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focal plane for Mil-Dot

Printed From: OpticsTalk by SWFA, Inc.
Category: Scopes
Forum Name: Tactical Scopes
Forum Description: Police and military tools of the trade
URL: http://www.opticstalk.com/forum_posts.asp?TID=8422
Printed Date: December/12/2017 at 00:44


Topic: focal plane for Mil-Dot
Posted By: tiny68
Subject: focal plane for Mil-Dot
Date Posted: November/08/2007 at 12:27
Figure you guys would know. I own several fixed power Mil-Dot scopes and used then for correction for windage and holdover for shooting in the field. I have owned one cheap Mil-Dot variable in which the Mil-Dot spacing was true only at one power setting (it was actually off quite a bit on that setting). I have been told that the upper-end scopes have the reticle in a different focal plane so that it magnifies with the power setting, so you have true Mil-Dot spacing with all scope settings. It this true? If so, which focal plane is it? I am considering a Zesis Conquest 6.5-20x50 MilDot which Zesis told me it has the reticle in the 2nd focal place and said it was not magnifying. They didn't directly answer my question, so I am not sure what that means.

I want to may sure I know what I am getting before I drop a grand or near on any scope.

Thanks for any clairification, tiny



Replies:
Posted By: Dale Clifford
Date Posted: November/08/2007 at 12:41
first focal plane will mill on any power, sfp are calibrated at some power. conquest is sfp,  the accuracy of the distance reading does not depend on ffp, or sfp, it is conditioned on the qualilty and accuracy of the reticle subtensions, 


Posted By: tiny68
Date Posted: November/08/2007 at 14:43
What is the cheapest quality variable Mil-Dot that has the reticle in the FFP? Most don't say in the specs.


Posted By: David
Date Posted: November/08/2007 at 15:07

Originally posted by tiny68 tiny68 wrote:

What is the cheapest quality variable Mil-Dot that has the reticle in the FFP? Most don't say in the specs.

 

Barska makes some affordable scopes with a FFP mil dot.

 

http://www.swfa.com/c-1046-barska-tactical-rifle-scopes.aspx - LINK



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Posted By: Urimaginaryfrnd
Date Posted: November/08/2007 at 15:36
Just a clue if it does not specifically say it is FFP (First Focal Plane) then it is not.  IOR makes a couple of FFP scopes and Leupold Mark 4 makes a couple of FFP scopes a 3.5-10x40 and an 8.5-25x50 with a 6.5-20x50 to be out sometime in a few months.  There is a big advantage as the hold over stays the same at all powers.  The TMR (tactical milling reticle) is very nice also.

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"Always do the right thing, just because it is the right thing to do".
Bobby Paul Doherty
Texas Ranger


Posted By: tiny68
Date Posted: November/08/2007 at 15:51
I have looked at Barska at gun shows.  I am not impressed with their optical clairity.  I see the different LR/T Marks.  Never picked up on that.  It seems that if you want the FFP you pay $200 for the same scope.  At least that is what I observe with the 3.5-10x40.  I don't what to spend more than a grand.  Anybody for a quality scope for less than a $1k in the 20-24x range and FFP?


Posted By: RONK
Date Posted: November/08/2007 at 22:34

 I have an IOR in a 2.5-10 with the MP-8 milling reticle in the Front Focal Plane. It ranges accurately at any magnification,  but it is kind of hard to differentiate the finer graduations when set much below 5 or 6 power anyway. If I had to do it over, I'm not sure that I wouldn't get one in the Second Focal Plane and just try to be sure it was set on the proper magnification when ranging. I don't generally use the reticle for holdover, though, preferring to dial.YMMV....

  It was less than a grand and is otherwise a GREAT scope, but not as much magnification as you're looking for.



Posted By: spreader
Date Posted: November/09/2007 at 20:37
Typical Mil-Dot ranging reticle (when properly made) in scopes with reticle in Second Image/Focal Plane (SFP) is designed to be used with magnification set at 10x. When you use MilDot in SFP scope at different magnification, you would simply need to take into account the fact that reticle doesn't get reduced or enlarged as you go through magnification range.

In other words, if you get a scope with MilDot reticle with magnification range 6.5-20x, it means that for correct ranging you will have to use it at 10x. Technically speaking, if you're using it at 20x, you will have to multiply your results by two and if you use it at 5x, divide by two.

So, if you see a two meter high target at 20x that measures four mildots (in other words is subtended by angle of four milliradian (mrad)), normal calculation tells you
2meters/4 * 1000 = 500 meters. However, since you measured at 20x, you need to multiply by two, therefore correct distance is 1000 meters.

Keep in mind though that there are quite a few manufacturers that seem to believe that making their MilDot reticle in their SFP scopes range correctly at highest magnification is a good idea. In other words, they will make a scope with magnification range of 4.5-14x and MilDot reticle in SFP that would be designed to give correct ranging at 14x, as opposed to standard 10x. This could be useful if that's the only scope you're using, but if you're using different scopes designed to use MilDot properly at different magnifications, then it is simply confusing. This is yet another reason why I personally believe that if you want MilDot reticle, it must be in First Focal Plane.






Posted By: RONK
Date Posted: November/10/2007 at 08:19

Originally posted by spreader spreader wrote:

Typical Mil-Dot ranging reticle (when properly made) in scopes with reticle in Second Image/Focal Plane (SFP) is designed to be used with magnification set at 10x. When you use MilDot in SFP scope at different magnification, you would simply need to take into account the fact that reticle doesn't get reduced or enlarged as you go through magnification range.

In other words, if you get a scope with MilDot reticle with magnification range 6.5-20x, it means that for correct ranging you will have to use it at 10x. Technically speaking, if you're using it at 20x, you will have to multiply your results by two and if you use it at 5x, divide by two.

So, if you see a two meter high target at 20x that measures four mildots (in other words is subtended by angle of four milliradian (mrad)), normal calculation tells you
2meters/4 * 1000 = 500 meters. However, since you measured at 20x, you need to multiply by two, therefore correct distance is 1000 meters.

Keep in mind though that there are quite a few manufacturers that seem to believe that making their MilDot reticle in their SFP scopes range correctly at highest magnification is a good idea. In other words, they will make a scope with magnification range of 4.5-14x and MilDot reticle in SFP that would be designed to give correct ranging at 14x, as opposed to standard 10x. This could be useful if that's the only scope you're using, but if you're using different scopes designed to use MilDot properly at different magnifications, then it is simply confusing. This is yet another reason why I personally believe that if you want MilDot reticle, it must be in First Focal Plane.




 

 I must admit those are some good points to consider. They also help build a good case for a fixed-power scope for many applications.




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