What's Most Important in Rifle Scope Quality?

[Ledd Slinger]

The parallax adjustment is not in the scope. Parallax is caused by the aiming eye being off the scope's optical axis. By definition, parallax is caused by an angle between two viewing axes. When the aiming eye is on the scope optical axis, there is no parallax regardless of where the scope range focus is set. Scopes' range focus mechanics (opposite the windage knob or the objective lens barrel twisting) move lenses only in its optical axis, does not move the reticle.

The parallax adjustment is moving your head to put your eye on the scope's optical axis.

I know the reticle isn't 'actually moving'. But that knob on the side is not just a "side focus". It is a focus, but it's main purpose is for eliminating the parallax effect by adjusting the focus lens fore and aft to bring the reticle into the same plane of focus as the target. When both are perfectly in tune, parallax is eliminated it doesn't matter where your aiming eye is on the rear ocular. You can position your head however you want and the reticle will not swim on the target.

 

[nakneker]

Reliability and repeatability. I would rather see my bullets going into the same hole with a mediocre sight picture than have a crystal clear picture of my bullet holes all over the place.

 

[Bart B.]

But that knob on the side is not just a "side focus". It is a focus, but it's main purpose is for eliminating the parallax effect by adjusting the focus lens fore and aft to bring the reticle into the same plane of focus as the target

Therefore, fixed range focused scopes cannot be adjusted for parallax.

Is there some other way to fix this problem? Maybe do the same thing to eliminate parallax with open or aperture metallic sights.

Factually, that side knob adjusts the erector lens position to focus the first image plane image on the reticle. That first image plane moves further back as target range gets shorter; over 1/8th inch is normal going from 1000 to 100 yards but only 1/67th inch going from 1000 to 900 yards.

Parallax is not mentioned in this excerpt from a Leupold variable power scope patent about the knob opposite the windage adjustment knob"

"The focus adjustment knob moves the erector lens unit in the longitudinal direction substantially parallel to the optical system axis to focus the image of the object being viewed on the plane of the reticle and adjust the focus as the viewing distance changes over a range of from infinity to approximately 50 feet, while maintaining a substantially constant magnifcation which varies less than about l0%."

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[Bart B.]

How does one test scopes below 30X?

Use a 6X to 10X monocular focused at infinity to look into the scope's eyepiece. It will magnify the reticle and target image as much as its power.

Another scope can also be used.

 

[Turbulent Turtle]

Therefore, fixed range focused scopes cannot be adjusted for parallax.

Is there some other way to fix this problem? Maybe do the same thing to eliminate parallax with open or aperture metallic sights.

Factually, that side knob adjusts the erector lens position to focus the first image plane image on the reticle. That first image plane moves further back as target range gets shorter; over 1/8th inch is normal going from 1000 to 100 yards but only 1/67th inch going from 1000 to 900 yards.

Parallax is not mentioned in this excerpt from a Leupold variable power scope patent about the knob opposite the windage adjustment knob"

"The focus adjustment knob moves the erector lens unit in the longitudinal direction substantially parallel to the optical system axis to focus the image of the object being viewed on the plane of the reticle and adjust the focus as the viewing distance changes over a range of from infinity to approximately 50 feet, while maintaining a substantially constant magnifcation which varies less than about l0%."

.

I love this place, I'm always learning about optics and riflescopes.

The above posts corrects a misunderstanding that I've had for quite some time now. See, I always thought that the side focus knob move a lens that was located between the objective and the first focal plane, back and forth to set the distance. I always thought that the ocular lens was used to set the reticle in focus for my eye and that the distance between the ocular and the second focal plane, the one after the erector and zoom lenses would remain constant once the ocular was set. Once the ocular was set, everything up to and including the FFP was fixed. The side focus would then move the lens between the FFP and the objective to put the target in focus on the FFP.

Now I have just learned that the side focus knob moves the whole erector tube back and forth to place the FFP of the erector tub in the focal plane of the target as presented by the objective. I've never noticed my ocular lens moving back and forth while playing with the side focus, but I'll check it again next time. If it doesn't move, I'm wondering how the reticle can stay in focus as the FFP/SFP move back and forth.

Now I know why I like the adjustable objective better than a side focus. So much simpler for me to understand.

So much to learn, so many questions engendered by more information.

 

[Ledd Slinger]

I love this place, I'm always learning about optics and riflescopes.

The above posts corrects a misunderstanding that I've had for quite some time now. See, I always thought that the side focus knob move a lens that was located between the objective and the first focal plane, back and forth to set the distance. I always thought that the ocular lens was used to set the reticle in focus for my eye and that the distance between the ocular and the second focal plane, the one after the erector and zoom lenses would remain constant once the ocular was set. Once the ocular was set, everything up to and including the FFP was fixed. The side focus would then move the lens between the FFP and the objective to put the target in focus on the FFP.

Now I have just learned that the side focus knob moves the whole erector tube back and forth to place the FFP of the erector tub in the focal plane of the target as presented by the objective. I've never noticed my ocular lens moving back and forth while playing with the side focus, but I'll check it again next time. If it doesn't move, I'm wondering how the reticle can stay in focus as the FFP/SFP move back and forth.

Now I know why I like the adjustable objective better than a side focus. So much simpler for me to understand.

So much to learn, so many questions engendered by more information.

Well...I don't know how it exactly works in FFP scopes.....

