Rifle scope focus vs parallax

A

Andy1

New member
I've always used rifle scopes with the standard two adjustment knob set up. I'm aware of scopes with the third knob on the left. To my memory they've always been listed as the SF or Side Focus knob. Self explanatory.

I was on Natchez today looking at scopes and I saw a Weaver 4-16x44 with a third knob listed as a SFP, Side Focus Parallax adjustment.

Is this a just a regular side focus knob? I've never really understood what parallax is, other than it has to do with long range shooting.

I live in PA where all my shots are under 100 yards, so I never bothered to learn about it.
 
Most shooters don't' need to ever worry about parallax. Most big game rifles are set for 150 yards, most shotgun scopes about 75 yards and those intended for 22's at 50 yards.

With any scope, at any range parallax will never be an issue as long as the shooter gets his eye directly behind the scope. But if your head is off to one side and you're not looking straight through the scope, and if you're shooting at 100 yards with a scope set for 150 or 50 yards your bullet will impact about 1/4" from where you aim, less than the diameter of most bullets. You'll be off between 1/2" to 3/4" at 500 yards.

It takes a pretty darn accurate rifle, a great shooter, and some distance before it starts to make any difference. And as long as you use good technique it makes zero difference.

The only time I find the ability to focus helpful is when shooting at close range with higher magnification. I often shoot a 22 at ranges up to 250 yards with a 4-12X scope. That scope won't focus at 50 yards if I go above 7X or so. As long as I keep it 7X or less it focuses fine. At 100 yards or farther I have no issues above 7X. I do have one rifle with a scope with parallax adjustment that will allow me to focus at 50 yards.
 
I have a scope with the "side focus" knob.
Problem is, it affects both focus and parallax at the same time and is seldom correct for both at once. The adjustable objective is not as handy for the snipper but is optically better.
 
In short, if your shooting 100yds you don't need it. watch this video from leupold, it explains it fairly simply

https://www.youtube.com/watch?v=Dy2QwkINuAY

my explination, it focuses the crosshairs at longer distances. Most rifle scopes have a set paralax of 100-150yds. most rimfire, shotgun, and muzzleloader scopes are between 50-70yds.

for example, a rifle scope with a paralax of 100yds is shooting a 300yds, the target will be in focus, but the crosshairs will not. The other issue this creates with the 2 separate focuses, image and crossharis, is if you move your head around the crosshairs will move around on the target. basically if you have inconsistent cheek weld it will move your point of impact around

all this to say, if all your shots are under 100yds, and most rifle scopes are set at 100yds you should be fine. its really not practical to try to range a target under 100yds and try to set a paralax while hunting unless your shooting farther out.

if you want to play with it, assuming rifle scope with 100yd paralax try this next time you shoot, set up a target at 50yds. use a sight in target with markings. line up like you would normally and move your head or eye up down left and right, the crosshairs will move around on the target. probably not more than 1/2 to 1/4 in in all directions if I had to guess. If you have a consistent cheek weld you will never notice it unless you are looking for it.
 
If it says side focus then obviously it is, but don't assume that any knob on the left side is for side focus. Sometimes it is for an illuminated reticle
 
Here's the design of a Leupold scope:

https://patents.google.com/patent/US4643542A/en

It shows and explains how the erector lenses adjust to focus the first target image plane on the second image plane where the reticle is. First image plane moves back about 2 tenths inch as the target range goes from 1000 to 50 yards; evidenced by observing adjustable objective lenses having to move forward that far to refocus target

There was a link that shows the eyepiece lens focusing the reticle on the eye retina. That's incorrect as the eyepiece lens acts like a magnifying glass sending the reticle and target image light rays parallel into the eye that focuses them on the aiming eye retina.

Scope makers should never have called that side knob a parallax adjustment. All it does is change the erector lens positions to focus the first image plane on the reticle as it moves back and forth as target range changes.

There will never be any parallax when the aiming eye is on the LOS axis that goes straight back from the reticle center through the eyepiece lens center into the aiming eye. Regardless of what range the objective lens system is focused at. The LOS angle goes through the objective lens several thousandths inch below and a few to one side of its center depending on the scope's alignment to the bore axis, amount and direction of LOF angle change during barrel time and bullet drop at target range.

Look up the unabridged dictionary definition of "parallax."
 
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Jim Watson said:
I have a scope with the "side focus" knob.
Problem is, it affects both focus and parallax at the same time and is seldom correct for both at once.
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it's designed to do both. Properly adjusted and used, it will correct both at once perfectly.

If you adjust the side knob until the reticle stays in place on the target while moving your eye around, perfect range focus happens and there's no parallax.

Rifle scopes are exactly like camera lenses because they're perfectly focused and parallax free at only one range.
 
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So, parallax is the misalignment between the focal point and the image. As you shift your head, the reticle moves on the object, causing a possible miss if you are misaligned.

LINK <- this won’t help!

IMO, it really only matters in cases of extreme accuracy.

So, you are looking at scopes with parallax adjust for 100 yd shooting? You are looking at the wrong scopes. If you buy a parallax adjusting scope, it will need adjusting. Look for something without the knob and it will have a larger “sweet spot of focus range.
 
Parallax exists when there's an angle between the eye and two distant items and one's further away.

That link explains why very good except it confused the issue mentioning "adjusting parallax."
 
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it's designed to do both. Properly adjusted and used, it will correct both at once perfectly.

