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Rifles, Reloading, Optics, Equipment
Long Range Scopes and Other Optics
Reticles on frontr rear focal plane, why?
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<blockquote data-quote="Randy in Va" data-source="post: 18902" data-attributes="member: 352"><p>Very good DCMI but I think you missed a major disatvantage to rear focal plane scopes. As USO states "Why front or rear focal plane placement? Question: What are focal planes and what is the difference between putting the reticle in the front or rear focal plane? Answer: Only in a variable power scope is the reticle placement a major problem. In the rear focal plane, or behind the power changing lens system (erector tube), was the first solution that occurred to optical engineers, and most American scopes are still being built that way. Unfortunately, this apparently ideal solution has a very serious flaw.</p><p></p><p>Any tolerance change in the centration of the lens system and their spherical/longitudinal movement with the power change, will shift the point of impact. A variation of one thousandth of an inch will move the zero point approximately one inch at 100 yards. Since the mechanical parts that hold the power changing lens system slide inside each other, (some allowances are made for temperature changes, manufacturing tolerances and wear), there must be some movement made to accommodate this. Consequently this lateral and vertical movement will often shift zero by as much as several inches as power is changed." This would be a good thing for a prarie dog over 300 yards!!!</p></blockquote><p></p>
[QUOTE="Randy in Va, post: 18902, member: 352"] Very good DCMI but I think you missed a major disatvantage to rear focal plane scopes. As USO states "Why front or rear focal plane placement? Question: What are focal planes and what is the difference between putting the reticle in the front or rear focal plane? Answer: Only in a variable power scope is the reticle placement a major problem. In the rear focal plane, or behind the power changing lens system (erector tube), was the first solution that occurred to optical engineers, and most American scopes are still being built that way. Unfortunately, this apparently ideal solution has a very serious flaw. Any tolerance change in the centration of the lens system and their spherical/longitudinal movement with the power change, will shift the point of impact. A variation of one thousandth of an inch will move the zero point approximately one inch at 100 yards. Since the mechanical parts that hold the power changing lens system slide inside each other, (some allowances are made for temperature changes, manufacturing tolerances and wear), there must be some movement made to accommodate this. Consequently this lateral and vertical movement will often shift zero by as much as several inches as power is changed." This would be a good thing for a prarie dog over 300 yards!!! [/QUOTE]
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Rifles, Reloading, Optics, Equipment
Long Range Scopes and Other Optics
Reticles on frontr rear focal plane, why?
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