Adjusting scopes for closer focus/parallax?

[Nobade]

Yes, it's a term from photography. You could set the F stop on your lense to control the amount of light coming through and adjust the depth of field. A small F stop, or larger number, would mean you needed a longer exposure for the film but it would give you a long depth of field, so focus was not as critical. The opposite, a large hole, lets in more light so exposures could be quicker but field depth is shorter so you had to be more careful with focus. The same principle applies to a rifle scope. A small hole will allow a wide focus range but makes the image dimmer so you need bright light to use it. It also allows you to adjust the parallax for closer so your head position is not so critical as it would be normally.

[ulav8r]

parallax definition from Duckduckgo: In optics, an apparent shifting of the spider-lines in a telescope-reticle as the eye is moved before the eyepiece: it is due to the non-coincidence of the threads with the focal plane of the object-glass.

Focus has nothing to do with focus. Per the definition above and several articles read over the last 60 years, parallax causes the bullet impact to be offset from the position of the crosshairs on the target. Though I have not seen any documentation, the above definition suggests that parallax adjustment would move the actual or apparent location of the reticle in relation to the focal plane. Does moving the front lens move the focal plane? I don't know. The few scopes I have focus by moving the rear lens.

[gloob]

parallax definition from Duckduckgo: In optics, an apparent shifting of the spider-lines in a telescope-reticle as the eye is moved before the eyepiece: it is due to the non-coincidence of the threads with the focal plane of the object-glass.

Focus has nothing to do with focus. Per the definition above and several articles read over the last 60 years, parallax causes the bullet impact to be offset from the position of the crosshairs on the target. Though I have not seen any documentation, the above definition suggests that parallax adjustment would move the actual or apparent location of the reticle in relation to the focal plane. Does moving the front lens move the focal plane? I don't know. The few scopes I have focus by moving the rear lens.

Yes, you can adjust the focus of a nonAO SFP scope a bit, by screwing the rear lens in/out. You can go do this right now. Point your rifle at a target that is just a tad too close to focus properly. Screw the rear lens out, and the target will come into focus. But the crosshairs will get fuzzy. So the crosshairs and the target will not be in the same focal plane, no matter how you adjust the rear eyepiece. And if your eye moves off center, the crosshairs and the target will move relative to each other, even if the rifle doesn't move. Set your rifle on a bag, and look in the scope. You can make the crosshairs wander all over the place, just by moving your head a tiny bit. The difference in movement is what will cause the bullet to miss the mark. Adjusting the rear eyepiece can help if you want to shoot close up. But it's a hack, and you have to be extra careful to keep your eye and your crosshairs centered in the FOV to minimize the parallax error.

Now screw your eyepiece back in to where the crosshairs are back in perfect focus and set your rifle on a bag, now aimed at a target that is the same distance as the scope's parallax. For a rifle scope that has a fixed objective, this is usually 100-150 yards. Now when you look thru it, the crosshairs stay anchored on the same spot on your target, no matter how you move your eye. The crosshairs and target move a bit, but they move together.



On an AO scope, you only need to adjust the rear eyepiece if you have a vision problem. You adjust it so the crosshairs are clear, then you leave it alone. When you aim at a target that is close up, you adjust the focus with the objective. Either by turning the front bell, directly, or by turning a knob on the left side of the scope (which adjusts the objective), depending on the style of scope.

Getting the focus spot on might be difficult to do at only say 4x magnification, because there might be a pretty big range where things look as good as it gets. So it helps to zoom in to max magnification, then focus. But if you get it just right, you'll have both target and crosshairs in focus... hence they're both in the same focal plane. When you move your eye, the crosshairs and target both move, but they move exactly together. So it looks like the crosshairs don't move, anymore. And wherever you put the crosshairs on said target, that's where the bullet will go (assuming you dope it correctly for how it's sighted in).

Parallax has everything to do with focus. Getting both target and crosshairs onto the same focal plane makes parallax error go away, even when your eye isn't perfectly centered. Whereas the fixed objective scope has no parallax error at one particular distance, the adjustable objective scope can be made to behave like that at any distance (to a point). And it comes down to putting the target into focus.

As said before, determining perfect focus can be difficult. A serious accuracy shooter will set that rear eyepiece where it needs to be, then never move it. Then he will test for parallax error at various distances, and take note where it disappears as he adjusts the objective. Then he will add his own marks to the focusing bell/knob. (It will already have yardage markings from the factory, but they're never exactly right through the entire range). Now, if he has a laser range finder, he can determine the exact distance, and then set his objective (focus) more perfectly to more completely eliminate parallax error. Or if he doesn't have a laser range finder, he can zoom in to max magnification and focus as best he can. And by looking at his markings, he'll have a more accurate estimate of range, for doping.

