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Scope Length, Advantage/Disadvantage?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/14/2010 at 03:12
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Optics GrassHopper
Optics GrassHopper


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This is a question for the technical folks.... Something I've always been curious about. Big Smile
 
What truly determines a scopes length?  And what benefit does longer/shorter have over the other?  Lot's of high power scopes are very long, but high powered short scopes do exist and continue to pop up (Burris Timberline, Bushnell 6-24x40 mildot, etc..).
 
Take a given scope, the 4.5-14x44 Conquest for example, shorten it 3-4 inches, and what happens?? Field of view change? Picture size? Depth of field? Parallax? Loss of sunshading?
 
Thanks!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/14/2010 at 04:58
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Optics Jedi Knight
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A scopes length is determined by the optical properties of the objective lens used. The objective lens curvature determines the focal length, usually about 3 inches in a normal scope. The ocular lens or eyepiece again focusses the image to the viewers eye.
 
Nothing changes if a scope is shortened. It only means that different curvature lenses were used to achieve the same result.
FOV is determined by the lens stop ring used and has nothing to do with the length of the scope.
Depth of field is determined by the legth of the focal point, which is determined by the lens curvature. The shorter the focal point, the more of an "out of focus " image will appear in focus. Parallax error is effected by the same issue, as the picture is focused either in front of or behind the stop. The reticule is fixed to the stop ring.
 
So, in short, you cannot just take a scope and squash it shorter. Nothing will work. You can take a shorter tube and use different lenses, which will make this scope work. If the same size field stop was used in a long scope and a short scope, the FOV would be the same, provided that the same focal length has been used. If a different focal length is used a different field of view will be seen.
 
The benefits of a shorter scope is mainly in weight and size.


Edited by 8shots - January/14/2010 at 05:10
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/14/2010 at 07:07
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Optics Jedi Knight
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A scope with a long focal point will be able to be focused more precise for a sharp image, with very little depth of field. So it will be very sensitive to distance for maintaining sharp focus.
This will require a longer scope tube, hence that high end scopes for precise focussing tends to have longer tubes.

Edited by 8shots - January/14/2010 at 07:09
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/14/2010 at 08:11
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Optics Journeyman
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I have noticed that most short and compact scopes tend to have smaller FoV's than the same power scope with a longer tube.
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