Paralax


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floatingblue
January 6, 2010, 07:48 AM
I have a couple questions about paralax. First, how is it spelled? I've seen it spelled a few different ways and want to get it right. Second, is it really that important? I'm shopping scopes and some that have features I like are set at 50 yards, most are set at 100 yards. From some of the stuff I've read, if it's not set at the distance I'll be shooting, I can expect to see my reticle wander around aimlessly and eventually spiral out of control. Of course, I'm being facetious, but you get the point. What's the skinny?

James Ridener

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Bart B.
January 6, 2010, 08:09 AM
Parallax is one of the most misunderstood aspects of rifle scopes. It only happens when the shooter's aiming eye is off the scopes optical axis AND the scope's front lens is focussed at some range other than target range. Keep your aiming eye fairly well centered on the scope's optical axis and with most high power rifle scopes focused at 100 yards (unless it has an adjustable objective lens you set at target range and note this is not a 'parallax adjustment' but instead a 'focus adjustment' exactly like a camera lens) and you'll do very well. Even with focus off a bit, nothing bad accuracy wise will happen unless missing your point of aim by 1/4 to 1/2 inch at 200 yards really upsets you.

The word 'parallax' is often mispelled as much as the military rifle designations stamped on their receiver hump. "M-1" and "M-14" are far more seen in print than what matches the stamped letters on those rifles as well as how they're spelled in military publications. There ain't no friggin' dashes!!!! Just plain M1 and M14.

Dave P
January 6, 2010, 08:20 AM
"I can expect to see my reticle wander around aimlessly and eventually spiral out of control." Partly true. Like Bart says, if your cheek weld is dead center every time, your parallax error is zero.

But most of us are not perfect: with a good scope that lets you adjust for parallax, and WHEN YOU HAVE PROPERLY ADJUSTED IT, you can move you eyeball anywhere behind the scope, and as long as the cross hairs are on target, that is where you will hit.

From EABCO site: PARALLAX ADJUSTABLE SCOPES -
Scopes with an adjustable objective (AO) allow you to set the distance at which they are parallax error-free. These scopes are also designated as Parallax Adjustable (PA), Side Focusing (SF), etc. It's misleading to call this feature "focusing" because its really parallax correction. Its also important to note that the distance markings on the parallax adjustment are quite often not exact.

rizbunk77
January 6, 2010, 09:30 AM
you can tell a really bad scope for paralax by looking through it at the bench at a target at 100yds or so. move your eye around slightly. If the scope crosshairs are moving on the target you have paralax. If the scope crosshairs don't move at all (assuming your holding the rifle steady) then you have no paralax at 100 yds. I think its important also to adjust the rear eyepiece for crosshair clarity before you use the scope. Follow the instructions that came with the scope for adjusting the eyepiece. Interestingly I have a russian scope that is paralax adjustable from the rear eyepiece. I didn't know that was possible but it is. I have found that adjustable objective markings for yardage are rarely spot on when it comes to adjusting the scope. Works alot better to focus on the target down range and stop adjusting when it becomes crystal clear. At least that is what works for me.

rangerruck
January 6, 2010, 10:45 AM
a scope with even slightly bad prllx can throw shots off more than an 2 inches at 100 yds , easy. Get a rifle all nice and comfy and sighted in, at 100 yds, rested on nice bags or a rifle rest or vice, etc., then stand or sit behind it. Get the best cheekrest pic you can, and put your crosshairs on the target.
Once you do that, lift up your head, and move it around in small circles, backward, forward etc., and see if your crosshairs are now moving around, off of dead center of where you origionally had your crosshairs; if it is moving around, that is your prrlx. Also, I bet you will notice that the crosshairs are moving 2 inches or more, off of dead center of where you had the crosshairs aiming before.
So it is big important to get a scope that is prllx adjustable; I like 50 yds or less even. Air rifle scopes go down to about 10 ft, or 7m, most of the time.

And above dude is right; consult you op manual, on how they say to focus the scope. and I allways focus the rear eyepiece , FIRST!!!

floatingblue
January 6, 2010, 09:09 PM
Thanks, guys. This is pretty much what I was beginning to believe. Now that I think about it, I seem to remember Kelly McCann doing a demo of a parallax-free sight in a video of his and he was showing how no matter where your eye is, as long as the dot is on target, the bullet will be too. This opens up my options considerably. For the record, I'm looking at doing a quasi-scout rifle and have been looking at LER scopes, most being for pistols, and those are what I'm seeing set to 50 yards.

