Can someone explain parallax?

The short and simple version -

Hold up your index finger in front of you. Close one eye and line up your finger with something further away such as a door, piece of furniture, window, whatever. Now without moving your finger, rotate your head from left to right - your finger will seem to move slightly as you turn your head. Voilà! you are seeing parallax.

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OpticsTalk has a nice discussion about it, much more than I could provide.

Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications. Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters. A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards.

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With the knowledge provide by the article, I'm prompted to ask, "By any chance, did the optics with parallax adjustment have a higher price than the optics without?"

James Burke, the BBC's science guru made it clear for me.

http://www.youtube.com/watch?v=v2kESSQxhvE&list=PL0RYY8t0gEFFBJ4xuL29RSGhx6GxjpjAl

The whole video is interesting, but he gets specifically to parallax at 20:30.

They guy behind the counter the other day told me I needed a parallax adjustable scope

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Pretty good chance he didn't even know what parallax is. Gun shop counter monkeys make me laugh. Alot.

As mentioned, for big game hunting, it's not necessary. For nailing the 10 and X ring at 1,000 yards, it's kind of important.

4thPointOfContact, that optics talk link has some of the best information I've read on scope construction .... thanks. It cleared up something that I've often wondered about. The parallax knob focuses the image on the same plane as the reticle thereby reducing parallax. So it really is a focus knob.

A cut and paste from a Leupold owners manual. Long story made short, it is just not much to worry about on a big game rifle. For a varmint or target shooter, maybe.

Parallax is the apparent movement of the target relative to the
reticle when you move your eye away from the center point of th

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continuation on the next page.

26
eyepiece. It occurs when the image of the target does not fall on
the same optical plane as the reticle. This can cause a small shift
in the point of aim. Maximum parallax occurs when your eye is
at the very edge of the exit pupil (Even in this unlikely event, our
4x hunting scope focused for 150 yards has a maximum error of
only 8/10ths of an inch at 500 yards). At short distances, effects
of parallax do not affect accuracy (using the same 4x scope at 100
yards, the maximum error is less than 2/10ths of an inch ). It is
also good to
remember that, as long as you are sighting straight
through the middle of the scope, or close to it, parallax will have
very little effect on accuracy.

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This was a great thread which was posted in the nick of time for me. I followed the links and created my own sticky.

Nice thread thanks.

My minimal physics background lets me understand parallax but I have never needed it for hunting as I am not a long range or precision shooter. Inside 300 yards or so it SHOULD NOT really be a factor but I know the snipers at Fort Benning deal with it a lot.

Most scopes are designed to minimize parallex at a certain range. The adjustment allows you to change that range.

If parallax did not matter, a repeatable cheek weld would not be considered a fundimental part of marksmanship, however there is a good chance that a scope for a particular application already has a reasonable set parallex optimization from the factory.

Mike

Here's a couple of web sites that go into all the detail you'll ever want to see:

http://www.snipercountry.com/Articles/Parallax.asp

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

Here's another site with a lot of info but it isn't laid out as well:

http://www.longrangehunting.com/forums/f18/parallax-vs-focus-1988/

That is nice to know about Nikon.....
And of course they probably do not print that anyplace...

^^^^Yep! Much like a spotting scope, an AO or "Side Focus" for parallax is a coarse adjustment in terms of focus; you may luck out and find a clear image when correctly dialed in for parallax or you may need to adjust the eyepiece. I suspect most owners simply turn the knob till the target is in focus and assume parallax has been corrected.

Easiest way to see parallax is to mount a rifle in a cleaning vise indoors and hang a target. Center the crosshairs precisely then adjust your cheek placement. The crosshairs will appear to aim all round the target. If you were sighting in that scope, each group might result in a different POI which may also be different than its POI when in a hunting stance.

One big reason for using a fixed (non-adjustable) scope is the difference in speed offered. In the woods it translates into missed opportunities and excess movement. None of my hunting rifles wear AO scopes because of this, though it wasn't always so. While fixed parallax scopes are set at the factory, most can be adjusted by the end user for alternate distances in a matter of seconds.

I can't ever remember any of my Leupold's failing in any respect, other than when I dropped them off a cliff, or had some other really stupid accident while hunting. But Leupold has taken care of me every time, even when it was my own stupid mistake.

You did good buying Leuplod glass!

GS

Walkalong said:

The parallax adjustment is for removing parallax by bringing the image and the reticule on the same plane. The eyepiece is for focusing on that image once it is on the same plane as the reticule. If you use the parallax adjustment to focus on the target first, you can still have parallax.

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from Optics Talk link said:

8 - The "Eyepiece". This optical group is like a jewelers loupe. Is is (or should be) a super fine magnifier. It's only job in the whole world, is to focus on the reticle.
Let me repeat that for those that live in Rio Linda...
THE ONLY JOB FOR THE EYEPIECE IS TO FOCUS YOUR EYE ON THE RETICLE!!!!

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The first thing I do once I have a scope mounted is focus the reticle using the eyepiece. I only do that once.

Skylerbone said:

While fixed parallax scopes are set at the factory, most can be adjusted by the end user for alternate distances in a matter of seconds.

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How?

On some scopes, Leupold being one, you simply remove (unscrew) the front (objective) lens to gain access to the lens that focuses for parallax. That lens can be screwed in or out while targeting the distance you wish to set parallax at.

