Parallax?

[Yellowhammer]

Since this is a place for stupid questions...

I vaguely know what this term is but it's never been terribly important to me because I'm not shooting terribly far distance. That said, I see it mentioned in almost every scope thread.

What is parallax?

Why does it matter?

Is there a minimum distance where parallax starts to matter?

How do I use these parallax knobs on the scope?

 

[Olen_4504]

in simplest terms, it brings your target and reticle BOTH into focus.

parallax knobs are labeled with yardages on them. so if your target is at 100 yards, set your parallax knob to 100. this will make your reticle and any object 100 yards away both in focus when you look through your scope. if you are shooting a target at 500 yards, set your knob to 500.

depending on the scope, the range of parallax adjustment will vary.

ex) my Ares BTR goes from 25 yards to 500 then to "infinity" so anything beyond 500 yards I just put it on the highest "infinity" setting.

you'll notice if your knob is NOT set to the correct distance your target is at, either your reticle will be blurry, or the target will be blurry depending which setting you have your parallax set at. (i've never paid attention enough to tell you which will be blurry if you are over or under on your parallax)

but yes parallax matters pretty immediately. if your parallax knob is at "infinity" (highest setting) and you are looking at a target at 50 or 100yds, something will be out of focus.

this is an un scientific explanation. lots of youtube videos explaining it way better than I can.

 

[Olen_4504]

also I would say it is not a dumb question:

before I got really into rifles/nice optics my dad gave me his .270 savage with a "nice to him" BSA Catseye scope on it. He swore it was the beesknees (later to find out his standards for scopes are very very low). there was this mysterious knob on the side that had yardage marks. I honestly thought that was a "cheater knob" that you would just put it to whatever yardage your target was at and it adjusted your elevation for you.

nope, just the parallax knob. i had no idea, never had seen one before in my life because my dad only ever bought cheap scopes

 

[Yellowhammer]

Will incorrect parallax alter the point of impact or is it just a function of how sharp the image you see will be?

Is there a procedure for setting parallax? I set the focus ring on my scope so the crosshairs are sharp, do you then adjust the parallax knob until the image is also sharp?

 

[Olen_4504]

also I would say it is not a dumb question:

before I got really into rifles/nice optics my dad gave me his .270 savage with a "nice to him" BSA Catseye scope on it. He swore it was the beesknees (later to find out his standards for scopes are very very low). there was this mysterious knob on the side that had yardage marks. I honestly thought that was a "cheater knob" that you would just put it to whatever yardage your target was at and it adjusted your elevation for you.

nope, just the parallax knob. i had no idea, never had seen one before in my life because my dad only ever bought cheap scopes

here read through this thread regarding POI change.

http://www.snipershide.com/shooting/threads/can-parallax-adjustment-affect-poi.100850/

as far as the proper process to set up the parallax. set the ocular lense first so the reticle looks crisp to you. then you just leave it alone.

then you adjust your parallax as needed based on the distance to your target.

read here:

http://www.accuracy-tech.com/ocular-lens-focus-and-parallax-adjustment/

 

[XLR308]

Inconsistent or out of adjustment paralax combined with inconsistent form as in not being in the exact same spot in the occular lens will cause impact shift.
If you look through the scope with the paralax out of adjustment and vary your eye position you will see the reticle shifting back and forth while doing so.
That is the impact shift I was referring to.

 

[pmclaine]

Your eye can only focus on one place (distance) at one time.

Your scope mechanically allows to have focus at two different distances.

First understand what parallax is.

Go stand by a light switch in your house.

Bend your dominant side arm at a right angle with index finger extended up.

Move that index finger so that it's tip is now covering the tip of that light switch. Line your eyebal up with tip of finger so line of sight is placing your finger tip on tip of switch but you are not actually touching the switch.

Your finger is on a focal plane halfway to the switch and the switch tip is on a focal plane a foot or so distant but when your eye is directly in line with the two objects the fingertip will cover the tip of the light switch.

Now move your head.

You will see that your fingertip and light switch will move opposite each other. Your finger tip (reticle) does not stay on the light switch (point of aim).

That is parallax error.

Now extend your arm so that your finger tip is touching the tip of the switch.

Move your head.

No matter where your eye is your fingertip (reticle) remains superimposed on the switch tip (point of aim).

The two focal points, finger (reticle) and switch tip (point of aim) are on the same focal plane and there is no parallax error.

Your scope allows you to take two objects, reticle and target, and superimpose their images on the same focal plane, elimination of parallax error.