James

Bill_Rights
May 24, 2011, 11:38 PM
When I see scopes listed for sale in catalogs, web pages, etc., I always know that an "Adjustable Objective" (AO) designation means the scope can be adjusted for parallax. It also means that you can get a crisp focus at really close distances, like less than 20 yards.

Does a scope with "Side Focus" designation work exactly the same way as AO? At least, does it adjust parallax in the same way? Does it adjust clarity of image at close distances? What ARE the differences between side focus and AO? There must be some differences, because they mechanically work at different locations along the optics path inside the scope, yes?

cottswald
May 25, 2011, 07:02 PM
Here's a pretty good article:

http://www.6mmbr.com/parallax.html


And here's a good example:

Wikipedia 2010: A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use an older-style "needle" speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.

Tim the student
May 25, 2011, 11:48 PM
Here is another question for you experts.

Why are many fixed power scopes not adjustable for parallax, but some are? What is the deal there?

Bill_Rights
May 26, 2011, 01:11 AM
cattswald,

Good article from U.S. Optics! That answers a lot.

Tim,

Good question, but I also ask: Why are many adjustable power scopes not adjustable for parallax?

OR

Why would anybody make a scope NOT adjustable for parallax? I guess it adds complexity and you do not need it if you actually hold your eye on the optic axis. So there ought to be some sort of guide or feedback to tell you that your eye is, indeed, on axis.

RugerMcMarlin
May 27, 2011, 03:50 AM
Cant see how parallax error could be off more than thickness subtended by crosshair. MOA? if 1" at 100yds, 2" at 200, etc?

BartB That is the simplest, clearest most understandable explanation I have ever read.

kingcheese
May 27, 2011, 05:10 AM
my scope, a barska 4-16x40m has AO
its a handy feature for the rifle i have it on, it allows me to adjust for most any range i can shoot at and expect to kill a coyote with 17hmr, it oes from 20-250 yards i think it is, and one final setting that is for past that

i can adjust the front eyepiece for clearness(also telling me general range of what i am looking at) and then the rear eyepiece for degree of magnification

Sako Shooter
May 27, 2011, 12:51 PM
As the above posters noted, parallax is a "potential" for error, based on various factors. Practically, in a fixed parallax scope, say at 60 yards for rimfire or 100 yards for centerfire, the further you move away from 60 and 100 yards, respectively, the larger the "potential" parallax error.

I recently converted all my rimfire scopes to AO. I had a Leupold rimfire scope with set parallax at 60 yards. My new VX3 adjusts down to 25 yards. The Leupold representive advised that the maximum parallax error from 25 yards to 60 yards was 1/4". To a hunter this is no big thing, to a fine target shooter it is everything. I cannot imagine a serious target shooter not having an AO scope. The image is also more crisper at every range with the AO.

Now, practically, I saw no reason to change my centerfire deer scope which is set at 109 yards (100 meters as it is European). Most of my taget shooting is done at 100 yards so this is perfect. I don't foresee fiddling with an AO objective if a deer is at 75 yards or 125 yards, it's just aim and shoot as quickly as possible. The fraction of an inch error which might occur is of no moment to me in that situation. Now, if I was taking 300 yards shots, that would be a different story.

In sum, fine target accuracy certainly demands an AO scope. Why put up with paralax errror if you don't have to. For hunting situations that don't deviate much from the fixed parallax distance set from the factory, it is not as important in my opinion.

natman
May 27, 2011, 02:11 PM
Parallax is larger:

At high magnifications
At short distances.

At any range the error only a small amount. So if you have a 4x hunting scope there's no point. If you have an airgun scope that's going to be used at short range and high magnification for targets, then you need an AO.

RugerMcMarlin
May 27, 2011, 04:00 PM
OK , if you can use AO to bring target into focus at range, can you run that backwards, unknown range, bring target into focus with AO , look at AO adjustmment on scope ,to come up with a useful range estimate? Thinking in like a hunting application. Cuz, that would have been handy a couple of times. I'm assuming you have already brought retical into sharp focus. (ocular)

That's pretty much how range finders work, isn't it?

