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© 2016 by Kacie Day, an independent contractor for Rex Reviews

  1. The REX CAVE
  2. Scope Questions
  3. Snell Refraction and hold over
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nobletitus
Dec 29, 2017

Snell Refraction and hold over

3 comments

Edited: Dec 30, 2017

Someone brought up an issue I had not heard about, it was Snell's Law on refraction and how it affects hold over. The video to illustrate this was: https://www.youtube.com/watch?v=yfawFJCRDSE

 

Although I'm sure no scope is probably impervious to this, I pointed out the shape of the lens may not be the same shape as the lens(es) in a scope. I also surmised that optics companies would be aware of this and correct/adjust for it. Also since the lens is mounted within a scope, essentially a tube, the extreme angles demonstrated in the above video may not apply.

 

Rex, would you be able to expound on the subject? Is Snell's law a consideration of manufacturers? How do they account/correct for it? How much error (and where) can the user expect to see? How much would it affect holdover - would it affect a lot of holdover that uses the peripheral part of the lens, is it negligible for most holdovers, etc.?

 

Thanks!

Frank

 

update: I was able to find a link to the copy of the document. I apologize in advance if this is something that is clearly fu fu - I’d just like more discerning eyes to help me understand it. Thanks in advance.

 

http://www.mountainshootingcenter.com/wp-content/uploads/2015/01/What-is-a-Mil-Snells-Law-and-Superior-Mirage-1123.pdf

 

 

 

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Geoff Glover
Dec 29, 2017

Frank,

 

I'm no scientist and darn sure I'm not Rex! But here are my observations on the demonstration, as it pertains to rifle scopes. The two other videos I found talked about flat surfaces, not curved as lenses are.

Nowhere have I ever been shooting in an area where a concentrated beam of light is the main source of illumination. I'm sure the beam makes demonstrating the effect easier, but it's not very realistic.

That piece of glass is a heck of a lot thicker than the glass of any scope. It is also on the wrong plane. Light can't enter a scope lens through the edge-unless the scope is really messed up- and I've not heard of a lens manufacuterer making flat lenses, yet.

A better way to demonstrate would have been to use an overhead camera, a round lens and etched flat glass to project the image on the paper.

What the effect would be on holding over I suspect depends many factors. The quality of the glass and coatings, how well the scope is put together, how true the reticle is to the units it subtends, distance and magnification used all come to mind. I'll leave it to those who's heads are a bit less block shaped than mine to expound further on how much effect there may be.

Rex explains at great length how light is brought back to the same focal point in quality scopes iusing multiple lenses in the 101 series.

 

 

 

0
nobletitus
Dec 30, 2017

If it helps, it was mentioned by Brien Ward, who wrote a lengthy document I downloaded to read. Just skimming it, it is a bit over my head with physics equations. He wrote the document in 2012. I can send a copy if necessary. It's funny because the link to the document was quickly taken down by the Facebook moderators of the group it was in. I don't know if he's bringing up a valid point or spreading some hocus pocus.

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Geoff Glover
Dec 30, 2017

I suspect that it's not a big deal for the most part, if the scope uses good quality glass. As a proportion of all the light entering the scope, the amount coming in from the pretty sharp angles is probably elatively small, and the curvature of the lenses is going to bend the light in any event.

My thought on the matter at this late hour is that it's something to put in the back of your mind. If you are going to use hold overs at distance, it's better to learn your rifle, loads, and scope. See what works and use it.

But it is an interesting thought,

 

0
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