Fixed Parallax - a real issue?

[JLH]

G'day

Got to talking today about fixed vs. variable parallax with a mate who's a new shooter, he had a bloke at one shop swear up and down that variable is required for deer hunting (sambar), whereas I know a bloke who's been hunting for many years that reckons it's a non issue.

I'm green and couldn't shoot anywhere close to 200 yards for an ethical kill yet - so I'm using a fixed Meopro. But it would be interesting to hear other seasoned hunters opinions.

[bigpete]

He's exceedingly wrong

[bladeracer]

G'day

Got to talking today about fixed vs. variable parallax with a mate who's a new shooter, he had a bloke at one shop swear up and down that variable is required for deer hunting (sambar), whereas I know a bloke who's been hunting for many years that reckons it's a non issue.

I'm green and couldn't shoot anywhere close to 200 yards for an ethical kill yet - so I'm using a fixed Meopro. But it would be interesting to hear other seasoned hunters opinions.

No, it's not a requirement.
If you have a good consistent cheek weld on your rifle then parallax becomes essentially irrelevant, especially for large targets at close ranges. Parallax is more important when you don't have a good cheek weld or no cheek weld at all, and you are shooting small targets at longer ranges. For large targets like deer just set it around 50-100m and you'll fine.

Now, if you are shooting deer at 300m or more, then parallax is more important to ensure good shot placement.

So, the true answer is that it depends on what you are using the scope for. And the reality is that you don't even need a scope for most deer hunting, try explaining that to this bloke that reckons you need adjustable parallax I've bought four rifles that I know came from enthusiastic local deer hunters, only one was scoped.

[Oldbloke]

Generally, for hunting at sensible ranges not required. Just something else to go wrong IMHO. As I found out.

In fact I would avoid it.

[JLH]

He's exceedingly wrong

which one? there's two people referenced in the story!

[womble]

The one trying to sell you the more expensive scope.

[deye243]

For hunting under 400y fixed its a non issue

[bigpete]

He's exceedingly wrong

which one? there's two people referenced in the story!

The bloke who reckons you need adjustable parallax

[JLH]

Generally, for hunting at sensible ranges not required. Just something else to go wrong IMHO. As I found out.

In fact I would avoid it.

That was my thinking too

[womble]

Well it is now after we’ve explained it to you.
There’s no need to avoid scopes with variable parallax either though. Not an uncommon feature. You don’t have to use it. I just leave mine on 25 yards.
In part because I don’t really understand parallax anyway.
And in part because your typical deer is rather shy. Not likely to have the patience for me to get out my range finder and dial in my parallax, roll out my mat, set up my bipod. Where did he go dammit.

[JohnV]

What I do is set any parallax adjustment on the zero range and I only move it for firing at targets much further out .
You can quickly set it by moving your head from side to side slightly as you adjust the range up and when the cross hair stays solid on the target as you move your head that's a good spot irrespective of whats on the actual dial . Most of my scopes don't have any parallax adjustment . When you initially set your scope up and first get the ocular focus right and then the eye relief right that helps fight parallax .
If the eye relief is too long or short the scope will have more potential parallax . Also having a stock comb that allows your eye to be kept in the center of the scope image when you have the correct cheek weld also helps stop parallax .
Fixed power scope are perfectly fine and what power suits you is a personal choice and it depends on how old your eyes are .
A good quality 4 power scope with quality lenses will give a better sharper image than a cheap one at 6 power .
If I were you when you have the money invest in a 2 to 7 variable for stalking . You get good field of view at 2 power for close shots in scrub etc. and at 7 power a 200 to 300 yard shot is very doable on a bigger target like a deer . They are light and compact .

[Die Judicii]

G'day

Got to talking today about fixed vs. variable parallax with a mate who's a new shooter, he had a bloke at one shop swear up and down that variable is required for deer hunting (sambar), whereas I know a bloke who's been hunting for many years that reckons it's a non issue.

