Rifle Scopes Parallax question

[4dds]

Could someone explain the effect parralax will have on your poi at 600+ yards. Most of the time when shooting I forget to adjust mine from 100 where it stays. Just wondering if I’m leaving something on the table or if it has little to no effect.


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[FourT6and2]

Yes, it will affect your POI. How much? Hard to say. It will be different for every scope and every set of eyes. Move your head around behind your scope and you'll see your reticle move. So... that's how much error it will induce.

 

[spife7980]

With it set for 100 position it to where you are centered on your target at 600. Without disturbing the rifle move your head left and right, up and down and you will see exactly how much effect it has.

edit: damn, beat me to it as I was typing

 

[4dds]

Thank you. Sounds like I need to start making sure not to skip that step.


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[koshkin]

Calculating maximum parallax error is not difficult, but it depends on the scope objective, magnification use, the size of your eye pupil and how well the eyepiece is set-up.

Or you could just try it and see like has already been suggested, which is a really good way to go from a practical standpoint. Keep in mind, that it is really important to have your eyepiece set-up correctly, or you can really see some cooky things.

ILya

 

[Deleted member 10043]

Isn't he in essence treating his scope like a 100 yard fixed parallax scope by leaving it set at 100. Shoot a 5-10 shot group on both settings to experiment the difference on target. Of course, don't change the dope, lol. I'm assuming correctly you are using the eye piece to focus on target? Otherwise, parallax set at 100 is going to give a shitty picture at 600. Time behind your scope will eventually tell you when you forgot to adjust parallax. The eye piece should be a one time and leave alone adjustment.

BTW, koshkin is being modest... http://opticsthoughts.com/?page_id=476

 

[Graywolf.260]

If your parallax is correct at 100 and your target is at 600, the max error is roughly [(600-100)/100] x objective radius. If you have a 50 mm objective, that means about a 5 inch error.

 

[goosed]

If your parallax is correct at 100 and your target is at 600, the max error is roughly [(600-100)/100] x objective radius. If you have a 50 mm objective, that means about a 5 inch error.

Ummm... are you sure it's 5 inches?

 

[BootyLord]

In the above formula is the second 100 a constant? For example if I had a fixed parallax scope at 125 yards would I still divide by 100? Also, am I correct in thinking the answer is to this equation is in mm?

 

[Graywolf.260]

The error term is the ratio of the difference between the zero parallax distance (100yds in the first case) and the actual target distance (600 yds) divided by the zero parallax distance. If your zero parallax distance was 125 and the target was at 600 yds then the ratio would be 475/125. That ratio is multiplied by the objective radius to get the actual max error. The radius of a 50 mm scope is 25 mm which is .98 inches. The error in mm is 25mm x (475/125).

 

[Baron26]

The error term is the ratio of the difference between the zero parallax distance (100yds in the first case) and the actual target distance (600 yds) divided by the zero parallax distance. If your zero parallax distance was 125 and the target was at 600 yds then the ratio would be 475/125. That ratio is multiplied by the objective radius to get the actual max error. The radius of a 50 mm scope is 25 mm which is .98 inches. The error in mm is 25mm x (475/125).

Great explanation Graywolf.

 

[Deleted member 10043]

That is spot on. Looks like an insurance person created a nifty calculator...

https://www.lelandwest.com/parallax-...calculator.cfm

 

[goosed]

That is spot on. Looks like an insurance person created a nifty calculator...

https://www.lelandwest.com/parallax-...calculator.cfm

While that link is a good theoretical estimate and a great starting point. Life is rarely that simple... this is no exception. Was really hoping Graywolf would continue to expand on this, hence my earlier question, so I could avoid going down the rabbit hole. Since he didn't and now a link doubling down has been introduced let's actually take a peek at the rabbit hole without going too far down...


Let us start with the formula provided on here "R x (T-P)/P"..

1.) We did not establish that we need the absolute value of "(T-P)" as the error can never be less than zero. For example 100 yard parallax with a 25 yard target would give us (-75)... That is impossible as there is no way to be more prefect than zero error. Thus we need the absolute value of "(T-P)" which for this example would be 75.

2) The formula as it sits ignores magnification. This is great if we aren't using magnification. If we are using a magnified optic the equation changes.

3) The formula assumes zero offset of your eyeball from center of the scope. If your eye is off any amount we also need to change the formula

4) R is equal to the radius of the objective in the above formula, however it is really equal to the 0.5 x EP(exit pupil). Not an important distinction in our basic formula as exit pupil is equal to objective diameter, but of increasing importance as we continue to add other variables.

I'll stop there for now as there are entire papers dedicated to the subject if one is really so inclined to go down this particular rabbit hole.

 

[4dds]

If your parallax is correct at 100 and your target is at 600, the max error is roughly [(600-100)/100] x objective radius. If you have a 50 mm objective, that means about a 5 inch error.

Wow. That’s way more than I was expecting.



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[4dds]

While that link is a good theoretical estimate and a great starting point. Life is rarely that simple... this is no exception. Was really hoping Graywolf would continue to expand on this, hence my earlier question, so I could avoid going down the rabbit hole. Since he didn't and now a link doubling down has been introduced let's actually take a peek at the rabbit hole without going too far down...


Let us start with the formula provided on here "R x (T-P)/P"..

1.) We did not establish that we need the absolute value of "(T-P)" as the error can never be less than zero. For example 100 yard parallax with a 25 yard target would give us (-75)... That is impossible as there is no way to be more prefect than zero error. Thus we need the absolute value of "(T-P)" which for this example would be 75.

2) The formula as it sits ignores magnification. This is great if we aren't using magnification. If we are using a magnified optic the equation changes.

3) The formula assumes zero offset of your eyeball from center of the scope. If your eye is off any amount we also need to change the formula

4) R is equal to the radius of the objective in the above formula, however it is really equal to the 0.5 x EP(exit pupil). Not an important distinction in our basic formula as exit pupil is equal to objective diameter, but of increasing importance as we continue to add other variables.

I'll stop there for now as there are entire papers dedicated to the subject if one is really so inclined to go down this particular rabbit hole.

Thank you. You’ve went further down the rabbit hole than I was ready for but an explanation is what I asked for and you have expanded on it quite well. Now time for some research.

 

[Ol Rob]

If your check weld is perfectly aligned in the same position for both groups you should see no variation. Parallax is where inconsistent check weld shows up. Please correct me if I have misspoken.
Rob

 

[goosed]

If your check weld is perfectly aligned in the same position for both groups you should see no variation. Parallax is where inconsistent check weld shows up. Please correct me if I have misspoken.
Rob

Correct, if your cheek weld, distance to target and other variables stay the same you will see no variation in groups. That doesn't mean there is no error in the system, but rather the amount of error has remained constant. When we change a variable, such as moving our cheek weld position, the amount of error likely changes hence we see it show up through POI changes.

Don't get too bogged down by this as the solution really is as simple as moving your head and turning a knob. Being a little bit off isn't the end of the world. Just go to any PRS match and pay attention to how often you see winning shooters change their parallax setting during a stage with multiple targets at different distances.

 

[Graywolf.260]

Yeah, being off in your setting isn't necessarily a big deal. That formula gives the maximum error that occurs when your eye is way off-axis (at the edge of the exit pupil). If you you have a perfectly reproducible cheek weld and your gaze is always centered down the optical axis (centered on your fovea), then you will have zero parallax even though your scope parallax setting doesn't match your target distance.