Scope FOV calculations
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GTA
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All Springer/NP/PCP Air Gun Discussion General
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Air Gun Gate
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Optics, Range estimation & related subjects
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Tpatner412
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Scope FOV calculations
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Topic: Scope FOV calculations (Read 409 times))
Cableaddict
Sharp Shooter
Posts: 831
yes
Real Name: Allan
Scope FOV calculations
«
on:
November 26, 2021, 08:34:22 PM »
------------------
Nevermind.
I'm going to do some more testing on this, then post again.
«
Last Edit: November 26, 2021, 09:06:17 PM by Cableaddict
»
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USA, NY, Suffern
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Scope FOV calculations
«
Reply #1 on:
November 27, 2021, 01:40:55 PM »
Ultimately, the Field of View primarily depends on the magnification.... A larger objective can help.... The further "off axis" you look, the greater the aberrations from the objective lense (sharpness drops off, coloured halos appear).... Telescope makers (including riflescopes) use what is called an "image stage", which is a circular opening (think washer), outside of which no light passes.... This is used to prevent you from trying to look way off center, and then complaining about the lousy clarity, or coloured fringes around the images.... Usually the reticle is mounted on this image stage, and moves with it, which is why the reticle stays centered in the circular image.... At least one manufacturer (Redfield I believe), used an image stage that was wider than tall, and called it a "wide field" scope, but in reality all they did was use a slightly larger circle and then cut the top and bottom off.... The "real" image from the objective lense, when the scope is properly focused for the target distance, is exactly at the same position as the image stage and reticle....
The eyepiece "looks" at the image, which is superimposed on the image stage and reticle, and because of its short focal length (compared to the long focal length of the objective) causes magnification.... The magnification is the focal length of the objective divided by the focal length of the eyepiece.... Zoom scopes either vary the focal length of the eyepiece, or by sliding the "erector tube" (without it, the image would be upside down) back and forth, vary the focal length of the light coming from the objective (like a Barlow lense in a telescope).... As you increase the magnification, the eyepiece looks at a smaller and smaller portion of the image, and the field of view decreases....
Bottom line is, the diameter of the image stage, relative to the objective lense, sets the outer limit of the field of view, at the lowest magnification.... While it might be possible to put a larger image stage inside a larger scope tube (eg. 30mm instead of 1"), that is often not the case.... Likewise, people say that a 30mm tube can give a brighter image.... In reality, large scope tubes are used primarily to give increased elevation adjustment....
Bob
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Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
JungleShooter
How ironic 🤦🏼♂️: All my life I had to be a jack-of-all-trades. And now since getting into airgunning all of a sudden they call me an
Expert
Posts: 1750
There is a bug. It's called AG. It bit me! Bad.
Real Name: Matthias
Re: Scope FOV calculations
«
Reply #2 on:
December 27, 2021, 12:30:03 AM »
Allan,
there is an app that calculates field of view:
GPC Ballistics Applications
A full suite, for Windows, Android, iOS, Linux
https://GPC.fotosoft.co.uk/Home.html
Here's a screenshot.
Cheers, 😊
Matthias
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Lima, PERU
rsterne
Member 2000+fps Club
Bob and Lloyd
GTA Senior Contributor
Posts: 26925
GTA Forums Person of the Year 2017
Real Name: Bob
Re: Scope FOV calculations
«
Reply #3 on:
December 27, 2021, 01:06:38 PM »
Correct me if I am wrong, but isn't that calculator designed to give you one of three results (selected by clicking on the dot circle), when all the other values are known?.... I don't think it has enough inputs to figure out what the FoV should be, given the actual mechanical specifications.... objective diameter, tube size, image stage dimensions, magnification, etc.etc.... I still think that is based mostly on the magnification and the size of the "image stage"....
Bob
Logged
Coalmont, BC, Canada
🇺🇦
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! 🇺🇦
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).
Cableaddict
Sharp Shooter
Posts: 831
yes
Real Name: Allan
Re: Scope FOV calculations
«
Reply #4 on:
December 27, 2021, 04:25:44 PM »
There's also the obvious "subjectivity" factor, which applies more to DOF than most other parameters.
If you saw my recent post regarding the 14x50mm fixed prismatic, specs don't tell the whole story.
That scope "focusses parallax" down to 6 meters, but at 10 yards, it never really seemed in focus, not even when perfectly adjusted for that tiny DOF window.
I couldn't stand it, but maybe some other users would be OK with it.
So you have to first decide how much parallax shift is acceptable, then you have to decide 'how slightly blurry" is still acceptable.
And clearly (pun intended) the two re not quite the same.
Logged
USA, NY, Suffern
JungleShooter
How ironic 🤦🏼♂️: All my life I had to be a jack-of-all-trades. And now since getting into airgunning all of a sudden they call me an
Expert
Posts: 1750
There is a bug. It's called AG. It bit me! Bad.
Real Name: Matthias
Re: Scope FOV calculations
«
Reply #5 on:
December 28, 2021, 02:20:30 AM »
Bob,
thanks for checking this...! I hadn't actually used this calculator.... 😄
And yes, your're right, this app is
not
really the way to calculate field of view (FoV). 👍🏼
🔶 The app is for checking and calibration of the magnification markings:
(1) Select a MIL reticle (with 20mil long crosshairs), or a MOA reticle (with 80moa long crosshairs).
Cf. the two screenshots below.
(2) Set the scope magnification you want to test (max., or min., or 10x, or whatever).
(3) Aim at a target with known dimensions (select between round or square in the settings), at a known range.
(4) Count and enter the reticle "ticks spanned" by the target.
(5) This will (when selected) calculate the real magnification at the magnification setting.
Matthias
Logged
Lima, PERU
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All Springer/NP/PCP Air Gun Discussion General
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Scope FOV calculations