For instance: you have a Leupold VX 3 set for 150 yds. mounted on a .22 lr. Your intended distance is 25 yds. so you set your target at the 25 yd. line and adjust the lens, checking for parallax as you go. This will not affect the sealed gas (argon, nitrogen) within the scope. I have done this on a Leupold and several Nikons without a problem. An old Tasco (Japan) was not adjustable but for the $20 I paid it worked well enough. Remember to use a lens cloth when touching the lenses and know what you're looking for with parallax before starting.

The parallax adjustment is for removing parallax by bringing the image and the reticule on the same plane. The eyepiece is for focusing on that image once it is on the same plane as the reticule. If you use the parallax adjustment to focus on the target first, you can still have parallax.

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Thank you for posting that. I've had so many people try to tell me that parallax was adjusted out when you saw a clear image that I bust out laughing sometimes. If you don't adjust your eye piece when you first get the scope you'll never get the parallax right and have a clear sight picture at the same time. Even with the eyepiece correctly adjusted sometimes you still don't see a clear image when the parallax is right.

Parallax adjustment done right has helped me win many, many shooting competitions. People just don't realize that if your eye position isn't exactly right you aren't aiming right if your scope isn't set up for parallax. And that includes shorter distances too. most people think shorter distances are less prone to parallax problems which just isn't true. In fact shorter distances have far more problems with parallax to the point that if you're shooting 50 yards and your scope is actually set to shoot 35 yards you will have a terrible time trying to hit what you're shooting at.

If your scope is set up correctly you should be able to move your head around to any position and the crosshair won't budge off the target (given you aim it at the target in the first place and you make sure the rifle doesn't move when you do). Even a slight bit of movement in where your scope points is going to cost you dearly in accuracy. For shooting rimfire benchrest with a strong scope (which all benchrest shooters use) you absolutely will not win unless you have your scope set correctly.

Another thing is the comment earlier about Leuopold scopes never having a problem with parallax or anything else. The thing is that certain Leuopold scope designs have built in problems that must be corrected very often when you adjust the parallax on the scope. You have to go all the way out to infinity to get the scope back on track if you happen to screw the parallax all the way down to as close as it gets first. The parallax adjustment will not adjust all the way down as far as the scope will and it pushes it out of adjustment every time you screw the scope down to minimum magnification. That's just true and I'm sorry if it offends Leuopold scope owners. Not all of their scopes have this problem and the problem is fairly easily fixed if you know to do it. They make great scopes otherwise for the most part. But it's hard for me to sit back and let it go when someone says they have zero problems. Not true. It wouldn't stop me from buying one of their scopes but I would have to think about it first.

Parallax is one of the keys to the successes I've had shooting especially for rimfire rifles. To be honest there have been situations where I didn't freely give out this info because I knew it might cause me to lose a contest. That's been rare though. I'm posting it here now ain't I?

Trust me if you want top quality accuracy parallax is an issue you need to know about and understand and know how to fix the problems. I am giving away trade secrets here so learn it while you can. Seriously it is a big issue.

Just for the record it generally isn't a big problem for hunting or plinking. It's only a problem for those trying to go from a .20" group to a .15" group mostly. But it can be a huge issue if your scope is seriously messed up.

Cee Zee said:

I've had so many people try to tell me that parallax was adjusted out when you saw a clear image that I bust out laughing sometimes. If you don't adjust your eye piece when you first get the scope you'll never get the parallax right and have a clear sight picture at the same time. Even with the eyepiece correctly adjusted sometimes you still don't see a clear image when the parallax is right.

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You need to explain please as you have lost me!

Optically when the image is focussed on the same plane as the reticule that the scope will have the parallax dialed out?

So having focussed our eyepiece on the same focal plane, which has an already focussed and clear image, what optical condition could arise to bring about a lack of clarity?

Cee Zee said:

I've had so many people try to tell me that parallax was adjusted out when you saw a clear image that I bust out laughing sometimes. If you don't adjust your eye piece when you first get the scope you'll never get the parallax right and have a clear sight picture at the same time. Even with the eyepiece correctly adjusted sometimes you still don't see a clear image when the parallax is right.

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You need to explain please as you have lost me!

Optically when the image is focussed on the same plane as the reticule that the scope will have the parallax dialed out?

So having focussed our eyepiece on the same focal plane, which has an already focussed an clear image, what optical condition could arise to bring about a lack of clarity?

They call it a "side focus" because in reality that is exactly what you are doing, focussing the image on the focal plane. If the image was out of focus then surely one could deduce that the parallax setting was incorrect?

It's only a side focus if your reticle is properly focused properly with the eyepiece prior to adjusting for parallax. If the eyepiece is not perfectly focused, bringing the image into focus won't help. That's why you adjust for parallax until the reticle stops moving in relation to the image. That removes parallax regardless of how focused everything is.

For the OP, the best simplified example I've heard is when you look at a car's speedometer from the front passenger seat. It often looks like the car is going faster because you are not lined up to see it like the driver. This isn't exactly parallax, but it's about the closest thing in the real world.

Without a scope or rifle you can demonstrate parallax with a window in your house. Take a grease pencil and make an X (crosshairs) on the window. Line up with something outside then move around, viewing the object from different locations in the room. You now have an example of extreme error, prevented in a scope by its limiting tube diameter which forces some alignment of your eye.

Now draw a picture on a sheet of paper and tape it to the outside of that same window centered in your crosshairs. No matter where you move in the room, the picture will display no more parallax than is allowable by the thickness of that pane of glass. This closely mimics the error in a fixed scope.

Take the picture down and draw crosshairs on the paper and note that they simply cannot move around as they reside in the same plane as the image drawn (you are now parallax free). Simple enough?