even out to 500 yards is rather small. I don't consider it an issue for big game hunting. An AO is a feature that is more useful for varmint and target shooting where even the tiniest error can result in a miss.
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Postby dave on Thu Jan 11, 2007 2:13 pm

Thanks for your very quick response and very useful information, but I need to understand this a little better before I decide what scope to order. Maybe it will help if I simplify my question. If I by a Zeiss 14x and set it to 9x and determine it is parallax free at 100 yards (ignoring all over point of impact variables you mentioned), do I essentially have a 3x9 scope with fixed parallax free at 100 yards – except my 14x would be more versatile? I understand that parallax error out to 500 yards is small, I am concerned with short range shots. If its set for parallax free at 100 yards, how far could my shot vary at 25 or 50 yards for both 9x and 14x? Thanks again for any additional information you can provide.
dave
 

parallax adjustments in general

Postby MrGman on Thu Jan 11, 2007 2:28 pm

If you buy the scope with the adjustable objective, you will not be able to just leave it set for 100 yards and use it down at ranges of 25 yards at 9X magnification simply because the target will be out of focus. In this respect it is not like a simply 3X9 scope without the AO feature.

If you have a 4.5X to 14X and you set it to 4.5X chances are that you would not see enough of an out of focus condition between 25 to 100 yards to actually have to make a focus adjustment, but at 9X you would.

The error in point of aim would grow as well down at 25 yards if you are set to 100 yards if and only if your eyeball is not centered down the tube. However you would notice that the target of interest in and around the 25 yard range was out of focus and somewhat blurry and you would be going for the focus ring to clear up the image anyway, especially at the higher magnification range.

So if you are thinking you want to use the higher power scope for hunting and targets may be appearing from 100 yards down to 25 yards, A: you wouldn't (shouldn't) be set to 9X, but if you were, you would have a hard time using the scope and B: To answer your direct question the parallax error of a 4.5X14 set to 9X and focused at 100 yards should have no greater than a 3X9 scope at 9X if you were using them both side by side at only 25 yards (assuming the 3X9) scope is the type that is parallax corrected for 100 yards and not one for 22 rifles or shotguns set to a closer range).

the advantage of the AO scope is that you can adjust the focus and thereby null out the parallax at 25 yards if needed to shoot at very small targets to obtain maximum accuracy from the scope/rifle combo.
MrGman
 

last question!!!

Postby dave on Thu Jan 11, 2007 2:57 pm

Mrgman, thanks a lot for your help. Since you are very knowledgeable on the subject and I am a very novice shooter, maybe you can help. If you were purchasing 1 scope for the next 25 years for a 30-06 and 79% of the time the scope would be used for deer hunting from a stand (between 60 and 120 yards) , 20% of the time for target shooting at 100 yards, and 1% of the time for shooting at varmints or big game at over 250 yards, would you buy a 3.5x10 fixed parallax or 4x14 AO? Obviously, I like to zoom as close as possible on things I shoot. Should I keep it simple and get the 100 fixed parallax, or will I regret not being able to adjust it?
dave
 

Postby opticsplanet.com on Thu Jan 11, 2007 3:04 pm

You are very welcome.

Yes, parallax free is parallax free, regardless of he scope or the magnification. Yes, the 14x will give you a bit more versatility, but whether you need it or not is still a matter of the ranges and the targets you expect to shoot. If it makes you more comfortable having the 14x available and you can afford it, go for it, but I don't think you need it and, in fact, having that AO may backfire on you. If you accidently have your AO set at infinity or forget to change it after setting it for a long shot, then suddenly find yourself shooting a deer at 50 yards, you have introduced far more parallax than the 3-9 parallax set at 100 yards. Besides, if you have a deer at 25 or 50 yards, are you going to have time to mess with an AO? Do you think you can adjust an AO at that range without being seen? Just not a practical option for deer hunting in my book.

I can't give you an exact number for parallax error when shooting a 100 yard parallax focused scope at 25 yards, but parallax is more of a problem at shorter ranges, yes. Shooters using AOs at these ranges, however, are shooting targets or small game. Still don't think you need for average deer hunting.
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deer and coyote hunting scope

Postby MrGman on Thu Jan 11, 2007 8:42 pm

Yes your welcome

The question "If you were purchasing 1 scope for the next 25 years for a 30-06 and 79% of the time the scope would be used for deer hunting from a stand (between 60 and 120 yards) , 20% of the time for target shooting at 100 yards, and 1% of the time for shooting at varmints or big game at over 250 yards, would you buy a 3.5x10 fixed parallax or 4x14 AO? Obviously, I like to zoom as close as possible on things I shoot. Should I keep it simple and get the 100 fixed parallax, or will I regret not being able to adjust it?"

The Answer, go with the 3.5X10X40mm for the deer hunting scope. most of the time you will be at 3.5X waiting for a deer to come into your field of view. You have an 8 inch diameter window of opportunity to take the deer out and parallax is not the issue.

Learn how your rifle shoots at 250 yards at 10X by practicing at that range and you will not really need the more powerful scope. But you cannot deer hunt at 60 yards all that well with a 4.5X14 power with AO. It will get in your way. I think you were interested in the Zeiss Conquest yes. That will go a long way to help you see clearly further out.
MrGman