The Firing Line Forums

Go Back   The Firing Line Forums > The Skunkworks > Gear and Accessories

Reply
 
Thread Tools
Old August 30, 2013, 04:19 AM   #1
trg42wraglefragle
Senior Member
 
Join Date: April 21, 2008
Location: new zealand
Posts: 826
What determines FOV in a scope?

Now I always thought that the field of view of a scope was directly related to the objective size of a scope. But I was looking at Leupold fixed powered scopes, specifically the 4x28 and the 4x33 FX series scopes and it quotes that the 4x28 has a wider FOV.

So what does contribute to the FOV of a scope?
trg42wraglefragle is offline  
Old August 30, 2013, 07:54 AM   #2
NoSecondBest
Senior Member
 
Join Date: December 7, 2009
Location: Western New York
Posts: 835
The power of the scope is actually a rounded up or rounded down number. Who ever puts the specs out has some liberty to do that and marketing has a shot at it also in writing up all the good features of the scope so you'll buy it. If two scopes actually had exactly the same power and same lens size, they'd have the same fov. Some scopes have a "margin" added around the outside edges as sort of a finish to hide the seal between the lens and housing. This can take a little away from the fov if present.
NoSecondBest is offline  
Old August 30, 2013, 08:37 AM   #3
wogpotter
Senior Member
 
Join Date: September 27, 2004
Posts: 3,002
Several things.
Quality of the design, Diameter of thre objective lens, magnification Etc.
__________________
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?

Elderly Hunter: “That's dashed unsporting. Probably Belgium.”
wogpotter is offline  
Old August 30, 2013, 10:13 AM   #4
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,509
You can make it a lot easier to understand by removing the complications inside a rifle scope and just considering a window in your house.

What determines the Field of View out your window?

Only two things, the size of the window (objective diameter) and how far you stand from the window (eye relief).

It's complicated somewhat in a scope (or telescope or microscope or camera) by the internal structures but those are the "Big 2".

In a scope, they sometimes use a Field Stop so you don't see the edges. Anything that makes the image pass through a space smaller than it actually is will decrease the FoV.
__________________
Still happily answering to the call-sign Peetza.
---
The problem, as you so eloquently put it, is choice.
-The Architect
-----
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.
Brian Pfleuger is offline  
Old August 30, 2013, 10:29 AM   #5
skoro
Senior Member
 
Join Date: April 30, 2008
Location: Texas
Posts: 1,686
What determines FOV in a scope?

Several things, mainly:

1. The magnification; higher x = narrower FOV

2. The geometry of the lenses, their radius of curvature, thickness, etc.

3. Number of lenses and the space between them.
skoro is offline  
Old August 30, 2013, 11:36 AM   #6
SSA
Senior Member
 
Join Date: August 1, 2010
Posts: 347
Field of view is determined by the overall design of the individual scope, especially the design of the eyepiece.

Scopes designed for shorter eye relief often have a slightly wider fov.
2 different 3-9x40s often have different fovs.
A bigger objective lens does not mean a wider fov. 1-4x20s have wide fovs. A 3-9x50 does not usually have a wider fov than a 3-9x40.
SSA is offline  
Old August 30, 2013, 05:08 PM   #7
trg42wraglefragle
Senior Member
 
Join Date: April 21, 2008
Location: new zealand
Posts: 826
Quote:
The power of the scope is actually a rounded up or rounded down number. Who ever puts the specs out has some liberty to do that and marketing has a shot at it also in writing up all the good features of the scope so you'll buy it. If two scopes actually had exactly the same power and same lens size, they'd have the same fov. Some scopes have a "margin" added around the outside edges as sort of a finish to hide the seal between the lens and housing. This can take a little away from the fov if present.
I had a look on Leupolds website and yes the 4x28 is actually a 3.6x28, so I was kind of right in my thinking.

How do Wide angle scopes work?
I have at home an old Nikko Stirling Gold Crown 4x32 Wide Angle scope, and it looks amazing look look through, It seems like it has a huge FOV and and very little tunnel vision.

The specs on a Nikko Stirling Wide Angle 4x32 says a FOV of 33.2 ft @ 100 yards, compared to the Leupolds is a huge difference. Unless its only a 3.1x magnification then they must have done something in the lenses to get it like that.
trg42wraglefragle is offline  
Old August 31, 2013, 08:43 AM   #8
wogpotter
Senior Member
 
Join Date: September 27, 2004
Posts: 3,002
A telescope, or telescopic sight is in essence 2 optical systems fitted back to back.
The front part (the objective lens group) is exactly like a camera lens. It collects incoming light & manipulates it into a cone where there is a point at the end of the cone & it all comes together in a pinpoint. That pinpoint isn't perfect so its actually a very small circle. (Called the circle of least confusion)

At one point in that cone is perfect focus, that's where the reticule sits inside the tube.

The rear (eyepiece lens group) glass collects the image of the front group & the reticule & projects it back towards youe eye.

If you design & build a wide angled front end (like a camera's wide angle lens) it collects image forming light from a wide area & bends (collimates) it into a cone, but with more bending it collects from a wider area.

Here's an illustration I found showing a zoom at wide & tele settings that might help clarify the way it works. This is a camera lens, so imagine a magnifying glass added to the output end on the right hand side, to convert to a telescope.



__________________
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?

Elderly Hunter: “That's dashed unsporting. Probably Belgium.”

Last edited by wogpotter; August 31, 2013 at 08:50 AM.
wogpotter is offline  
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:10 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2014 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Contact Us
Page generated in 0.07223 seconds with 7 queries