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Page AR-15 » Optics, Mounts, and Sights
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Posted: 12/18/2010 1:50:06 PM EST
I have a m-4 Del-ton with a fsb.

I removed my m4s from it & installed a Nikon Buckmaster 6-18x40 scope.

the mount is an Armalite with 1" rings,  1' to center of rings.

when I look from the front I can see that the fsb is taking up quite a bit of space on the front of the scope, scope is right on top of the fsb

but when I look through the rear of the scope I can only see cross hairs & my target, no front sight

the front sight can not be seen at all.

how does that happen.

always told to co-witness, mounts must be 1.5" c/l

but I have no fsb on my 24" so no problem there. I like to have my scope sitting low on the receiver
Link Posted: 12/18/2010 1:57:47 PM EST
[#1]
Anything over 1.5 - 2x magnification will blur out the FSB and make it invisible.  If you want to be able to co-witness you have to use much less magnification.
Link Posted: 12/18/2010 4:32:58 PM EST
[#2]
Quoted:
Anything over 1.5 - 2x magnification will blur out the FSB and make it invisible.  If you want to be able to co-witness you have to use much less magnification.


That is correct!  The higher the magnification, the smaller the depth of field which is the distance things are in focus.  

So a low magnification optic will have a relatively large depth of field i.e. allowing you to focus and see both your target and the very close FSB.  A high magnification optic having small depth of field will allow you to see the target and things that are close to it, but since the FSB is so far away from the target, it is completely blurred out to the extent that you will not see it at all.
Link Posted: 12/18/2010 6:06:22 PM EST
[#3]
Optics 101...focal distance.

ETA - OMFG, it can be true that optical physics are real.  
Page AR-15 » Optics, Mounts, and Sights
AR Sponsor: bravocompany
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