Why do 1x prisms even use a prism inside?

[Goloco]

Why do 1x prism optics still need to use a non-magnifying prism when true 1x can be achieved by a flat pane of glass? It seems like many 1x prisms on the market do not really achieve true 1x or have heavy distortion when moving one's eye off the center point due to an imperfect prism.

 

[Gawernator]

Why do 1x prism optics still need to use a non-magnifying prism when true 1x can be achieved by a flat pane of glass? It seems like many 1x prisms on the market do not really achieve true 1x or have heavy distortion when moving one's eye off the center point due to an imperfect prism.

A flat pane of glass can transmit an unmagnified view, but simply etching a reticle onto it does not create a functional optical sight.


The central problem is collimation. An etched mark on a flat window physically exists only a few inches from your eye. Your eye must focus on either the nearby reticle or the distant target, and any lateral movement of your eye changes the reticle’s apparent position against the target by a large amount. It would behave more like looking through a marked windshield than through a properly corrected aiming optic.


A 1× prism sight is essentially a compact telescope adjusted for unity angular magnification:

1. The objective lens forms an image of the outside scene.
2. The etched reticle is placed at or near an internal focal plane.
3. The prism erects the otherwise inverted image and folds the optical path, allowing the sight to remain short.
4. The ocular lens presents the target and reticle together as a collimated or nearly collimated virtual image, usually adjustable with the diopter.

That is why the prism is present even though the finished optic is nominally 1×. It is not primarily there to magnify. It is part of the image-forming, image-erecting, and packaging system. Prism designs can place the etched reticle directly on a prism surface and fold a relatively long effective optical path into a compact housing.


A reflex sight solves the same basic aiming problem differently. Its LED reticle is reflected from a curved, partially reflective lens that collimates the reticle, making it appear at optical infinity. The front lens may look like a simple window, but optically it is not merely a flat pane with a dot painted on it.


Your criticism of many 1× prisms is valid, however. “1×” usually means approximately unity magnification along the central optical axis, not that the entire field is geometrically identical to naked-eye vision. Designers must balance:

• field of view;
• eye relief and eyebox;
• optic length and weight;
• edge distortion;
• chromatic aberration;
• pincushion or barrel distortion;
• manufacturing cost.

A sight can be nearly true 1× in the center while producing substantial “fishbowl,” rolling, or image-shift effects as the eye moves off-axis. That is not necessarily caused by the prism being “imperfect” in the sense of defective workmanship. It can be an inherent consequence of the complete lens-and-prism design, although inexpensive glass, loose tolerances, poor alignment, or inadequate correction can make it considerably worse.


So the basic answer is:


A flat window gives you a natural 1× view, but not a collimated, focus-compatible, reasonably parallax-controlled etched reticle. A 1× prism is a complete unity-power telescope, not merely a window that happens to contain a prism.

 

[Goloco]

ah thanks, I suspected that might have been the reason why.

 

[Gawernator]

ah thanks, I suspected that might have been the reason why.

np say thanks to ChatGPT, I am way too tired to write that all out myself lol

 

[pancakes]

A flat pane of glass can transmit an unmagnified view, but simply etching a reticle onto it does not create a functional optical sight.


The central problem is collimation. An etched mark on a flat window physically exists only a few inches from your eye. Your eye must focus on either the nearby reticle or the distant target, and any lateral movement of your eye changes the reticle’s apparent position against the target by a large amount. It would behave more like looking through a marked windshield than through a properly corrected aiming optic.


A 1× prism sight is essentially a compact telescope adjusted for unity angular magnification:

1. The objective lens forms an image of the outside scene.
2. The etched reticle is placed at or near an internal focal plane.
3. The prism erects the otherwise inverted image and folds the optical path, allowing the sight to remain short.
4. The ocular lens presents the target and reticle together as a collimated or nearly collimated virtual image, usually adjustable with the diopter.

That is why the prism is present even though the finished optic is nominally 1×. It is not primarily there to magnify. It is part of the image-forming, image-erecting, and packaging system. Prism designs can place the etched reticle directly on a prism surface and fold a relatively long effective optical path into a compact housing.


A reflex sight solves the same basic aiming problem differently. Its LED reticle is reflected from a curved, partially reflective lens that collimates the reticle, making it appear at optical infinity. The front lens may look like a simple window, but optically it is not merely a flat pane with a dot painted on it.


Your criticism of many 1× prisms is valid, however. “1×” usually means approximately unity magnification along the central optical axis, not that the entire field is geometrically identical to naked-eye vision. Designers must balance:

• field of view;
• eye relief and eyebox;
• optic length and weight;
• edge distortion;
• chromatic aberration;
• pincushion or barrel distortion;
• manufacturing cost.

A sight can be nearly true 1× in the center while producing substantial “fishbowl,” rolling, or image-shift effects as the eye moves off-axis. That is not necessarily caused by the prism being “imperfect” in the sense of defective workmanship. It can be an inherent consequence of the complete lens-and-prism design, although inexpensive glass, loose tolerances, poor alignment, or inadequate correction can make it considerably worse.


So the basic answer is:


A flat window gives you a natural 1× view, but not a collimated, focus-compatible, reasonably parallax-controlled etched reticle. A 1× prism is a complete unity-power telescope, not merely a window that happens to contain a prism.

That was beautiful thank you