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Scope on limit of adjustment |
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Gerry Atric
Optics Journeyman Joined: April/21/2011 Location: Sweden Status: Offline Points: 340 |
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Posted: March/24/2019 at 06:27 |
Read about parallax and black ring, was taught that "trick" at young age, I was also taught that one should never use the last part of turret adjustment because the image get deteriorated at the scope edges. I have lived by that "rule" by using adjustable bases and Burris Signature Zee rings but what happens optically with a deviation from the optical axis and how bad is it? The question is on my bucket list of knowing-before-dying, have not planned a hasty departure but....... Gerry Atric
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Gerry Atric
Optics Journeyman Joined: April/21/2011 Location: Sweden Status: Offline Points: 340 |
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Should I leave this fantastic life without an answer to above mentioned question ?
Gerry Atric
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bugsNbows
Optics God bowsNbugs Joined: March/10/2008 Location: North Georgia Status: Online Points: 11201 |
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PM Ilya.
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If we're not suppose to eat animals...how come they're made of meat?
Anomymous |
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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Here are two references that should help you out:
ILya's article on this is an excellent treatise on how things really work.
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Opinion,untempered by fact,is ignorance.
There are some who do not fear death... for they are more afraid of not really living |
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Gerry Atric
Optics Journeyman Joined: April/21/2011 Location: Sweden Status: Offline Points: 340 |
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Thank´s bugsNbows and Kickboxer I will PM my headaches to Ilya. Have read both references before, it could be that I´m struggling with a foreign language but I don´t see the answer to my question in any of those excellent references.
Gerry Atric
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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There is a mechanical limit to the adjustment range of a riflescope. The erector tube assembly moves inside the maintube and there are stops that at some point arrest its movement in all four directions.
There is also an optical limit to how far you can move the erector tube and still get a good quality image. Ideally the mechanical limit and the optical limit should match and in some scopes they do. However, in a many scopes, at the end of adjustment the image gets vignetted or distorted. However, many times it is still OK for aiming although edge performance suffers. This is really specific to a particular design. Optically, this is sorta what happens or at least I think this is a good way of explaining it. Objective lens creates an image of a particular size in the front focal plane. The erector system samples a small portion of that image. Depending on where inside the larger image created by the objective that is, the erector is looking at a different segment of the FOV. Essentially imagine a small circle (image sampled by the erector system) inside a large circle (image created by the objective). When you spin the turrets, a small circle moves inside the large circle. If it never gets close to the edge of the large circle, image quality does not suffer (much), but as it gets close to the edge it starts picking up aberrations. ILya
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Gerry Atric
Optics Journeyman Joined: April/21/2011 Location: Sweden Status: Offline Points: 340 |
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Thank´s Ilya
Now I can hunt in peace in heaven (or hell) Gerry Atric
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tpcollins
Optics Journeyman Joined: January/12/2009 Status: Offline Points: 428 |
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I had a conversation with Vortex once when I was maxing out the elevation turret on my crossbow’s XBR scope, and the poi moved to the right.
He said if the scope rail isn’t perfectly centered with the centerline of the shooting rail, then the windage turret would need to adjusted to correct this - as is normal with most weapons. If the erector tube is set to the left of center, and as the turret starts to max out on the up elevation, the left front edge of the erector tube hits the inside of the scope tube and pushes it over to the right. This moves the reticle at the back of the erector tube to the left, and to bring it to the target you have to move the crossbow’s line of sight to the right which cause the poi to be further right than it was with less elevation. |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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That is correct for cheap scopes. Better scopes usually have internal adjustment limiters, so that the adjustment range is rectangular. ILya
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tpcollins
Optics Journeyman Joined: January/12/2009 Status: Offline Points: 428 |
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The Vortex PST XBR 2.5-10x44mm with illumination originally retailed for about $800 and sold for $600 before they were discontinued. Not sure if that’s considered a cheap scope or note.
Here a link - same exact scope, just different reticles, but with 1 MOA per click, I still own two. Competitor link removed.
Edited by koshkin - April/11/2019 at 10:41 |
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koshkin
MODERATOR Dark Lord of Optics Joined: June/15/2004 Location: United States Status: Offline Points: 13182 |
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I am familiar with that scope. When Vortex first released, it had some unforeseen issues, so I ripped it to shreds. They ended up being forced to re-design the eyepiece entirely. It was still a bit of a failure with centerfires, but it found some success in the crossbow world. Generally, in that price range there should already be adjustment limiters. ILya
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