first focal plane scope - does POI change if you adjust parallax

Johnf66

Johnf66

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have a Optisan Optics CP 4-16x40 F1 FFP Non Illuminated F1MOA16 Side Focus Rifle Scope,

I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.

Question I have is will my point of impact change if I alter the focus/parallax out further to say 40 yards?

getting lots of mixed answers so thought I'd ask you guys.
 
Johnf66 said:
have a Optisan Optics CP 4-16x40 F1 FFP Non Illuminated F1MOA16 Side Focus Rifle Scope,

I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.

Question I have is will my point of impact change if I alter the focus/parallax out further to say 40 yards?

getting lots of mixed answers so thought I'd ask you guys.
Click to expand...
You shouldn’t if your eye is centred on the eyepiece, but if your eye is not centred correctly then you will get a slight POI change if the parallax setting is different to the target distance.
 
Johnf66 said:
have a Optisan Optics CP 4-16x40 F1 FFP Non Illuminated F1MOA16 Side Focus Rifle Scope,

I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.

Question I have is will my point of impact change if I alter the focus/parallax out further to say 40 yards?

getting lots of mixed answers so thought I'd ask you guys.
Click to expand...
It should not !!

In FFP even the tension doesn’t change during magnification.
 
Stephen@ said:
You shouldn’t if your eye is centred on the eyepiece, but if your eye is not centred correctly then you will get a slight POI change if the parallax setting is different to the target distance.
Click to expand...

No it shouldn’t, but also adjusting the parallax to the correct range (ie sharp focus) minimises the effect of poor eye position too. As Stephen says, if it isn’t sharp, then you have the parallax incorrectly adjusted and head position will make a different. This would apply to FFP and SFP scopes equally…
 
thanks guys that has been really helpful.
I think I might need to drop my cheek piece a little bit as my eye might not be central enough on the scope and do a little more testing.
 
This is yet another of my gripes with FFP scopes, as above, the answer is "it shouldn't", but because so many of these scopes try to cover large mag ranges we end up with reticules which in many cases are absolutely useless for fine detail like we use in air gunning when we are talking mm's for a target, as opposed to inches for full bore :rolleyes: So using one for a 15mm kill zone at 30m is a bit different from a 5inch dear heart at 300m Shrug tt
 
@jesim1 Not trying to be argumentative and happy to be corrected but isn't 15mm at 30m demanding the same accuracy as 5" at 300m?

For simplicity, let's say we are looking at 12.5mm (half an inch) at 30m, then using similar triangle rules (from my very rusty geometry store in an ageing brain)
10 x 30m = 300m
10 X 0.5" = 5"
 
GrumpyGrandpa said:
@jesim1 Not trying to be argumentative and happy to be corrected but isn't 15mm at 30m demanding the same accuracy as 5" at 300m?

For simplicity, let's say we are looking at 12.5mm (half an inch) at 30m, then using similar triangle rules (from my very rusty geometry store in an ageing brain)
10 x 30m = 300m
10 X 0.5" = 5"
Click to expand...
It is, but the problem is the reticule, if you buy a medium to high mag FFP scope then the reticule at low mag is all but useless, and since we use air rifles at closer ranges, then less mag is needed, making trying to judge the spacings on a FFP scope difficult - so why buy one?

1742807706171.png



You can see the problem here, at high mag you can see, and judge, the spacing in the ret, but at low mag it all just blends into one, which is not a problem on a large target, but smaller ones really show it up - and we shoot smaller ones ;)
 
Johnf66 said:
I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.
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I don't understand this. Are you stating the focus dial is off somehow or is the target a bit out of focus?

Shouldn't you just adjust the focus control to get the target pin sharp regardless of what is actually on the focus dial? Is that what you have done and that is the result?

POI is more likely to change if you don't have a reliable head position.
 
Johnf66 said:
I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.

Question I have is will my point of impact change if I alter the focus/parallax out further to say 40 yards?
Click to expand...
I'm not sure anyone has answered the question, not because they are wrong but because of the way you've framed the question.
If you are asking, "Will the POI at 30yds change?" the answer is., "No." If you mean, "Does the holdover for 40yds change when you change parallax?" the answer is, "No." Does the zero change?... No. This all assumes the scope is perfect and you aren't inducing any parallax error. But, of course, the POI at 25yds is not the same as the POI at 30yds or at 40yds. Once the scope is correctly zeroed, changing the parallax/focus shouldn't change the size of the target image, the size of the ret or the intercepts.

I can't understand why you would want to set the parallax at 25yds then zero at 30. Shrug tt
 
BallisticBill said:
I can't understand why you would want to set the parallax at 25yds then zero at 30. Shrug tt
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thanks for your reply Bill very informative. I say its 25 yards as that's what it reads on the knob. The image is clear at 25 and at 30 but knowing the scope manufacturers these side markings are probably not that accurate and are only a guide. I think my main issue is that I might have my cheek piece a little to high and not having my eye central with the scope. I only shoot at that weekends so going to adjust the cheek piece and try a few more shots during the weekend. As Andy said head position is important to get the same position each time
.
 
