That’s my question. 
Peace,
mangeorge
Doesn’t the telescope project the cross hairs at an infinite distance? So if the optical axis of the scope and the rifle bore are aligned, i.e. boresighted when the cross hairs are on the target the rifle bore is properly aimed.
I’m certain a better reply will come along soon enough but I think that the rear sight estblishes parallelness with the barrel. The scope is bolted to the rifle (and thus the barrel) and then ‘tuned’ in such a way that it will only work when viewed straight down the barrel.
Your eye does need to be aligned to the optical axis of the scope but this is pretty much self correcting as moving it very far laterally will place your pupil outside the scope’s exit pupil so that you don’t see anything. In most shooting with moderate power scopes the error is something you can live with. You can test for parallax error by placing a scope in a fixed rest when aimed at a target. Moving your eye laterally can cause the crosshairs to appear to move against the target if the scope is not adjusted for that specific range. High magnification target scopes will have an adjustable objective lens so that parallax can be adjusted for the specific range of the target.
Zero magnication scopes aren’t supposed to have parallax at any range but my BSA red dot does. It looks like a short fat scope but instead of crosshairs it has an internally reflected red dot from an LED that works kind of like the HUD in a fighter plane. The shooter sees a red dot over the target image but nothing is projected to the target. The newer desigs do away with the tube and multiple lenses, just having a single piece of flat glass and I think these may totally eliminate parallax error.
While the above answers are correct, I think it might be more helpfull to look at the question slightly differently.
A rifle scope does have a rear sight: the rear aperture of the scope. Similarly, the front aperture is the front sight on most modern scopes. The magnification of the scope serves to make its effective length (the effective distance between sights) much longer – usually much longer than the actual barrel.
Though most people think of the rear “v-notch” and front “pin” (or ramp) when they think of barrel mounted “iron” sights I’ve seen “ring” or “hoop” sights, with various small embellishments on the right (e.g. small tabs marking the vertical and horizontal “middle” of the ring to allow rough adjustments for drop or windage). These were not terribly uncommon in the late 19th/early 20th century, and were widely used on heavier guns (e.g. anti-aircraft guns) in my lifetime [e.g. Jane Fonda’s Vietnam footage). Think of the circular rear aperture or eyepiece as a “rear ring sight”, and the front aperture as a front ring sight. The Tube simply occludes everything that lies outside the front ring sight, making sighting easier.
You may have seen old scopes with a reticle on the eyepiece and another on the front aperture (I’m sure at least som inexpensive modern scopes still do this, brcause it is cheap and has certain advantages). Again, the rear lens ring or reticle is no different in function than a rear iron sight in the same design.
Since I’m already here and on a roll let me pick that nit for you. :rolleyes:
Rarely are sights aligned perfectly parallel with the barrel. Aside from recoil which can have some odd and non-intuitive effects in handguns a bullet travels in a more or less parabolic path. This illustration is grossly exxagerated but shows the bullet path from a rifle. The M-16 for example has sights that are unusually high above the centerline of the barrel so this form of parallax is is taken into account when sighting in. The sights can be zeroed at short range which corresponds to the first intersection on the illustration, the rear sight has a special setting of this distance and zero is set with the front sight. Once this is done the rear sight settings for various distances on the rear sight drum will be correct with standard ammunition.
Suppose the scope is boresighted so that its optical axis intersects the trajectory of the bullet at 200 yards in calm air.
When you look through the scope you are always looking along the optical axis no matter where your eye is relative to the scope, if you can see though it at all, because of the way the scope is designed.
Ergo, no rear sight is needed to keep the sightline in proper alignment with the trajectory.
I may not have been clear but this is not the case. It isn’t untrue that you are looking down the scope’s axis but you can certainly look through one slightly off axis which can give you a misleading picture of where the scope is aimed.
For starters the exit pupil is always larger than your eye’s pupil so there is some margin for eye position. The 36x target scope I often use is more difficult to line p than a typical 3-9x but there is margin. A scope with no pupil margin would be virtually impossible to use.
When you do have your eye slightly off axis the apparent image of the reticle inside the scope can move relative to the target image. An adjustable objective lens, the front optic group, allows the target and reticle to be ajusted to the same apparent distance. When this is correct you can shift your eye position slightly and the reticle will not move relative to the target.
I’m not saying it;s a big error but it does exist. That’s why target scopes have adjustable objective lenses and hunting scopes typically do not.
All that is moot anyway since you can’t aim down the optical axis of a maodern scope. Modern scopes are mounted usually parallel to the barrel but you could never hit a target aiming that way. The reticle is shifted inside so the aiming point changes. Old fashioned scopes were aimed by moving the entire thing in adjustable mounts and they are aimed by the optical axis.
It was my red dot sight that prompted this questin. I noticed that if I changed the angle at which I looked through it, the dot didn’t move much at all on the target (my doorknob ;)). Then I tried to repeat the experiment with my binoculars, but locating the recticle, a spot on the lens, turned out to be problematic.
I’ll try some more tomorrow at the range with my red dot sight.
