Rifle Scopes Explanation of Parallax

[3bnRanger]

I'm wishing to understand the use of this feature as well as the terminology regarding my 5.5-22x50 NXS. My NF manual defines parallax as "the apparent movement of the reticle in relation to the target as the shooter moves his eye across the exit pupil of the riflescope, caused by the target and the reticle being on different focal planes".

Taking the above definition into consideration I've become more confused in my endeavor to understand this concept....for example: I shot a short range stress drill yesterday which involved running a .5 mile trail run with your equipment and taking two UKD shots, which ended up being 196 and 65 yards respectively. The event was timed which added another "stress" factor. The longer shot was first so I had my NXS cranked to 22x due to shooting "ringed" targets with a very small 10 ring. I had to adjust the parallax knob slightly before taking the shot for target clarity. As I ran to the 65 yard FP I dialed down my dope as well as my power to 10x. When I went prone I noticed my sight picture was incredibly blurry; which brings me to my main point. The NF definition of parallax seems to have NO mention in regard to "image clarity" however whenever I use the knob that is the effect I'm gaining. How do the two relate? Hopefully I explained this with enough clarity. Thanks.

 

[prplhaz72]

Re: Explanation of Parallax

http://www.opticstalk.com/what-exactly-is-parallax-anyway_topic5026.html

 

[dcjs]

Re: Explanation of Parallax

When the parallax is not adjusted for the range in question, the target image is not in the same plane as the reticle. While this causes parallax errors when the eye is moved behind the scope like described in the manual, this is also the very definition of the target image being out of focus aka blurry. In fact, the parallax knob focuses the target image to the reticle just the same as a camera focuses the image to the film/sensor plane for different distances. If parallax isn't adjusted properly, the image will be blurry to some degree, depending on magnification and how far the parallax is off.

 

[Plange]

Re: Explanation of Parallax

3bnRanger,
The easy and short answer is even though parallax and focus are 2 different things they do overlap optically. Typically when you adjust parallax it is as NF stated in their manual, BUT as you do this the image naturally becomes clearer as a product of being parallax free. Keep in mind that in some instances the focus may be clear but you are still not parallax free and vice versa, but there is an overlap there and the main thing is being parallax free.
Hope this helps,
Paul

PS: This is overly simplified maybe David S or Ilya or a few of the other optic Gods can really delve into it for you, but this is the short and sweet of it.

 

[mouse07410]

Re: Explanation of Parallax

David,

Could you please explain the following phenomenon I'm experiencing:

My eyes are myopic. I prefer to shoot without corrective eyeglasses so I adjust the ocular diopter to focus on the reticle without eyeglasses. So far so good. Now when I "focus the target" with the Parallax knob - the target comes in focus when the marking on the Parallax knob shows distance much further than the actual (measured) range to the target.

If I put corrective eyeglasses on - then both the reticle is in focus, and the Parallax marking when the target is in focus corresponds to the real distance.

What causes the effect that I'm observing? Is it normal? Is it OK - i.e. is the parallax really taken care of when the target is in focus without eyeglasses (since the distance the scope "thinks" the target is at is wrong)? I'd really prefer to avoid shooting with corrective eyeglasses if possible...

Thanks!

 

[dcjs]

Re: Explanation of Parallax

Mouse,

I'm afraid I can't really explain how the eyes with/without corrective glasses are interacting with the parallax setting, but the "correct" parallax setting (meaning the one that brings the target image and reticle into the same plane) should always be the same no matter what's happening behind the scope.
If you want to be sure to remove the possible POI shift caused by parallax, you'll have to move your head side to side/ up and down and watch for movement of the reticle relative to the target. If the reticle doesn't move, there is no parallax error. I've heard it before that in some cases the sharpes image doesn't necessary coincide with the parallax free setting, and I don't have a good explanation for this.

Regards,

David

 

[Lowlight]

Re: Explanation of Parallax

Parallax and Focus are not the same... you can be in focus and out of parallax, and because parallax is a function of distance the shooter's eye sight will determine the correct focus.

Look at it like this, shooting iron sights, the target is not in focus, you focus on the front site and this along with consistent cheek weld minimizes the error, because there is natural parallax in that system.

Movement of the head / eye has to be without causing shadowing of the sight picture. The reticle will then float on the target if there is parallax in the system. Away we demonstrate this, is, take a pencil and put it between you and the screen, aiming at my Avatar... if the pencil is between you and screen, it has parallax, now to adjust it out, move the pencil point to the screen and touch it. Now the pencil and target, my logo is on the same focal plane.

It is not uncommon for the target image to be slightly out of focus. It's like going to the eye doctor when he asks, better or worse, while changing the prescription.