FFP reticles reside in 'front' of the magnification lens. SFP reticles are behind it. So there may be differences in how the side parallax focus works from FFP to SFP scopes

 

[Bart B.]

I love this place, I'm always learning about optics and riflescopes

Learn more by reading this patent for a Leupold scope with a side focus knob...

https://patents.google.com/patent/US4643542

 

[Bart B.]

FFP reticles reside in 'front' of the magnification lens. SFP reticles are behind it. So there may be differences in how the side parallax focus works from FFP to SFP scopes

What lens is "the" magnification lens? All convex (surface curves outward) lenses magnify.

Both FFP and SFP type scopes work the same. Both focus target images at those two focal planes. Where the reticle is doesn't matter. Erector lens mechanics with the side focus knob can do most anything to move lenses in erector tubes to focus on the FFP to focus its image in the SFP. The optical engineer designs lenses to do that as they move back and forth. The mechanical engineer uses lens data to design then make small lens tubes move in the erector tube to satisfy optical formulas to move them.

Read that link I put up on a scope patent. Some dimensions are in 4 decimal places.

 

[Turbulent Turtle]

Learn more by reading this patent for a Leupold scope with a side focus knob...

https://patents.google.com/patent/US4643542

Thank you. Very interesting. This is for an SFP scope only as the objective image has not yet merged with the reticle plane.

 

[Bart B.]

This is for an SFP scope only as the objective image has not yet merged with the reticle plane.

Why hasn't it merged?

All scopes with fixed objective lenses move their image focal plane (The first focal plane) back as target range decreases. Erector lenses can move to focus that FFP image on the eye piece focal plane (SFP).

 

[Ledd Slinger]

I'm not gonna play a game of splitting hairs with terminology Bart, you know what I meant...

On another note. I've been seeing the Dual Focal plane reticle technology picking up a little bit of speed. I think DFP will be the wave of the future for hunting and tactical scopes.

 

[Turbulent Turtle]

Why hasn't it merged?

All scopes with fixed objective lenses move their image focal plane (The first focal plane) back as target range decreases. Erector lenses can move to focus that FFP image on the eye piece focal plane (SFP).

Well the drawings clearly show the reticle plane to be after the erector focus adjustment lens. The erector is only moving the objective image, it is not moving the merged reticle and FFP image.

 

[Bart B.]

I'm not gonna play a game of splitting hairs with terminology Bart, you know what I meant...

No, I didn't know what you meant. I'll assume it's the erector lenses.

 

[Ledd Slinger]

No, I didn't know what you meant. I'll assume it's the erector lenses.

Call it whatevet you want. Your assumption shows you did know what I meant.

 

[Bart B.]

Well the drawings clearly show the reticle plane to be after the erector focus adjustment lens. The erector is only moving the objective image, it is not moving the merged reticle and FFP image.

I mentioned in post #30 the erector lenses could be used to focus on a front focal plane image.

 

[Bart B.]

View attachment 1038808
That's the first time I've seen a scope diagram with the erector lens group also called a magnification assembly.

Same could be said for the ocular lens; it magnifies the SFP image and reticle, if in it. The ocular lens' focal length and objective/erector lens group determines the scope overall magnification power. Objective/erector combined group focal length divided by ocular lens focal length equals scope power. Conversely, multiplying ocular lens focal length (near 2 inches) by scope power (30X, for example) equals an objective/erector lens combined focal length of almost 60 inches.

 

[Turbulent Turtle]

I mentioned in post #30 the erector lenses could be used to focus on a front focal plane image.

Be that as it may, in the pdf of the patent, figure 2 is quite explicit in the position of the reticle plane as item 18 between item 14, the erector lens unit and the ocular. There is no reticle plane for a front focal plane that I could find.

 

[Bart B.]

There is no reticle plane for a front focal plane that I could find.

The front focal plane is between 32A and 32C, depending on target range. The reticle could be moved when range focus changed.

Here's a scope that does it another way.

https://patents.google.com/patent/US20130033746

 

[Turbulent Turtle]

The front focal plane is between 32A and 32C, depending on target range. The reticle could be moved when range focus changed.

Ok. but the text of the patent says otherwise. There is no reticle plane in 32A to 32C. There is only the front focal plane, which is what I said earlier. This is for an SFP ONLY. You can't go around moving the reticle as you will lose the ocular focus on the reticle and end up with a mess.

Here is the text that plainly says it's for focusing the objective image on the reticle focal plane 18, which is an SFP.

"The image plane of the object being viewed is shown by dashed lines 32 in FIG. 2. As the object being viewed moves from infinity to 50 feet towards the telescopic sight, the image plane 32 of such object moves from initial position 32A to intermediate position 32B, and to final position 32C toward the erector lens. If the erector lens unit 14 were fixed in position, the image planes 32A, 32B and 32C would not all be in focus on the reticle plane 18. However, by moving the erector lens unit 14 in longitudinal direction 30 for focus adjustment, each of the image planes 32A, 32B and 32C may be focused on the reticle plane 18."

 

[Turbulent Turtle]

Here's a scope that does it another way.

https://patents.google.com/patent/US20130033746

I don't have much time to look at this one but a cursory glance shows this to be a reticle that changes shape depending on the magnification used, but it still stays at the same position.

Irrespective of the focusing used in a riflescope, the prime concept remains the same. The distance from the ocular to the reticle must remain the same while you can try all sorts of things to make the objective image plane coincide with the fixed reticle plane. The reticle doesn't move around.