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Field of View

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Old Wednesday 26th February 2020, 16:00   #1
JoeJ
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Field of View

I have a question that possibly someone here has the knowledge to answer. I was looking at a thermal night scope that expressed the FOV as 3*X2.5* with an optical magnification of 6X with a lens diameter of 75mm. I would like to know the formula to determine the FOV in feet/meters at 100 yards.
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Old Wednesday 26th February 2020, 17:12   #2
henry link
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Presumably the real field is rectangular or oval with a width of 3 degrees and a height of 2.5 degrees.

Multiplying by 52.5 will give a close enough approximation of FOV in ft. at 1000 yards, 157'w x 132'h at 1000 yards. Divide by 3 for meters at 1000 meters.

Multiply by 6 for an approximation of the apparent FOV, 18x15.
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Old Wednesday 26th February 2020, 18:23   #3
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Thank You Henry,

This thermal has a base optical setting at 6X with additional 2X and 4X digital zoom - so, at 100 yards I'm only observing 15' of terrain at base optical magnification? Probably have to look for a 1.5X or 2X optical base magnification for a decent FOV at 100 yards and closer.
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Old Wednesday 26th February 2020, 18:53   #4
etudiant
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Quote:
Originally Posted by JoeJ View Post
Thank You Henry,

This thermal has a base optical setting at 6X with additional 2X and 4X digital zoom - so, at 100 yards I'm only observing 15' of terrain at base optical magnification? Probably have to look for a 1.5X or 2X optical base magnification for a decent FOV at 100 yards and closer.
It is not easy or cheap to make a thermal sight with decent resolution on a wideish FoV.
Pilots need special training to use thermal of night vision gear because the FoV is so limited, even though this is military hardware, where money is less critical.
My guess is that you can find some way to compensate for the hardware constraints once you really try. I don't think there is an economical hardware solution.
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Old Wednesday 26th February 2020, 19:38   #5
jring
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Hi,

I reccommend the following example at 4x magnification:

https://de.wikipedia.org/wiki/Leopar...rmal_image.jpg

The rectangle in the middle is the field of view for the 12x setting - which has a 5 degree true field.

Unfortunately the device is a bit bulky and the price is irrelevant as it is not available to civilians... on the plus side it comes with a free 12x daylight optics full stabilization and a laser rangefinder...

Joachim

Last edited by jring : Wednesday 26th February 2020 at 19:41.
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Old Thursday 27th February 2020, 19:38   #6
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If they tell you the size of the field of view in degrees then you can use an online tool to express this in yd’s/1000yds etc. It won’t tell you the apparent field of view as this depends on the screen size and eye piece optics. It’s possible to have the same detector and front optics and have different magnification, however the real field of view will be the same.

Peter
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Old Thursday 27th February 2020, 20:17   #7
jring
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If they tell you the size of the field of view in degrees then you can use an online tool to express this in yds/1000yds etc. It wont tell you the apparent field of view as this depends on the screen size and eye piece optics. Its possible to have the same detector and front optics and have different magnification, however the real field of view will be the same.
Hi,

No need for an app or page:

true field in feet/1000yards = true field in degrees * 52.5 (as previously stated by Henry)

or for the rest of the world:

true field in meters/1000m = true field in degrees * 17.45 (or divide the above by 3 for a good approximation as stated by Henry).

The true field of view is determined by the field stop diameter of the eyepiece or sensor of the camera and the focal length of the objective.

true field in degrees = field stop diameter of the EP / focal length of the objective * 57.3

Joachim
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