I hope anyone with doubt about what parallax is could read this thread without getting bored and skipping over the important bits. It's long. I hope it's not too annoying and maybe clears some things up. This isn't about being right, it's about shooting better! I didn't know about parallax error when I was a kid, and explanations were too confusing for me to understand. And since I was shooting BB guns at 15 yards and closer 99% of the time, and only tried fixed objective scopes, I never trusted scopes and only used iron sights. My eyes aren't that good anymore to use irons. I eventually learned how to use scopes properly and hope this helps you and Popper.

[Rapier]

Never take apart a scope unless you have the tools and equipment to purge and fill the tube.
Suggest you find a close parallax adjustable new scope. Most times it will be a rimfire scope. Just forget the standard everyday hunting scopes.

If you have never seen one, the Russian P-1 or P-2 for the Mosin sniper rig is pretty special, they focus from 0-1,000 yards without tuching anything, very simple setup.

[popper]

parallax for closer so your head position is not so critical as it would be normally.
That is NOT parallax! That is object 'image' position. If you eye is not on the lens system centerline, you get mechanically vignetted.

[gloob]

parallax for closer so your head position is not so critical as it would be normally.
That is NOT parallax! That is object 'image' position. If you eye is not on the lens system centerline, you get mechanically vignetted.

For Popper, specifically. For others, the terminology might be strange:

If you're trying to hit an 8" kill zone at 10 yards, then sure. Your object image position error doesn't matter. If your expectations/hopes are to shoot a single hole group or a mouse at 10 yards (something that you typically might do with an airgun, rather than a powder burner), your object image position error will be huge when you're slightly mechanically vignetted, and you won't be able to tell how mechanically vignetted you are when shooting offhand or in low light. It is also much more difficult to get the crosshairs on the target AND the crosshairs centered AND to break your shot, all simultaneously, when you're not on sandbags. Especially impossible for a scoped pistol where you have no cheek weld. Especially difficult if you have a time constraint before your target takes off.

You'd be much better off using iron sights or even just a bead at this distance vs a fixed objective rifle scope. But iron sights don't work in low light.

So just switch over to a red dot, which we know doesn't have object image position error. But at this short a distance, make sure to compensate your hold over/under for "parallax error" of your red dot sight, based on how high it is mounted over the bore axis and the distance where it's been zeroed.

At ranges that are longer than the focal distance of a fixed scope, the amount of object image position error is much smaller in terms of MOA. But at longer ranges, a small error by MOA will mean big misses. So long range snipers/hunters are also invested in using AO scopes to minimize this error. Long range-only shooters could easily get the impression that parallax - oops, I mean optical image position error - is not related to focus. Because the difference in focus from 100 yards to infinity is very miniscule, and our own eyes will automatically adjust this much, to make a crisp image. Adjusting the objective this miniscule amount will not necessarily change the focus, as you experience it. If you shoot at shorter ranges, it will be obvious that focal distance and "object image position error-free" distance are the same thing.

I enjoy this terminology, Popper! Am I using it correctly?

Completely unsarcastically, I agree that by the definition of the word "parallax," it is 100% correct and appropriate to use it in the context of sight-to-bore axis. But in the world of scopes, most people use it to describe what you call "object image position error." (It is also 100% correct and appropriate to use the word, here, although it's a bit less directly understandable, as to why this is). This is a source of confusion, misunderstanding, and miscommunication. If you look up "parallax error" most of the explanations you get from shooting experts/writers have some errors and inconsistencies between each other, even to this day. But if I am speaking your language, correctly, my post should make sense to you.

Shooting airguns at short distances can teach you all you need to know about the nuances of scope "optical image position error" with a lot less time and effort versus doing super long range shooting. Just putting out your targets is a major workload, learning how to shoot at super long range. The doping/ballistics and effect of wind, of course, has to be done the hard way. But you'll be making the most of that time and effort and learn quicker, having already mastered the "optical error" part of the equation.

[popper]

Red Dots have the same problem with object image position and some often call it parallax. Kinda strange, dot is 'on-the-target' but the target image moves. I won't go into the reason but happens in scopes too. Cheek weld is important. The scope entrance pupil is large to gather light, exit pupil is small to get better depth of focus. Diopter is the name for reticle focus.