You guys have that AO deal on your bug shootin guns dont you.

sscoyote
May 27, 2011, 05:46 PM
I have an interesting experience with parallax. Had a Burris 3-12x mounted on a custom specialty pistol. For the life of me i couldn't get a load to shoot less than 1.5-2 MOA. I was sitting at the bench 1 day trying to null it out. Finally thought to check parallax, and sure enuf the parallax was as big as the group i'd just shot. I then adjusted the parallax out, shot a 3/4MOA group with same load and done.

Now it's possible that the LER scopes are harder to center your eye on the optical center of the scope to help minimize parallax--i don't know this, but it sure made a difference that day.

RugerMcMarlin
May 27, 2011, 06:01 PM
OK went and read the article, and I'm starting to think the confusion, has to do more with the TERMs used, and the understanding of those terms, and the number of different terms for the same thing.

So for the sake of my understanding, and if I'm correct maybe someone elses.
I'm going to try and explain my understanding of Parallax with using as few terms as possible.

Parallax = 0 , when there is 0 difference, between distance to target plane, and distance to reticle plane. Turning the Adjustable Objective lens back and forth,
is running the focused RETICLE image plane out and back. Between target and objective lens.

It would have been easier, if instead of Parallax, they called it Retical Plane Distance. So I'm gonna just call it that RPD.

An example I think, would be like sitting in your Living room , with a Mini maglight. You have 3 walls all at different distances, what the distance is doesn't matter as long as they are different. When you shine the light on each wall, in order to have a crisp clear edge circle of light, you have to rotate the front end of the light, focusing the light. In each, case matching the distance to the wall( target) with the distance to the sharpest circle of light(RPD). The plane where the circle of light is sharpest is a variable adjusted by the end of the maglight.

I'm saying the crisp circle of light, is roughly equivalent to a crisp,sharp reticle image.

The adjustable bell on the maglight, is like the AO adj. on the scope,
turning either runs the crisp circle of light or the crisp reticle image out and back as an optimum adjustment. Matching with the wall or target plane.

And if thats right, I want to change my answer on optimum scope mounting height.

Optimum scope mounting height is where it centers optical axis with the height the eye is off comb with solid repeatable stock/cheek weld. We can get rid of one variable right there. Proper paralax adjustment should make the centering of optical axis a little less critical.

I apologize if it's an over simplification, simpler was my goal.

If I still don't have it,, I cordially invite all and sundry to jump on it as I am determined to understand this.

kludge
May 27, 2011, 08:41 PM
Parallax is simply a difference in the location of the reticle and the focal plane of the object in the scopes view. Zero parallax means the rays of light coming into the objective lens are focused in a pinpoint of light on the reticle as seen here:

http://upload.wikimedia.org/wikipedia/commons/1/12/Telescopic_sight_internals.png

http://en.wikipedia.org/wiki/File:Telescopic_sight_internals.png

When you set up a scope you first adjust the reticle focus for your eye... you look at a blank white wall or the sky and turn the ocular lens until the reticle is in sharp focus.

Then when you adjust the objective lens you are adjusting the it to focus the light in exactly the same plane as the reticle. When you do this there is zero parallax.

I'm not quite sure I understand what you wrote, but no, you are not moving the reticle, you are changing the focal point of the objective lens to be coincident with the reticle. The reticle remains stationary.

ETA: In some scopes the reticle is located in the first focal plane, but on many scopes it is in the second focal plane

RugerMcMarlin
May 27, 2011, 09:59 PM
I don't think you understood what I wrote either. I dont see with the exception of windage and elevation there is any way to move the reticle.!?

What I was moving was the point where the reticle is focused or sharp. that point will change by turning the AO. If it doesn't its worthless.

None of the planes I used were internal to the scope. I only used planes as a way to illustrate values of distance between scope and target.

kludge I see what your saying I was saying "reticle" when I meant "focused reticle image" Like usual I knew what I meant. just didn't get it across.

I see what your saying, your moving focused image of target in to focused reticle image.

I was taking focused reticle image out to focused target image. either way we'll arrive at 0 parallax

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