There have been a few threads about scopes. One thing I'm a bit fuzzy on is the term 'paralax error'. I know from analog meters, it is the issue of not having the needle line up with the scale and giving a false reading, but how does it apply to scopes? How is it that some scopes are set for less than 100 yards, and some at 100-300?
Want to end up with a scope that'll be happy at 25-75 yards.
imagine looking at that same analog meter, from 3 ft away, and at a 45 degree angle to the left, which is where you are standing, from it. How accurate do you think you would be able to say that the needle is pointing at what number?
Not so much. Prrlax can be seen in a scope the same way, if you are looking through it, right behind it, the crosshairs are pointing dead on, on your target.
If you move your head slightly up/down/left/right, do the crosshairs move off of exactly where you wanted it? If so , this is prrlax. mostly you can adjust this out, with the AO bell, up front, to get very good focus. Also , even if this helps , it may not be enough, especially for close up shooting, under 100 yds.
Most centerfire scopes do not become prllax free until a distance of 100 or even 150 yds. Most airgun rated scopes are prllax free at 15 yds, some even less than that. So You want to do your best , to find out what distance you want a scope to be prllax free, most centerfires are fine at 100 yds, unless you are doing competition target shooting, at that distance or under.
When you adjust the "focus" at the back of the scope you are adjusting it for the cross hairs (if you follow the direction packaged with the scope). So those cross hairs are in a focal plane that is fixed. The way lenses work, distant images form inside the scope at different distances from the front surface of the front lens element dependent on how far they are in front of the scope. Closer objects form images farther from images from farther away objects. So those object images only fall on the plane of the cross hairs for one distance. All other distance objects form in front or behind that cross hair plane. When you look through a scope at an image at that "focus" distance and if you move you head from side to side then the cross hairs WILL NOT move on the target. When you look through a scope at an image at some distance other than the "focus" distance and if you move you head from side to side the cross hairs WILL move on the target. The latter case is a paralax case.
To have the best of all worlds get a scope with an adjustable objective to get the target image to focus on the cross hairs. Most powerful scopes and some rimfire target scopes come with that feature. Few below a 3 to 9x scope have adjustable objectives as the effect of paralax is less pronounced with lower power scopes.
Scopes made for rim fire usage are pre-focused at 50 yds usually whereas scopes for centerfire hunting guns are usually pre-focused at about 150 yds, where most hunters get their kills.
Last edited by LDBennett; 02-25-2008 at 03:49 PM..
Hmmm..very interesting. I have always used the rear ocular adjustment to focus the hairs and target to match the requirements of my eyes. I wear glasses, but on my scopes, they (rear ocular) is adjusted so that I don't have to wear glasses when I shoot.
I have also "thought," and I use that term cautiously, that scopes (fixed power) that were made for 22 rimfires were parallax free at 50 yds, and those that were fixed power for centerfire rifles were parallax free at 100 yds.
Any adjustable power scope was parallax free to 100 yds, say a 3x9x40 without an adjustable front ocular. Those that have the capability to adjust out the parallax were/are adjustable from as close as 50 ft out to infinity. Funny thing is, I can't shoot to infinity. LOL
I think if drag out the instruction that came with any of your scopes you'll see that they tell you to point the scope at the sky and focus the crosshairs. They tell you that so you won't focus on the target image rather than the cross hairs.
Manufactures of scopes have no industry guidelines to follow when it comes to where the target image is focused. The number I suggested are generalities and not all maufacturers use them. But in genrtal rime fire scopes are focused under 100 yds because they are rarely used beyound 50 yds and hunting scope are typically image focused somewhere betwen 100 and 200 yds. I prefer high powered scopes and most of them have adjustable Objectives and I like it that way!