I'm green and couldn't shoot anywhere close to 200 yards for an ethical kill yet - so I'm using a fixed Meopro. But it would be interesting to hear other seasoned hunters opinions.

Now that particular Gunshop Dealer would never be able to sell any "scruples" cos he just doesn't have any.
A typical rip off artist.

[JohnV]

One time at Morsely Park gun shop I was overhearing the sales guy pushing a 7 mm Magnum in a fairly light rifle off onto a slightly built first time gun owner . The salesman went off to let him think about it so I sidled in and whispered don't do it it will kill you with recoil if you are not used to it . Tell him you want 308 Win in the same rifle . When the salesman came back and the guy said I have changed my mind I want a 308 win he looked straight at me and I acted all disinterested and split . It's their job to sell what they have and push what they are having trouble selling . It's our job to know better .

[animalpest]

One time at Morsely Park gun shop I was overhearing the sales guy pushing a 7 mm Magnum in a fairly light rifle off onto a slightly built first time gun owner . The salesman went off to let him think about it so I sidled in and whispered don't do it it will kill you with recoil if you are not used to it . Tell him you want 308 Win in the same rifle . When the salesman came back and the guy said I have changed my mind I want a 308 win he looked straight at me and I acted all disinterested and split . It's their job to sell what they have and push what they are having trouble selling . It's our job to know better .

I wouldn't have been quite so discrete.

[Die Judicii]

One time at Morsely Park gun shop I was overhearing the sales guy pushing a 7 mm Magnum in a fairly light rifle off onto a slightly built first time gun owner . The salesman went off to let him think about it so I sidled in and whispered don't do it it will kill you with recoil if you are not used to it . Tell him you want 308 Win in the same rifle . When the salesman came back and the guy said I have changed my mind I want a 308 win he looked straight at me and I acted all disinterested and split . It's their job to sell what they have and push what they are having trouble selling . It's our job to know better .

Ahhh, Classic.

[Oldbloke]

This article seems timely.

https://sportingshooter.com.au/wisdom/p ... olve-them/

[straightshooter]

Awwww, why can't I have an adjustable parallax scope like all the big kids have?
Because!
Do you actually know what is the maximum parallax error that you might possibly experience at any given range.
Try this calculation
((radius of objective lens) x |(target distance)) - (scope parallax focus distance)| / ((2 x (scope parallax focus distance))
Now to get that amount of error you would have to position your eye an impractical distance off center with your head nowhere near the stock.
It also suggests that parallax error is proportional to target distance
If you are numerate and perceptive you will have noticed that the degree of parallax is proportional to the diameter of the objective lens. So your huge objective that "gathers more light" also makes the scope more liable to parallax error.

[bladeracer]

Awwww, why can't I have an adjustable parallax scope like all the big kids have?
Because!
Do you actually know what is the maximum parallax error that you might possibly experience at any given range.
Try this calculation
((radius of objective lens) x |(target distance)) - (scope parallax focus distance)| / ((2 x (scope parallax focus distance))
Now to get that amount of error you would have to position your eye an impractical distance off center with your head nowhere near the stock.
It also suggests that parallax error is proportional to target distance
If you are numerate and perceptive you will have noticed that the degree of parallax is proportional to the diameter of the objective lens. So your huge objective that "gathers more light" also makes the scope more liable to parallax error.

That's because the exit pupil is larger - the diameter of the "hole" through which the light passes to your eye - thus the window for placement of your eye behind it is also wider.

[JohnV]

That's true the bigger the exit pupil the more room for head position error but the faster you can mount and find the reasonable sight picture .
Smaller the exit pupil the more touchy it can be to get the right sight picture . Objective lens diameter divided by power = exit pupil size .
Like a 4 x 50 would have a 12.5 mm exit pupil . Real easy to find the sight picture . But a 12 x 50 would have a 4.2 mm . Still easy to use but more precise . A 25 x 50 would be 2mm so not the best scope for running game .
When the exit pupil size matches the size of the eye retina at the time , that's when the scope appears to be the brightest .