[gloob]

I agree, red dots will also have some "object image position error," because the lens does not (significantly) affect the image, but the curvature of the lens does effect the position of the red dot as you move your eye around, and it depends on how far your eye is from the sight, as well as the accuracy of the shape of the lens. So theoretically, when using a specific eye relief for a target at a specific distance, the dot will not move relative to your target.

So when shooting with a red dot, consistent eye relief does matter a teeny bit. And centering the dot (or consistent dot position, wherever you choose, for a weirdly shaped red dot sight) does matter even more. (If you have a pneumatic airgun with a removable stock, you will usually need to adjust the red dot when changing from rifle configuration to pistol, where the sight will be much farther from your eye. But in terms of cheek weld on a rifle, the exact eye relief is not very sensitive.)

This is why I use a "stop" like Nobade describes with red dots mounted on a rifle. In this case, it just helps to center the dot quicker/easier, rather than actually eliminating this error. The small hole goes on the muzzle end of the tube dot sight, not the near side, so you can still see the dot anywhere on the lens.

[45_Colt]

Never take apart a scope unless you have the tools and equipment to purge and fill the tube.
Suggest you find a close parallax adjustable new scope. Most times it will be a rimfire scope. Just forget the standard everyday hunting scopes.

So true when disassembled, but when adjusting the parallax distance on a Leupold, this isn't an issue. Removing the bell has no effect of the scope seal. Moving/rotating the objective lens (sets the distance), has no effect on the seal.

Although, if the objective lens carrier is removed, uh-oh.

45_Colt

[Willbird]

So true when disassembled, but when adjusting the parallax distance on a Leupold, this isn't an issue. Removing the bell has no effect of the scope seal. Moving/rotating the objective lens (sets the distance), has no effect on the seal.

Although, if the objective lens carrier is removed, uh-oh.

45_Colt

Hmm how do they stop the objective from fogging on the inside if that part of the scope is not nitrogen purged ?? I never saw a scope fog ever but also never used the old lyman target spot or Unertyls in crappy weather either.

[45_Colt]

Hmm how do they stop the objective from fogging on the inside if that part of the scope is not nitrogen purged ?? I never saw a scope fog ever but also never used the old lyman target spot or Unertyls in crappy weather either.

The end bell is just decorative, removing it doesn't open the scope seal. Calling this piece the end bell is probably not entirely correct. It is the furthest forward piece on the front of the scope.

The objective lenses sit in a carrier. Which is threaded into the scope body. Moving/rotating it still leaves the o-ring seals in place. And moving it is how the parallax distance is adjusted.

45_Colt

[TurnipEaterDown]

The end bell is just decorative, removing it doesn't open the scope seal. Calling this piece the end bell is probably not entirely correct. It is the furthest forward piece on the front of the scope.

45_Colt

Objective bell is what is being referred to, I believe. All the words are there, just trying to be helpful not corrective.

This thread has been informative. I never considered how scopes are put together in this manner, and I never had much reason to.
Makes perfect sense though, as adjustable objective scopes just allow the user quick adjustment by rotating the objective bell and changing the relative distance to the objective lens. Makes sense at least that the objective lens isn't sealed to the rest of the "guts", as older parallax adjustable scopes would then be subject to failure on a routine basis as rotating seals degraded and yet they don't seem to be prone to doing that (in my limited experience).

[gloob]

I've managed to grind a piece of 20-25 bandsaw blade to fit the slots. Slight turn out to shorten the focal/parallax distance. On really large objective (long wrench), you may need to fold a piece of sheet metal over the spine to reinforce it against bending.

I wonder if there an easier way to turn the objective? Foam rubber pressed directly against the lens, or something? You wouldn't even need to remove the decorative endcap, possibly. Do gunsmiths ever do this enough there are special tools available? Most rifle shooting, you wouldn't want to do this. 100 yards is the magic number where focus is still crisp for most peoples' eye all the way to infinity, at least with reasonable magnification. And you can just buy an AO scope. Or you can buy cheap 22 or crossbow scopes set for 50 yards.

