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1st FP, why is it better than 2nd?

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DWilly View Drop Down
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    Posted: February/16/2019 at 23:55
 Not talking about the ranging and hold over on any power, i understand that part. what i would like to know is mechanically, why is the 1st FP better than the 2nd?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 06:38
The information below is from U.S. Optics:

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. 

A better solution is to place the reticle in the front focal plane, or ahead of the power changing lens system. The movement of the erector system will, optically, have no effect on the point of aim here. So why don’t all scope manufacturers build them this way? The downside of this method is that Americans typically do not like reticles that grow in size when the power is turned up. There is no actual growth in the reticle size. As the magnification increases, so does the reticle along with the objects in the field of view. A one inch dot reticle will still be one inch, at any power, be it low or high. It is only the appearance that is altered. If the power is turned from 2x to 4x, or doubled, the size of the objective image is doubled, and so is the reticle along with it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 21:49
thanks, but i still don't understand why the point of impact doesn't change with ffp. the reticles are rigidly mounted in the scope. the point of impact changes when the erector tube moves the picture in relation to reticle. it seems to me that there would still be variation whether the reticle is in front or back of the erector tube? this is what i'm not understanding.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 22:21
With ffp the reticle is in front of the magnification system. So it is not physically affected/changed when you turn the mag ring.  So it stays still, no movement.  With sfp it is in the power change system. So when you turn that mag dial the reticle is physically moving foward and backwards. So it can cause point of impact changes because of that movement. 

Modern scopes and machines can make them pretty good today so i dont think its as big of a movement as it used to be. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 22:26
Lifetime warranty and excellent customer service don't mean a thing when your gun fails during a zombie attack.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Urimaginaryfrnd Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 22:36
Originally posted by DWilly DWilly wrote:

thanks, but i still don't understand why the point of impact doesn't change with ffp. the reticles are rigidly mounted in the scope. the point of impact changes when the erector tube moves the picture in relation to reticle. it seems to me that there would still be variation whether the reticle is in front or back of the erector tube? this is what i'm not understanding.


When you look through a second focal plane scope and zoom from 3x to 9x as the image increases in size going from low to high the reticle always looks the same.  Did you notice that as the image increased in size the field of view side to side becomes increasingly more narrow yet at the same time the reticle appears to remain exactly the same. 

With a First Focal Plane scope as you zoom from 3x to 9x as the image increases in size the reticle proportionately appears to also increase in size.  Yes the field of view becomes increasingly more narrow as magnification increases.  The fact that the reticle size is tied to the current magnification means that it is always a true representation of the units in which it is designed to measure either MOA or MIL. 

If you still don't get it just use a fixed power scope and you don't have to worry which it is because its always the same.

Your actual point of impact can be the result of many things that influence the course of the bullet like wind, gravity.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/17/2019 at 23:46
Urfrnd, i understand all of that, it's not what i'm asking. 

173, i don't think the reticle is mounted in the erector tube on 2fp. i think it's behind the tube which is what causes the variation. i get it, the changing magnification, moving parts. but i still don't understand why it doesn't affect the ffp the same as it would a rfp.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 09:59
I didnt say it was in the erector tube. Its back in the magnification system by the magnification ring. So it is in the part of the scope that physically moves. The reticle of ffp is forward closer to the objective on the complete other side of the erector system. So it doesnt move. 

Google ffp vs sfp and look at images. There are pictures that will show you the difference. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 12:02
my understanding from what i've read is that the magnification lenses are in the erector tube. not trying to be confrontational, just saying what Steiner scopes stated. it still seems to me that if it moves, it wouldn't matter if the reticle was in front or back, it would change the poi.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 12:17
No worries. The reticle only moves when it is in the sfp position. In ffp it is not in a part of the scope that moves. It is in a fixed position in front of all the moving parts. 

I guess i am stuggling to see how u are not understanding this. when the reticle is in a position in the scope that causes it to physically move it can affect poi. When the reticle is built into a place where it does not physically move it doesn’t affect poi. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cbm Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 15:45
 
Maybe I am not getting what your aasking, but as you can see in this picture, you can't use the 2nd focal plane the same as the first on 10x or 5x. Because of the size of the targt compared to the reticle size, the distances change for every power range. Where on a 1st focal plane it doesn't. I don't know what mechanically makes POI shift, but the size of the reticle compared to the target is pretty easy to understand how they both can't be used in the same way.