Padeye, you were right. This is fun, but the old eyes ain’t as before 
Peace,
mangeorge
What kind of red dot are you using mangeorge?
It’s a BSA RD30. I’m taking it to the range today for the !st time. I have trouble seeing the front sight against the target, so I thought I’d give this a try. It was only 30 bucks @ Walmart. Visibility is definitely better, we’ll see how it does in action.
Later I may go for a low power (2-3X?) scope for longer shots. For that I’ll be a little more careful in my selection.
You get what you pay for!
The new scope Leopold sells, Rifleman is just such a scope. one would have to put a rear sight on it to shoot it well(that could be done with a hylighter pen and putting a small dot in the center of the rear lense) . You want a Leopold buy the V-III
In theory this shouldn’t happen. The objective focuses the target to an image plane, and the reticle should be at the same plane. No matter how you look at it, the relative position between reticle and target shouldn’t change.
If it does change in reality, it’s because the scope isn’t perfectly focused. (I’m not saying you’re using it improperly, I’m just saying it’s a practical limitation and not a theoretical one.)
As for red-dot sights, they create a broad parallel beam of red light and superimposes it with the target. Since the beam is parallel, moving the eye doesn’t change the direction it comes from. The light always appears to come from infinity. I don’t know about gun sights but the red-dot finder on my telescope uses a single curved piece of glass to do everything. The glass is curved but has no power. Its concave surface collimates the light from a red LED (i.e. takes a divergent light from a point source and makes it parallel), and it’s transparent so you can see the target through it.
By george I think he’s got it.
You are correct sir. The reticle needs to be focused to the same plane as the target to eliminate apparent shift. One question: how does the factory know how far away my target will be? It isn’t a matter of quality. The Austrian craftsman putting together a Swarovski scope have have no better idea than the laborer in a Chinese factory. Don’t take my word for it, this is easy to demonstrate with a rifle scope placed in a fixed rest.
Note there are two places to focus a scope. focusing the ocular is so the shooter sees a clear image and is useally tweaked a bit for near or far sighted shooters. The objective is where parallax is adjusted.
Some shooters such as hunters can live with a small error so they do fine with a scope adjusted for a midrange distance. Some actually have non-adjustable scopes set to a specific distance by a repair service. Those who demand greater accuracy buy scopes with an adjustable objective lens.
gbrohman, I haven’t uses a Leupold Rifleman but you won’t fix anything with dot on the lens since your eye can’t focus there to make an image. What range were you using the scope at? Most non-adjustable are set for around 100 yards so you may see big parallax errors when shooting in closer.
Just got back from the range. Sure 'nuff, the bullet goes where the red dot lies, no matter the angle. If you change any angle much, the dot disappears. But if you keep it in sight the dot remains on the bulls-eye.
I was shooting from about 35ft, using a rest.
BTW; the red dot sight is pretty cool, IMO.
Yet another plea for more precision when asking GQ questions… 'cause you could have been asking about a proctoscope, which offers nothing butt (heh heh) rear sights. Unfortunately I was unable to find a link to a picture that would meet board rules.
And for a target a long way off, say 50 yards or more, the light rays to the scope are virtually parallel both target and reticle are focused at an infinite distance and you can move around all you want and they won’t change relative positions as long as you can see through the scope at all.
So which do you use to adjust for different target distance? The objective, right? If you focus the ocular so that the reticle is in perfect focus, then adjust the objective so that the target is in perfect focus, then the image plane and reticle should coincide. I can understand if this is difficult in practice, though. The human eye has a built-in autofocus mechanism that compensates for a small amount of focus error.
By the way, if this offset is so crucial, why don’t they put a frosted glass focus screen with a reticle printed on it? This should eliminate the offset error completely.
“Effective infinity” can vary a lot. Fifty yards is pretty much the same as infinity focus for our eyeballs which have very low magnification - that is the ratio of the object size to the apparent size on the retina - and small relative apertures. It’s a different matter at high magnification where a few hundred yards is not the same as infinity.
I think you’re absolutely right. The eye corrects so automatically and quickly that the reticle and target can both look perfectly sharp while there is still parallax error. I generally setup a scope by adjusting the ocular so I can see the reticle clearly as I am slightly nearsighted. I then put the rifle in a rest and adjust the objective so there is no parallax shift when I move my eye. Ideally this should match the markings on the objective bell but often it doesn’t.
That certainly could be done. You’d pretty much have an SLR camera wihtout the mirror and it’s basically a mechanical version of how some night vision scopes work. Unfortunately it solves one problem while creating another as groundglass screens scatter and waste a lot of light. At high magnification the objective lens diameter of most scopes works out to a very high f number/small effective aperture. The image would be too dark to see.
It’s not equally critical for every kind of shooting that’s why no one has seen a need to find different ways to work around it. Someone shooting a deer at 130 yards can easily live with a minor parallax error as it won’t keep him from making a clean kill. A varminter trying to hit a prairie dog at 300 yards or a .22 benchrest shooter aiming at a target the size of an asprin at 50 meters has to be more precise.