Fixed Power Scopes

Close, but not quite correct:
A plain Jane fixed scope only has two lenses-one at each end. A fixed power with AO adds one more. A plain variable has at least three lenses, and the fourth for the AO or paralax ring.
Click to expand...
A fixed power scope needs at least 3 lens groups. Better quality scopes have at least 2 individual lens elements cemented together in each group.

8293461901_26ea269198_z.jpg


The front group called the objective group focuses a reversed image in front of the middle lens group often called the erector group. Sometimes this objective group can be moved back and forth like a camera lens to focus the target image at different ranges at precicely the same place in front of the erector lens group behind it.

The erector group focuses the image from the objective group reversed again, or "erected" behind it in front of the rear group called the eyepiece group where the reticule is usually located. The inner tube holding the erector lenses is what's moved by the elevation and windage adjustments.

The eyepiece group focuses on the reticule where the now erect image is and it's adjusted for the shooter's eye's optical characteristics so it and the reticule appears sharp and easy to see.

Variables have two sets of lenses in their erector group. Those two groups move back and forth different amounts changing the image size on the reticule to make it appear closer or further depending on the magnification selected on the zoom ring. As their lens mount mechanics have tolerances, so do their alignment with each other. As both lens groups move back and forth as the scope's power is changed, those two groups will not do so exactly the same. So, the image will move around a bit as the scope power changes. Good variable scopes will have no more than about 1/4 MOA image shift on the reticule in a figure 8 pattern as the power's changed. Others will have as much as 3/4 MOA or more.

For absolute precision, use a fixed power scope. If one must use a variable, then a small amount of slop's going to exist in how the target image is focused on the reticule, so set the scope to maximum power to hold the erector lens groups as solidly as possible when best precision is desired.

A scope's magnification is calculated by dividing the focal length of the combined objective and erector lens groups by the focal length of the eyepiece group. The eyepiece group for all rifle scopes has about a 2 inch focal length which is good for most human eyes. So, a 20X scope's objective-erector lens focal length is 40 inches. But the optical properties of telescopes are such that the physical length of them is shorter than their longest focal length set of combined lens groups. That's how a 20X rifle scope can be only 15 inches long.
 
Last edited:
For absolute precision, use a fixed power scope. If one must use a variable, then a small amount of slop's going to exist in how the target image is focused on the reticule, so set the scope to maximum power to hold the erector lens groups as solidly as possible when best precision is desired.
Click to expand...

So are you saying the reticle will appear to move slightly, or will there be subtle differences in parallax at different powers?
 
Coyota1, the target image for a given power is focused on the scope's reticule. Both the objective and erector lens groups combine to do that for scopes with reticules in the second (rear) image plane. If any of their lenses or groups are moved out of alignment with each other, the target image will move on the reticule. This happens even when the objective lens group is set to focus the target image exactly on the reticule and no parallax exists.

Lens group misalignment causes lateral shift (up, down, left, right) of the target image on the reticule. The objective lens group focusing on the target only changes the front to back movement of the target image on the reticule and if it's not on the reticule, then parallax will occur when the aiming eye's off the optical axis of the scope.

If you put an optical collimator in the barrel, adjust the scope W and E knobs to put the scope reticule on the collimator reticule, changing power on a variable will cause the scope's reticule to move around and off of the collimator reticule. That's caused by the mechanical slop in the two erector group's two lens mounts that move back and forth in the erector tube. Two cams are typically used to do that; one for each lens barrel. There has to be a tiny amount of clearance so no binding will happen. Some scopes are better/worst than others in this regard. Precision machining of lens mounts costs money if done to minimum tolerances. With a 10X scope, for every .0014" of elevation or windage misalignment of the target image on the reticule, there'll be about 1/4 MOA of error. This is why benchresters prefer fixed power scopes.

And yes, there will subtle parallax problems when changing power if both zoom lens groups don't move exactly the same amount front to back. Such errors cause their focal length to change a bit and that will refocus the target image either in front of or behind the reticule.

I'll gladly take a scope with excellent mechanics and average image quality to a scope with average mechanics and even the very best image quality. Image quality is easy to discern. Mechanical quality is a totally different thing and impossible to measure accurately by shooting unless you shoot your stuff zero MOA all the time.

Check the image in my earlier post for details of a rifle scope's internal stuff.
 
Last edited:
Fixed power provides an economical advantage when shooting at at known "range" of distances where the appropriate magnification can be chosen.

At 1000, 20x might be an issue if there is a strong mirage.

However, a variable can always be dialed down, and there are circumstances where you want the MOST magnification you can get at long range.

The SOCOM PSR Program has selected the S&B 5-25-x 56 PMII for it's Precision Sniper Rifle.

http://www.schmidtundbender.de/en/news/news.html
 
Bart B,this old Bushnell 4x has a moving reticle, so when I sighted it in, the reticle is not directly above the bolt on my 722. My concern is that if sighted in at 100 yards, the bullet might cross and hit to the left further out. It was too windy at the range for accurate 200 yard shooting. I might have to wait for spring. I still like the fixed power idea.
 