Johnf66 said:
have a Optisan Optics CP 4-16x40 F1 FFP Non Illuminated F1MOA16 Side Focus Rifle Scope,

I have zeroed at 30 yards. with my parallax set on the side for roughly 25yards.

Question I have is will my point of impact change if I alter the focus/parallax out further to say 40 yards?

getting lots of mixed answers so thought I'd ask you guys.
Click to expand...
1. you've zeroed at 30yds but your parallax says 25ds .... get out the tap measure check the real distance to the target . confirm the range .

2. if you alter the parallax ring to 40yds but shoot at a 30yd target . the target will be out of focus .


what magnification are you shooting on . x4 . x6 . x12 . x16
at lower mag you will have more leeway on focus at low mag . whack the zoom right up to focus on the target . focus then turn the mag down
 
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Someone can correct me on this test.

lock the sight in place (or gun) so that you can move your eye around, without moving the sight at all.
Set a visual target with a good number of features, like large text on a magazine, put the ret on a very identifiable point of some character, then circle your eye around the optical centreline of the scope, does the reticle move off the exact point you started with?

Parallax correction relies on the distance being set correctly, so we must set the objective (reticle) focus first, or we will see the target 'focused' at an incorrect parallax distance.
I wear varifocals so add in an extra complication, but to me the goal is to have the target in focus, exactly at the parallax correction range, or the sight will be sensitive to head movement.

That is what I remember from back in the day explanations of fixed parallax, but I have not tested enough modern scopes, to see if it is indeed possible, to dial-out parallax completely.
 
Technically it shouldn't if the scope is well made and it certainly shouldn't where you're only talking about a 5yd difference in what the scope has been set to and the actual distance you're shooting at.
If your scope is set to 30 yds and you then shot at 50 yds say then yes the POI could shift slightly due to the possibility of parallax error if your eye is not central in the eye piece.
But even then the POI shift would or should miniscule.
Obviously if you zero the scope at one distance then shoot at another distance the the POI will alter due to the fact pellets don't have a perfectly flat trajectory and have an arced trajectory.
For example a scope zeroed at 30 yds will have the same POI at around say 10 yds.
Shooting at any other distance the POI may oe will alter slightly and depending on what that distance is the POI may or will be slightly higher or lower.
And of course calibre i.e .177 or .22, the type of pellet and its weight will also affect how much the difference is.
For example two sub 12ft.lb guns one say in .177 and one in .22 both with scopes zeroed at the same distance the difference of the pellets trajectory will be diferent and the .22 will have a more curved trajectory so shooting at any other distance than what the scope is zeroed to will require more holdover/holdunder than the .177.
The difference would be even more pronounced between a .177 and a .25 calibre
This is why when you zero a scope at a certain distance and shoot at a distance different to that you have to employ a certain amount of holdover or hold under for the pellet to strike the desired POI.
If your scope is focussed/parallax corrected to 30 yds and you shot at 25 or 35 yds and your eye was not central in the eyepiece the amount of possible parallax error would be 1.5mm or less.
 
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I just tried a Hawke SFP, and was checking if my 'focussed' distance, was also the exact parallax distance, it was, nearly.
Once the parallax is set at the correct distance, the reticle was static (in relation to target), with head movement, but my focussed distance was nearly always a tad off.

Target at 25y and parallax set to 40 the POI of the scope will not move, but is subject to parallax error, so will appear to move move, if the eye is not exactly in line with the optical centre.

I think this would be true for first, or second focal reticles. On both there will be a correct setting, but depending on our eyes, and any corrective lenses, it might be hard to ensure we set that, from the appearance (focus) alone.

So in answer to the OP question, the POI does not move if the parallax is not set at the right distance, but anything other than the correct distance, opens the POI to head movement error.

I did try and record it, but it is hard enough filming through the eyepiece, without moving it around to show parallax. Distant tree 75 yards, close about 20.
The 20y twigs move when set at 75, less so when set at ~20.

View attachment 711168
 
Thanks Guys, that was fairly comprehensive. I guess ffp scopes are much more expensive?
 
Mongoose said:
Thanks Guys, that was fairly comprehensive. I guess ffp scopes are much more expensive?
Click to expand...

They are a little, but don't need the break the bank if you want to try.
I have a Vector Veyron (or two) and they are decent optics, FFP fairly compact scopes.
I cannot use the 3-12x44 much below about 6x as my eyesight cannot distinguish the fine reticle at those mags.
Nothing wrong with the scope, but the detailed reticle it becomes equivalent to single black hair on a patterned carpet, when out in the wild.
If you can, try a few FFP scopes, as it is a hard balance to have it usable across a range of magnifications (as @jesim1 above).
With decent (younger) eyesight, and your intended use, a scope like the Veyron might even work well across its zoom range.
 
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