 

[dcjs]

Re: Explanation of Parallax

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">Parallax and Focus are not the same... you can be in focus and out of parallax, and because parallax is a function of distance the shooter's eye sight will determine the correct focus.</div></div>

Frank,

this is the part that bugs me. As I said, I've heard this before (not achieving perfect focus at the parallax-free setting), but in <span style="font-style: italic">theory</span> the only thing that has to do with eyesight is the diopter setting. You adjust the diopter to get the reticle in focus, and then focus the target image to the reticle using the parallax/focus knob. Reticle and target image should be in focus and parallax error eliminated.
Obviously, there are cases where this doesn't play out like it should in practice, and I don't know wheter this is a result of misadjustment of diopter and focus or some problem with the calibration of the optics. From experience, how common would you say this problem (not achieving good focus and parallax-free setting at the same time) is? Depending on the shooter (same problem with different scopes) or scope (all shooters have the same problem with one scope)?

Regards,

David

 

[timelinex]

Re: Explanation of Parallax

Your knob can not correspond to the actual distance for more than one reason. I know that on one of my scopes I always have to set the parallax knob 20-30 yards farther than the actual distance for the picture to be in focus and reticle 'wobble free'. So sometimes the knob is just not precise.

Obviously I might be wrong because many of you disagree but I was under the impression that the 'focus' setting is to focus the RETICLE while the parallax focuses the image. The only way I can see a mismatch in parallax and image focus is if the scope was calibrated for 20/20 vision and the user has much worse vision.

 

[Lowlight]

Re: Explanation of Parallax

David,

I would think it is mostly because the system is using a lens to "cheat" and not actually move the objective. So it would be a problem within the system. the question is, does adjusting the parallax via the objective lens cause the same issue ?

I understand what you are saying the optical design should not produce this, but each person's eye sight being what it could potentially be, there is definitely room for error.

 

[mouse07410]

Re: Explanation of Parallax

What would you recommend me then from the practical point of view? My options wrt. Parallax setting are:

1. Adjust the knob so that the target image is in focus (or closest to focus) and the reticle doesn't move when I move my head.
2. Adjust the knob so that the indicated distance is roughly correct and the reticle doesn't move when I move my head.

(Wearing eyeglasses would introduce yet another lens between the scope optical system and the eye, which confuses my already confused analysis.)

 

[Lowlight]

Re: Explanation of Parallax

Give time and opportunity you want to be parallax free... any other time image focus is enough. A consistent cheek weld will help with any remaining parallax in the system.

 

[dcjs]

Re: Explanation of Parallax

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">David,

I would think it is mostly because the system is using a lens to "cheat" and not actually move the objective. So it would be a problem within the system. the question is, does adjusting the parallax via the objective lens cause the same issue ?</div></div>
Frank,

this isn't "cheating" in an optical sense, it's just a different way to achieve the same thing. There are scopes that adjust for parallax by moving the whole objective assembly and some just move an internal lens that is part of the objective system, and both focus the target image to the reticle. Similarly, there are camera lenses that move the whole objective and those that move an internal lens/group to focus, and bot will crate a sharp image on the film/sensor plane.

Using your example with the pencil and the computer screen, if the pencil tip is on the screen (no parallax), how could one appear blurred and the other one sharp? It bugs me that it does seem to happen though.

In the same context, different optical designs (even at the same magnification) may result in different perceived depth of field, so with one scope, the image may be sharp even though parallax adjustment is slightly off to either side, while others will blur much faster beyond the optimal point of focus. As most of the time, there are advantages and disadvantages to both. More depth of field will allow rapid target recognition/acquisition when there is not enough time to fiddle with parallax to get it "just right". Less depth of field will make it easier to dial out parallax using image sharpness as an indicator (provided sharpest image and no parallax coincide).

Definitely some interesting things going on with parallax.

Regards,

David

 

[Sespe]

Re: Explanation of Parallax

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">...Away we demonstrate this, is, take a pencil and put it between you and the screen, aiming at my Avatar... if the pencil is between you and screen, it has parallax, now to adjust it out, move the pencil point to the screen and touch it. Now the pencil and target, my logo is on the same focal plane.
</div></div>

Holy crap! I finally got it. The whole concept finally makes sense.

(embarrassed I never understood it before)

 

[Jumper]

Re: Explanation of Parallax

I have a Premier 3-15x55 and I find it extremely difficult to know when all the parallax is adjusted out. I think the broad focal range of the design has something to do with it because through the entire adjustment range, the focus is only blurry at the extremes. So I have to do the head bob with the reticle on-target looking for relative image shift and it gets to the point where I can't tell if the image is shifting or if its me imparting movement to the rifle. I'm about ready to change over to a fixed 10x and forget parallax.

 

[dcjs]

Re: Explanation of Parallax

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jumper</div><div class="ubbcode-body">I have a Premier 3-15x55 and I find it extremely difficult to know when all the parallax is adjusted out. I think the broad focal range of the design has something to do with it because through the entire adjustment range, the focus is only blurry at the extremes. So I have to do the head bob with the reticle on-target looking for relative image shift and it gets to the point where I can't tell if the image is shifting or if its me imparting movement to the rifle. I'm about ready to change over to a fixed 10x and forget parallax.</div></div>
Jumper,

that's what I was getting at with my above comment, different designs for different purposes. This scope was designed as a combat optic, and as such, the most important function is to be able to see the target under all circumstances, quickly and without having to fiddle with settings. You trade a little comfort at the bench for more usability under adverse conditions. Same with parallax adjustment that goes down to something like 10m by the way, it surely has it's advantages under certain conditions, but if the parallax setting is left at such short ranges accidentally, you'll see <span style="font-style: italic">nothing</span> at long range.