Edited by cbm - February/18/2019 at 15:52
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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 19:01
Originally posted by DWilly DWilly wrote:

my understanding from what i've read is that the magnification lenses are in the erector tube. not trying to be confrontational, just saying what Steiner scopes stated. it still seems to me that if it moves, it wouldn't matter if the reticle was in front or back, it would change the poi.

That is partially correct.  

POA/POI changes when the reticle moves within the image.  In FFP scopes, the reticle is superimposed on the image coming from the objective lens prior to the erector system.  In SFP scopes, the reticle is superimposed onto the image after the erector assembly.

In FFP scopes, if the erector assembly misalignment moves the target image around a little, the reticle image is already in there, so they move together and POA/POI does not change.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 19:07
Originally posted by supertool73 supertool73 wrote:

No worries. The reticle only moves when it is in the sfp position. In ffp it is not in a part of the scope that moves. It is in a fixed position in front of all the moving parts. 

I guess i am stuggling to see how u are not understanding this. when the reticle is in a position in the scope that causes it to physically move it can affect poi. When the reticle is built into a place where it does not physically move it doesn’t affect poi. 

In either case, the reticle does not move when you change the magnification.  The image of the scene moves around as it propagates through the erector lens system (between the turrets and the magnification ring).

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 22:04
In FFP scopes, if the erector assembly misalignment moves the target image around a little, the reticle image is already in there, so they move together and POA/POI does not change.

ILya

so if the reticle image doesn't change with regards to the image coming through the erector tube, how would you change the POI? the turrets don't move the reticles, just the image.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 22:30
The image of the scene moves around as it propagates through the erector lens system.
 Ilya

i'm having trouble understanding why this doesn't move the POI. the image is moving and the reticle is not. this is what i've been trying to grasp!

please understand i'm not arguing with anyone, i'm just trying to understand this concept
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 22:48
Thanks for explaining it Ilya. My bad for explaining it incorrectly.  Sorry DWilly
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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2019 at 23:59
Originally posted by DWilly DWilly wrote:

The image of the scene moves around as it propagates through the erector lens system.
 Ilya

i'm having trouble understanding why this doesn't move the POI. the image is moving and the reticle is not. this is what i've been trying to grasp!

please understand i'm not arguing with anyone, i'm just trying to understand this concept

Think of it this way: the movement of the image of the reticle with respect to the image of the target is what is important.  

If the reticle is in the front focal plane, it does not move with the respect to he target and POI does not shift.

If the reticle is in the second focal plane, it moves with respect to the target and POI shifts.

With high quality scopes, the movement of the image is well controlled, so there is no POI shift, but it still happens some time with scope that are less well optimized.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: February/19/2019 at 00:04
Originally posted by DWilly DWilly wrote:

In FFP scopes, if the erector assembly misalignment moves the target image around a little, the reticle image is already in there, so they move together and POA/POI does not change.

ILya

so if the reticle image doesn't change with regards to the image coming through the erector tube, how would you change the POI? the turrets don't move the reticles, just the image.

The turrets move the erector tube together with the reticle cell that is either in front of it in the FFP scope or in the back of it in the SFP scope.

The objective lens system creates an image in the front focal plane that is comparatively oversized.  The front of the erector tube moves around within that oversized image when you adjust the turrets.

The movement of the image that I was talking about earlier is what happens when you adjust magnification.   In a FFP scope, your POI can not shift with magnification.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: February/19/2019 at 08:08
The movement of the image that I was talking about earlier is what happens when you adjust magnification.  Ilya

i understood this, was just trying to understand the difference between magnification movement and erector tube movement. they seem to be the same thing as relates to the reticle. if you could elaborate on where exactly the reticles are positioned, that would be a big help. i've always envisioned them rigidly attached to the outside tube, either in front of or behind the erector tube. but i'm beginning to realize they are in/on the erector tube. if that's the case, what you are saying makes sense to me. also, i've been looking at this as my eye looks through the scope and that's backwards. viewing the light as it comes from the front and goes to the rear is helping to clarify things.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/19/2019 at 09:10
Here is a Swaro SFP scope.


 I would have posted this earlier but couldn't get it to go on my phone.  This is why I was thinking it the reticle was moving with the magnification adjustment.



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