second image plane

[bjlooper]

Hi
I'm looking at a Swarovski and the description says "reticles in the second image plane". What does this mean?
DR B

 

[Paul Wyatt]

here's a link to us optics explanation of the two focal planes

http://www.usoptics.com/sub_pages/reticles.php

 

[Bart B]

Good link Brass Bender. Too bad it doesn't tell folks that reticules in the first focal plane don't stay centered when adjustments are made if the reticule is moved separate from the erector tube. Some rifles have their scopes mounted so far off parallel to the bullet's initial trajectory that they are way off towards the edge in the field of view.

One has to mount such installed front focal plane reticule scopes such that they can be boresighted to a bit below bore axis by moving the mounts. Otherwise, the off-center reticule is a way of life.

If the reticule in a front plane installation is mounted in the erector tube, then it will stay centered because it stays in the center of the scope's optical axis.

 

[Chrismadrid]

[ QUOTE ]
Good link Brass Bender. Too bad it doesn't tell folks that reticules in the first focal plane don't stay centered when adjustments are made if the reticule is moved separate from the erector tube. Some rifles have their scopes mounted so far off parallel to the bullet's initial trajectory that they are way off towards the edge in the field of view.

One has to mount such installed front focal plane reticule scopes such that they can be boresighted to a bit below bore axis by moving the mounts. Otherwise, the off-center reticule is a way of life.

If the reticule in a front plane installation is mounted in the erector tube, then it will stay centered because it stays in the center of the scope's optical axis.

[/ QUOTE ]

ALMOST nobody makes FIXED image (non centred reticle) scopes any more - and certainly nobody in USO's zone of price. USO/S&B/KAHLES/ZEISS etc mount the FFP reticle in the erector internal tube.

With the exception of some older russian designs and "made to order" scopes for mounting on "classic" guns - every FFP scope I've seen has a permenantly centred reticle. One exception Sheperd - but his FFP is deliberately fitted in the fixed tube.

EVEN so a FFP can have FOCUS induced POI shift. Esp devices with ROTATIONAL front focus. It's RARE but possible.

 

[Bart B]

[ QUOTE ]
... a FFP can have FOCUS induced POI shift. Esp devices with ROTATIONAL front focus. It's RARE but possible.

[/ QUOTE ]Agreed. That happens when the objective lens isn't mounted square with its rotational axis; it's cocked a little. The focused image is a bit off the mechanical axis and its center will be seen to rotate about the reticule center as the objective lens barrel is rotated.