[straightshooter]

bladeracer and JohnV
The comments about exit pupil size in relation to parallax might seem intuitive but they are wrong.
Look at the formula. It does not include in the calculation either exit pupil size or magnification.
A large exit pupil diameter allows a fair amount of lateral variation in eye positioning without vignetting which in itself leads to potential parallax error.
In the precision shooting world "mirage caps" are now making an appearance. Their main function is to reduce the depth of field of the scope and it would also appear, as the formula explains, that they provide a side benefit of reducing the extent of potential parallax error.

[bladeracer]

bladeracer and JohnV
The comments about exit pupil size in relation to parallax might seem intuitive but they are wrong.
Look at the formula. It does not include in the calculation either exit pupil size or magnification.
A large exit pupil diameter allows a fair amount of lateral variation in eye positioning without vignetting which in itself leads to potential parallax error.
In the precision shooting world "mirage caps" are now making an appearance. Their main function is to reduce the depth of field of the scope and it would also appear, as the formula explains, that they provide a side benefit of reducing the extent of potential parallax error.

The calculation includes objective lens diameter, which dictates exit pupil diameter, regardless of magnification. Exit pupil is objective diameter divided by magnification. A 4x50 has EP of 12.5mm, a 4x32 has EP of 8mm. The larger objective thus gives you more room to move your eye around behind the exit pupil, thus more room to be offset from the centre of it. My 40x56 has an EP of 1.4mm, very difficult to move your eye at all and still see down the "tube" of light, thus very difficult to have any parallax error at all.

To use your formula ((radius of objective lens) x |(target distance)) - (scope parallax focus distance)| / ((2 x (scope parallax focus distance)):
I'm assuming that using the standard objective sizes which are given in millimeters that the answer is also in millimeters, regardless of your units of distance.
40mm objective at 100m with 100m parallax, then at 200m without changing the parallax.
((20) x |(100)) - (100)| / ((2 x (100))
1900/200=9.5mm at 100m
Could there really be 10mm offset at 100m just from parallax despite having the parallax set to the correct range? It seems unlikely to me. With a 40-power scope I would think 20mm of "float" around the target would be very obvious to me, so obvious that I would be looking to adjust the parallax.

((20) x |(200)) - (100)| / ((2 x (100))
3900/200=19.5mm at 200m
This is believable but seems less than I'd expect to see with the parallax set at half the target distance, I would expect to see a lot more "float" across the target than 40mm.

((28) x |(100)) - (100)| / ((2 x (100))
2700/200=13.5mm at 100m

((28) x |(200)) - (100)| / ((2 x (100))
5500/200=27.5mm at 200m

[straightshooter]

It looks like I didn't do a very good job presenting the formula because I was a bit lazy so now I did it in McPaint.
So look at the attached picture of the equation.
a = the scope objective radius
b = the difference between the target distance and the scope parallax focus distance
c = twice the scope parallax focus distance

I think the way I presented element b within vertical lines denoting absolute value was the source of the confusion.

[JohnV]

What you don't understand is we are not dealing with the maximum parallax any lens system can have . We are dealing with the typical amount of parallax that most shooters experience and that's way smaller than predicted by your formula . It's not applicable to the average sighting errors experienced , if it was then super small groups would be impossible to shoot and there not .

[straightshooter]

You are indeed fortunate to possess the power to know what I understand or don't understand.
Has it occurred to you that the purpose of my posts may have been to enable interested readers to be able to apply some sort of numeric value to the magnitude of parallax error that may possibly be encountered and thus make their own decisions regarding the need for adjustable parallax compensation.
To quote Lord Kelvin
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."

[Oldbloke]

I agree, measuring is always good.
But I suspect for many, just average hunters who restrict ranges to say,,, 200yards, just have a look through through the scope at a target, preferably with a grid at a measured distance (perhaps 100yards) and move your head/eye left to right will be good enough.