If you have the time, you can dial the parallax out completely (if the shooting position isn't stable enough to do that, chances are the remaining bit of parallax won't make a difference anyway), but if you don't, you still have a clear sight picture and can overcome parallax with a good cheek weld. Setting the parallax at something like 300m will result in a very practical "combat setting" that allows the use of the scope at various distances without having to readjust parallax.

I don't really understand what gain you are expecting from changing to a fixed 10 (without parallax adjustment)? The only thing you could forget there is being able to get rid of parallax even when you have the time.


Regards,

David

 

[dcjs]

Re: Explanation of Parallax

double tap, sorry

 

[livetohunt]

Re: Explanation of Parallax

I have found that parallax is much harder to adjust on a scope that has really GOOD GLASS.I wear glasses and just because the sight picture is clear that does not mean that all the parallax is out of the scope.For example a cheap scope is much easier to adjust than a zeiss victory because the glass is so good you can be way out of adjustment and still have a Great sight picture.Just my experience not sure if others see it the same way.

 

[sniperaviator]

Re: Explanation of Parallax

The key point to concentrate on is that you want the scope to NOT HAVE parallax issue. The way to test to see if it does is to focus on a horizontal line and bob your head up and down. Now adjust the parallax focus knob and see if it improves the unwanted movement of the horizontal line with respect to the horizontal reticle. If it does leave the parallax knob where it is and adjust the ocular and see if you can bring things into focus. You need to test this at all distances and magnification and reach a compromise (it will be a compromise) where you get the least unwanted movement of the horizontal line with the horizontal reticle.

Basically you have 3 adjustments and distance. Magnification, ocular and parallax (and distance). Manipulate those in such a way to give you the least parallax issue at the distances you shoot.

 

[Jumper]

Re: Explanation of Parallax

Good info, thanks. The 10x scope comment was because most fixed power scopes don't have parallax adjustment.

 

[Mute]

Re: Explanation of Parallax

OP, how did you adjust your diopter? I wear glasses also (nearsighted with slight astigmatism in left eye). I adjust all my scope diopters in the following manner and I never have problems with my reticles and target being out of focus:

1. Set your scope to maximum magnification.

2. Turn diopter all the way in.

3. Look into the scope at a cloudless sky or a big blank wall if skies are cloudy.

4. Adjust diopter until reticle is sharp and in focus.

5. Leave diopter alone.

I adjust all my scopes while wearing the eyewear I'd use while shooting. Hope this helps.

 

[mouse07410]

Re: Explanation of Parallax

Mute,

Do you shoot wearing corrective eyeglasses? Or do you adjust the ocular for your diopter and shoot without glasses?

 

[Heltsley]

Re: Explanation of Parallax

I did not read all the responces but the focus and corrective lenses is a very complicated situation that is dependent on the individual shooters eyes.

The Focus on the retical is a simple spherical system that works for the majority of people but is far from what most people need. Being Hyeropic or Myopis is not the propblem, it is that the eye has in most people 2 focal planes. This is caused by the lens and the cornia causing a condition known as astigmatism. The spherical focus on a scope can only work in one focal plane. Thus causing a condition where with corrective lenses works due to there only needing to be a fine tunning of the system, depending on how acurate your refraction is.

With out glases a patient with an astigmatism will see the vertical or horizontal lines in the retical be in focus when different focal powers are being dialed into the retical.

Sorry it I got off topic and spelling errors... racing to work.

 

[Mute]

Re: Explanation of Parallax

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mouse07410</div><div class="ubbcode-body">Mute,

Do you shoot wearing corrective eyeglasses? Or do you adjust the ocular y=for your diopter and shoot without glasses? </div></div>

I shoot with corrective lenses, so I make my adjustments while wearing my corrective eyewear.

 

[3bnRanger]

Re: Explanation of Parallax

David, Frank, Plange, and others....thanks for the input. I believe I'm slowly grasping the concept.

Frank, your analogy regarding iron sights makes perfect sense due to the importance of concentrating on your front sight post...much like quickly acquiring your front sight in handgun drills.

Also, using your example regarding different focal planes (i.e. your logo and the pen) I would think it would be easy to discern parallax....if not, I must be missing it. Either way, what I'm gathering from the posts above parallax has to do with target distance and the way the reticle is INTERNALLY adjusted to compensate for the different focal planes. This would make sense due to the varied distances I was shooting (196 and 65 yards). If I'm way off, I apologize. So now, is it possible to adjust a variable powered scope for a "happy medium" while involved in a stress course or competition?