OpticsTalk by SWFA, Inc. Homepage SWFA     SampleList.com
Forum Home Forum Home > Scopes > Rifle Scopes
  New Posts New Posts RSS Feed - How an Erector Tube Works in Variable Mag Scope
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Visit the SWFA.com site to check out our current specials.

How an Erector Tube Works in Variable Mag Scope

 Post Reply Post Reply
Author
Message
GarageShopIt View Drop Down
Optics Apprentice
Optics Apprentice


Joined: May/23/2014
Status: Offline
Points: 50
Post Options Post Options   Thanks (0) Thanks(0)   Quote GarageShopIt Quote  Post ReplyReply Direct Link To This Post Topic: How an Erector Tube Works in Variable Mag Scope
    Posted: May/24/2014 at 11:44
I've been studying ray traces and cross sections for scopes to learn more about how they work, how they're designed, and how they're built. Erector tubes, though, have been confusing me for a while. I've been thinking on it and I reckon I might have worked it out. Well see. 

Magnification is found by dividing one focal length by the other. So how would moving the lenses back and forth some how change their focal length? Stumped me for like a week. It wasnt until Id looked over a dozen or so ray traces that I think I found the answer. Usually ray traces show the incoming light as being perfectly parallel from the source. But some show the rays as divergent from the source. The discrepancy was frustrating until I read a description of astronomical scopes, where the article mentioned that light from stars was coming in from so far away that it was practically parallel. The obvious jump there being that light from infinite distance is in fact perfectly parallel, and all the ray traces I've seen showing it as such are really just describing perfect/simple world physics. 

This then means that light from closer sources is somewhat divergent and the exact focal point of the light depends on the severity of it's divergence. The wider the spread, the further the focal point. So the lenses are actual moving back and forth to allow the light to diverge more and to influence the focal length of the lens. I dont think it would be able to influence it a whole lot, but its a change that gets multiplied through the the remainder of the four lens system so I'm sure it adds up pretty quickly.

My gogole fu hasnt been able to find a good technical description of how an erector tube works but I know there are a lot of optical design/engineering folks on here (your posts on parallax have been invaluable) so I'm sure someone will be able to chime in with a right/wrong/explanation.

Thanks guys
Back to Top
koshkin View Drop Down
MODERATOR
MODERATOR
Avatar
Dark Lord of Optics

Joined: June/15/2004
Location: United States
Status: Offline
Points: 13181
Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2014 at 22:09
Back to Top
Kickboxer View Drop Down
MODERATOR
MODERATOR
Avatar
Moderator

Joined: February/13/2008
Status: Offline
Points: 23679
Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: May/25/2014 at 22:29
By the way, ILya, I meant to tell you yesterday... that is an EXCELLENT reference book.  I've been trying to get them to put it out on Kindle, but have never heard anything back.  Well worth the price as a reference tool.
Opinion,untempered by fact,is ignorance.

There are some who do not fear death... for they are more afraid of not really living
Back to Top
Gappa View Drop Down
Optics Apprentice
Optics Apprentice


Joined: May/21/2012
Location: SC
Status: Offline
Points: 218
Post Options Post Options   Thanks (0) Thanks(0)   Quote Gappa Quote  Post ReplyReply Direct Link To This Post Posted: May/28/2014 at 21:12
if you are really interested go to wally world and buy a $40 cheapo and tear it down. It's a tube inside the main tube that pivots usually on a ball joint at the ocular end. There are screws on the top and right side for the elevation and windage opposed by leaf springs on the bottom and left side.
Back to Top
Dale Clifford View Drop Down
Optics Jedi Knight
Optics Jedi Knight


Joined: July/04/2004
Location: United States
Status: Offline
Points: 5087
Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: May/29/2014 at 09:13
my kids got one of those, except he forgot to put in the lenses
I love little league baseball-- it keeps the kids out of the house
Yogi Bera

Back to Top
GarageShopIt View Drop Down
Optics Apprentice
Optics Apprentice


Joined: May/23/2014
Status: Offline
Points: 50
Post Options Post Options   Thanks (0) Thanks(0)   Quote GarageShopIt Quote  Post ReplyReply Direct Link To This Post Posted: June/04/2014 at 19:58
Okay, so I bought the book, got it shipped, and spent a couple hours reading it today (it's only a couple hundred pages and half that is devoted to optics systems that are outside my interest).

There is a section specifically dealing with variable magnification. The kicker is that it still didnt answer my question. It never explicitly states HOW the magnification is achieved. It says that the lenses move back and forth to change the magnification but it never explains what effect that movement has on the light that would cause it to magnify that image. There is a depiction of a zoom tube in various stages that seems to support what I think is happening but Id prefer a hard yes or no from learned heads. Implicit support of an amateurs interpretation isnt exactly what I thought I was buying.

It wasnt a total waste of money though, far from it. I had a lot of other follow on optics questions that the book answered. The copy I bought was even hand signed by one of the authors (Paul). It did however leave me a few unanswered questions, one of which:

The book describes two methods of erecting an image. The two lens erector tube we all know and love, and a one lens method that chokes up on the focal point of the objective lens to take advantage of the inversion caused by having an image inside the focal length of a lense. The book doesnt explain why the one lens style fell out of favor. What are the draw backs to a one lens erector that caused it to be discarded in favor of two lenses? 


Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.

This page was generated in 0.154 seconds.