Coyota1, if your scope's 1/10th inch off center to the left, zero it to center groups 1/10th inch to the left of the aiming point. It'll be 1/10th inch off the aiming point from here to the moon.

I've seen scopes and metalic sights mounted 2 to 3 inches off center to the left for left eye dominant right hand target shooters. All they do is make a slight change to their windage zeros for each range.
 
Keep in mind that another disadvantage to fixed power scopes is that you are unable to enlarge the field of view to re-acquire your target.

While proper form does certainly mitigate things like bipod "hop", long range targets can be re-acquired much more easily with a quick dial-down and then zoom back to the desired magnification.

If you're looking for a high-quality, fixed power scope, check out the Sightron SIII's on closeout at SWFA.

http://swfa.com/Sightron-20x42-SIII-30mm-Riflescope-P45296.aspx
 
For hunting, I believe in general, that the best scope ever made was a 2 x 7 variable. I have tried a lot of different scopes and still hold these as the best all around. I have used the Leupold 2 x 7 for probably 35 yrs.

I don't like bigger variables except maybe for a 2.5 x 8 tops. I find most 3 x 9's to not have enough field of view on 3 power for fast running shooting, however I used to use 12 power scopes on pistols in competition so I am not against higher powered scopes when think I need them. And I guess I could get by with a fixed power 4 for most of my hunting, and I bought into the fixed 6 power 6 x 42 Leupold for a fine 25-06 Model 70 that I have, but I recently lost a coyote, that I would have killed if I had been using my .308 BLR with the 2 x 7 power set on 2x.

I like high power and I can use it too, when I have time to watch game for a little bit, but I want low power so I can shoot at moving game at close range or even running game at a distance, I do better with a low powered scope.

But even a 4x fixed has too little field of view for my taste on the low end.

But what I really want to know is who the dammed idiot is at Leupold that ruined my favorite scope the 2 x 7 vari-xII or shall I say now days the VX-II. The current crop of VX-II's do not have a locking ring on the eye focus adjustment. It's a fast focus set up. This is stupid. Who needs a fast focus adjustment on a simple little 2 x 7 power scope. The darned thing keeps moving on me if I am not careful when changing the power setting ring. I called an complained, and hope others do, to get this great scope back to it's roots as a fine hunting scope, where you set the eye focus and forget it. I just can't believe how stupid Leupold was on this move.
 
Last edited:
But what I really want to know is who the dammed idiot is at Leupold that ruined my favorite scope the 2 x 7 vari-xII or shall I say now days the VX-II. The current crop of VX-II's do not have a locking ring on the eye focus adjustment. It's a fast focus set up. This is stupid. Who needs a fast focus adjustment on a simple little 2 x 7 power scope. The darned thing keeps moving on me if I am not careful when changing the power setting ring. I called an complained, and hope others do, to get this great scope back to it's roots as a fine hunting scope, where you set the eye focus and forget it. I just can't believe how stupid Leupold was on this move.
Click to expand...


Blue Duck,

I'm with you on this, I hate the fast focus eyepiece that seems to be the standard on most scope manufactured today.
I'll take a lock ring eyepiece any day over the fast focus crap.

Here's a fix I used on my fast focus eyepiece scopes.
I took an intertube from a 10 speed bike, cut some 1 inch rubber bands, slipped them over the fast focus eyepiece on to the scope tube and eyepiece which makes it much more difficut to turn the eyepiece.
Not as good as a lock ring, but close.

Best Regards
Bob Hunter
www.huntercustoms.com
 
This is what industry does, think about it, in 10 years or so Leupold will bring it back as the "Classic" model, and itll sell like hotcakes.;)
 
This is what industry does, think about it, in 10 years or so Leupold will bring it back as the "Classic" model, and itll sell like hotcakes.
Click to expand...

hooligan1,

Yes sir, you are so correct.
A good example of this is Colts 1911 pistols.
Colt will charge you more for a new series 70 style gun over the 80 series when the 80 series requires additional machine work and parts.
But the 70 series is a "Classic".

Back to the subject of scopes, I've never seen a need to focus my scopes in the field once I had them focused.
Of course this don't hold true with the fast focus feature as the eye pice seems to be getting moved out of focus all the time.

Best Regards
Bob Hunter
www.huntercustoms.com
 
Bob, if your fast-focus eyepieces don't stay in focus and they've not physically moved, I don't think the scope's at fault. It's your aiming eye that has the problem.
 
Bob, if your fast-focus eyepieces don't stay in focus and they've not physically moved, I don't think the scope's at fault. It's your aiming eye that has the problem.
Click to expand...

Bart B,

I'll be the first to admitt my old eyes are not as good as they once was.
My issues with the fast focus eye piece is they get moved, I 've moved them by just slinging the rifle over my shoulder.
I much prefer the lock ring eyepiece, even with my old eyes I don't seem to have issues with it.

Best Regards
Bob Hunter
www.huntercustoms.com
 
You must log in or register to reply here.
Back
Top