If you adjust the side knob until the reticle stays in place on the target while moving your eye around, perfect range focus happens and there's no parallax.
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Well, it doesn't, on either of two examples of Major Big Name Brand scope.
Get the parallax out and the image is fuzzy. Clear up the focus and the crosshairs "move" with head position.

Maybe if I twiddled the eyepiece focus more. Turning the whole eyepiece on fine threads is tedious and you might fatigue your eye into apparent focus before you get it right.

I had a cheaper scope with neither of those problems. The adjustable objective was settable for parallax (even though it was at the 77 yard mark when right at 100 yards) and the binocular type eyepiece was easy to focus for clear image.
 
Parallax varies greatly from one scope to another, set the gun in the rest solidly and move your head around behind the eyepiece and watch for movement of the reticle on the target.
I disagree about it not mattering on any scope including big game rifles at least here in the west where shots are longer, that commonly repeated phrase that you just have to have your head in the same spot behind the scope everytime doesn't account for shooting in field conditions. Thick jackets and bulky clothing, shooting from any position other than off the bench where you sighted in the gun will effect point of impact.
I've sold several fixed focus/parallax scopes because they've had a ridiculous amount of parallax in them at normal hunting ranges.
 
Jim Watson said:
Well, it doesn't, on either of two examples of Major Big Name Brand scope.
Get the parallax out and the image is fuzzy. Clear up the focus and the crosshairs "move" with head position.

Maybe if I twiddled the eyepiece focus more. Turning the whole eyepiece on fine threads is tedious and you might fatigue your eye into apparent focus before you get it right.

I had a cheaper scope with neither of those problems. The adjustable objective was settable for parallax (even though it was at the 77 yard mark when right at 100 yards) and the binocular type eyepiece was easy to focus for clear image.
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That happens every time when all scopes are set up using your sequence of adjustments.

First, adjust the eye piece to focus the reticle for your eye. Do this aiming the scope on a blank area (clear sky or wall) and stop looking after 2 seconds, reticle must appear sharp instantly. Tighten the eye piece locking ring.

Then adjust range focus such that there's no movement of target image relative to the reticle as you move your eye around.

Is this mentioned in the scope instructions? What make and model and I'll look them up.

All scope objective lens systems adjust target focus on the reticle and nothing else optically. They also control the LOS angle and direction it leaves the scope to the target.

Focal length tolerances in objective lenses is usually why the scope focuses at 100 yards when the focus knob's set to 77. Assuming the eyepiece is correctly adjusted.
 
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Double K said:
Parallax varies greatly from one scope to another, set the gun in the rest solidly and move your head around behind the eyepiece and watch for movement of the reticle on the target.
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Not if their eyepiece is focused on the reticle and they're all focused at the same range. Then they'll all have zero parallax at that range.

Yes, Andy1, that knob on the left is the target range focus one.
 
"Not if their eyepiece is focused on the reticle and they're all focused at the same range. Then they'll all have zero parallax at that range."

Therein lies the problem, they're not all focused at the same range. I have half a dozen 4x12 scopes w/o adjustable objectives or side focus, same mfg. two of them are set the same the rest are different. Those those are hunting scopes so I sight them in at 200yds, it seems to be less of an issue the farther downrange you go.
I've had several other scopes that when focused on the target had parallax, those got sent back to the mfg for repair.
 
Double K said:
I've had several other scopes that when focused on the target had parallax, those got sent back to the mfg for repair.
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That's impossible in my opinion.

Did you set the focus to some range mark? They're typically not exact because the lenses have tolerances in their optical properties. Range marks on the focus knob are only approximate.

Did the maker explain how they were fixed? My guess is the front objective lens was moved a few thousandths so all the range marks had minimum focusing error to minimize parallax error.
 
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First, adjust the eye piece to focus the reticle for your eye. Do this aiming the scope on a blank area (clear sky or wall) and stop looking after 2 seconds, reticle must appear sharp instantly. Tighten the eye piece locking ring.

Then adjust range focus such that there's no movement of target image relative to the reticle as you move your eye around.
Click to expand...

OK, I looked up the maker's instructions and that is what they said.

One question they and you do not cover: When adjusting the eyepiece focus on the reticle, what position should the side parallax knob be in? Or does it matter?
 
"Did you set the focus to some range mark? They're typically not exact because the lenses have tolerances in their optical properties. Range marks on the focus knob are only approximate."

If you read my post #15 the scopes are w/o adjustable objectives or side focus, in other words without.
I wasn't going to mention the mfg but I'm so disappointed in their product lines, quality control and custom shop in the last 10 years I might as well, Leupold. The exact scope is the VX-1 4x12.
If you were around in the 80's and 90's buying and using their products you'll remember what a fantastic company they were before the bean counters took over the company.
 
Double K said:
"Did you set the focus to some range mark? They're typically not exact because the lenses have tolerances in their optical properties. Range marks on the focus knob are only approximate."

If you read my post #15 the scopes are w/o adjustable objectives or side focus, in other words without.
I wasn't going to mention the mfg but I'm so disappointed in their product lines, quality control and custom shop in the last 10 years I might as well, Leupold. The exact scope is the VX-1 4x12.
If you were around in the 80's and 90's buying and using their products you'll remember what a fantastic company they were before the bean counters took over the company.
Click to expand...
Yup, I overlooked that.

Were you able to determine what range they were focused at?

Keep in mind that no scope has parallax problems when optically focused on the target. Several yards error on the range scale is common and the scope focus may need be set to 200 yards to be parallax free at 100 yards.
 
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