If you reload, think of your balance scale......if your eye is not exactly level with the readout marks than you will not get a true weight. If your eye is high the reading will show low and if your eye is too low your weight reading will be high. This is parallax error also.

The second example is what you would experience if you have a fixed parallax scope. Why it's important to always get your eye directly in line with the reticle cross hair.

 

[Yellowhammer]

Thanks all, good stuff.

So after setting the ocular focus then I focus on the far image and adjust the knob until it is sharp as well? Seems pretty simple.

 

[Yellowhammer]

Also, hookedonbrass, the pawn shop 30-30 and bushnell scope I hunted with as a kid (everybody had one of those I think) had the same adjustment on it with range hashes. I too thought it was for adjusting for range...didn't know any better. FWIW, I still have my pawn shop 30-30 with the bushnell scope.

 

[XLR308]

Thanks all, good stuff.

So after setting the ocular focus then I focus on the far image and adjust the knob until it is sharp as well? Seems pretty simple.

That's basically it but as pmclaine described with the finger and light switch example I always verify by checking for reticle shift by varying my eye position before firing when I change distance.

When adjusted properly you should see no reticle shift when slight variations of eye to occular lens movements are made although you will still need to be as precise as possible in your shooting form.

 

[Sheldon N]

So after setting the ocular focus then I focus on the far image and adjust the knob until it is sharp as well? Seems pretty simple.

I would say that it's not really about what's in focus or not or whether it's sharp. Focus and the image appearing sharp are sort of a byproduct of getting the parallax right. PMClaine's description of parallax is more accurate as to what the issue is.

Here's the simple thing you should do when setting up to shoot. Put the gun on the bench or bipod/bag so that it can sit with the crosshairs on target without you touching the gun at all. Next you "hover" over the gun looking through the scope and move your eye left/right or up/down - again without touching the gun. If the crosshairs move on target when your eye moves then you have parallax error, regardless of whether the target looks sharp. Adjust the parallax dial until the crosshairs hold perfectly still on target when you move your eye around.

Ideally you can set up your eyepiece ocular focus so that the reticle is sharp, the target is sharp, and the parallax is gone all at the same point when adjusting the parallax knob. Then in practice you can just dial until the image is sharp, then shoot. I still make sure to completely dial out parallax when shooting groups at 100 yards. The parallax is much more sensitive and has a bigger effect at close ranges than at long distance.

 

[Yellowhammer]

That makes sense Sheldon, particularly the part about setting the rifle down and moving your head around. I've seen people do that in you tube reviews and were wondering what that was about.

 

[JohnMWoo]

What Sheldon said... Parallax error is the error you would get in your scope when the reticle and the focused image of the target are not in the same plane. So you use your parallax adjustment on your scope to put the reticle and the image on the same plane. If they are not on the same plane, then as you move your eye around the scope (left, right, up, down) the image will appear to move in relation to the reticle. If they are on the same plane, they no matter where you eye is, the image will not move.

Presumably with good scopes, when the image is in focus, it is also on the same plane as the reticle. In some scopes, this may not be the case, and you would want to adjust your scope to remove parallax error - leaving the image out of focus.

It does make a difference over longer distances. The more parallax error, the greater possibility of shooting to different points of impact, because the reticle will point at different places on the target as your eye settles in slightly different places in relation to the scope.

Now, if you were able to put your eye in relation to the scope in exactly the same place every time, then parallax error would not matter.

Hope that makes sense... (at least that is my understanding, please educate me if I am mistaken)

R,
jmw

 

[pmclaine]

Sacrifice image quality for correctly adjusted parallax.

You may have atmospheric, eye or scope manufacturing issues that when you have your best target image you may "duck" your head and still see parallax error.

If you adjust the side focus and get a slightly blurred image but you note all parallax error is gone that is the adjustment you want to use.

 

[Yellowhammer]

Sacrifice image quality for correctly adjusted parallax.

You may have atmospheric, eye or scope manufacturing issues that when you have your best target image you may "duck" your head and still see parallax error.

If you adjust the side focus and get a slightly blurred image but you note all parallax error is gone that is the adjustment you want to use.

Good to know, thanks all---I've never worried about parallax before so this is all good info.

 

[Snuby642]

Had trouble couple of days ago with poi shift and focus, bad groups, dejected.
My son had adjusted parallax when he shot.

I completely forgot to set my scope to infinity before adjusting ocular.
Going out to set it properly right now, and I'm using the setscrew this time.
Setting ocular in bad lite is not as easy if your eyes are old.