For airgun shooters, a lot of the AO scopes only adjust down to 12-15ish yards, despite what the markings say, and a slight tweak could bring the adjustable range a bit closer for indoor shooting and close range pesting. The factory yardage markers don't seem to be reliable enough to care about, anyway. The Leapers/UTG Bugbuster is a name you will hear a lot, with airgun shooters. It adjusts down to 5, at least, out of the box. The markings claim 3 yards. And it has mildots, which is very useful to manage the sight-bore-offset at very close range and pellet drop out farther.

edit:
"Makes sense at least that the objective lens isn't sealed to the rest of the "guts", as older parallax adjustable scopes would then be subject to failure on a routine basis as rotating seals degraded and yet they don't seem to be prone to doing that (in my limited experience)."
Humm... I examined my sidewheel AO scope. And the objective DOESN'T appear to move when I adjust it!? I suppose the middle lens moves, instead. The end result is the same. The wheel clearly adjusts the focal distance in the exact same way as my scopes where you turn the objective ring. This particular sidewheel scope is a FFP, and I don't understand scopes enough to know if this makes a difference in how parallax adjustment works. Perhaps sidewheel scopes may be better as far as keeping in the nitrogen?

Oh, I have another sidewheel that is SFP. Also can't perceive any movement of the objective when I turn it.

[Texas by God]

Thanks for the idea, Nobade. It does help on the 40 yard targets.


Sent from my iPhone using Tapatalk

[gloob]

Great idea. But now you need a cover for the cover?

I have a scope that has a screw-in aluminum cover, so you could put the hole in that, and slip a flip-on over it. Except it's already AO.

Here's one of my "peep" red dots. This sight comes with a screw-in sun shade I replaced with the peep hole. Especially helpful in lower light, where you can't see your entire FOV to center the dot. Parallax error on a red dot is way, way less than on a rifle scope that is too close, but it does make a difference.

[Willbird]

I've managed to grind a piece of 20-25 bandsaw blade to fit the slots. Slight turn out to shorten the focal/parallax distance. On really large objective (long wrench), you may need to fold a piece of sheet metal over the spine to reinforce it against bending.

I wonder if there an easier way to turn the objective? Foam rubber pressed directly against the lens, or something? You wouldn't even need to remove the decorative endcap, possibly. Do gunsmiths ever do this enough there are special tools available? Most rifle shooting, you wouldn't want to do this. 100 yards is the magic number where focus is still crisp for most peoples' eye all the way to infinity, at least with reasonable magnification. And you can just buy an AO scope. Or you can buy cheap 22 or crossbow scopes set for 50 yards.

For airgun shooters, a lot of the AO scopes only adjust down to 12-15ish yards, despite what the markings say, and a slight tweak could bring the adjustable range a bit closer for indoor shooting and close range pesting. The factory yardage markers don't seem to be reliable enough to care about, anyway. The Leapers/UTG Bugbuster is a name you will hear a lot, with airgun shooters. It adjusts down to 5, at least, out of the box. The markings claim 3 yards. And it has mildots, which is very useful to manage the sight-bore-offset at very close range and pellet drop out farther.

edit:
"Makes sense at least that the objective lens isn't sealed to the rest of the "guts", as older parallax adjustable scopes would then be subject to failure on a routine basis as rotating seals degraded and yet they don't seem to be prone to doing that (in my limited experience)."
Humm... I examined my sidewheel AO scope. And the objective DOESN'T appear to move when I adjust it!? I suppose the middle lens moves, instead. The end result is the same. The wheel clearly adjusts the focal distance in the exact same way as my scopes where you turn the objective ring. This particular sidewheel scope is a FFP, and I don't understand scopes enough to know if this makes a difference in how parallax adjustment works. Perhaps sidewheel scopes may be better as far as keeping in the nitrogen?

Oh, I have another sidewheel that is SFP. Also can't perceive any movement of the objective when I turn it.

I agree that the side focus is moving a different part of the scope. Rotating the objective has it's own issues because nothing in the world is exactly concentric, so rotating the objective has the issue of changing the POI. Scope makers apparently do a good enough job that we never notice that issue . We know that there is some error but it must be very tiny....I have never even noticed it in scopes like the BSA Platinum which were like an $80 scope in 1995.

[Longfellow]

I’ve done this with a couple of Leupolds. It is not difficult but to be safe I did make a special spanned to turn the locking ring without harming anything. You start spinning until there is no visual parallax at your chosen distance. Leupold will also do this for you.

[GhostHawk]

I've had great luck with Simmon's Scopes. And the easy solution here is to move your target out to 50 meters instead of 50 feet.

If you truly need to shoot at 50 feet look at the airgun scopes. Many of them are sold with adjustable parallax and focus.