FOV specs - reliable? comparable? red-herring? (1 Viewer)

[tx2ad]

In looking at various manufacturer specs, the stated FOV in meters and feet seem to (generally) not actually agree.
The stated FOV in meters is generally larger than the state FOV in feet (if a manufacturer provides the spec in both units).
I saw one example where they FOV was stated at 118m (which, should be about 387 feet) but instead they listed it as 354 feet.
33' is a not inconsiderable difference. Seems like most of the industry is calling 1m = 3'; though I've seen one use around 2.9';
the actual conversion should be 1m = 3.281' - no?

This makes me question numbers I've seen on 3rd party sites (say B&H) that have stats for nearly every bino currently on sale.
I know that the best policy is to do direct personal A/B comparison - but when there's not many samples one can lay hands on directly
using a site with specs is (or I had hoped would be) invaluable as a starting point of comparison/information.

 

[Ted Y.]

The FOV is expressed as 118m at 1000m. Or 354feet at 1000yards.
Not just 118m, nor just 354feet.

Redo the math and the numbers will agree.

 

[Binastro]

A metre is not a yard and a yard is not a foot.

It is a problem of math and of the obscure notion of so many feet at 1000 yards.

I use angular measure, i.e degrees and decimals of degrees.

Most good binocular makes are fairly accurate, although some are either deliberately or mistakenly wrong.

But post #1 is just a mistake in math.

Regards,
B.

 

[tx2ad]

As one example, straight from the mfr spec:

 

[tx2ad]

The FOV is expressed as 118m at 1000m. Or 354feet at 1000yrd.
Not just 118m, nor just 354feet.

Redo the math and the numers will agree.

Could you remind me the angles/math here?

 

[Binastro]

135m at 1000m is about 7.7 degrees.

404ft at 3000ft is also about 7.7 degrees.

404ft at 1000yds is the same as 134.7ft at 1000ft.

1000yds is 3000ft.

It is a question of ratios or fractions in simple terms.

B.

 

[tenex]

Could you remind me the angles/math here?

When you want to convert m/1000m to ft/1000yd, just multiply by 3, not 3.3. You're not converting meters to feet, because FOV is dimensionless; just adjusting for the fact that yards are 3 feet. Manufacturers sometimes get this wrong themselves as you saw. The simpler alternative (as B. said) is just to state degrees instead but that seems to have become less common today.
404ft at 1000yd:
circle of r=1000yd has circumference *2*3.1416= 6283.2yd =18850ft, that's 360° so 1° is 52.4ft/1000yd (easy enough to remember), 404ft makes 7.7°
(or in meters, without the 3x conversion from yd to ft, 1° is 17.5m/1000m; of course it's also 17.5yd/1000yd, but we use ft/yd so *3)

 

[Ted Y.]

Here are a few steps

118 m at 1000 m
118*3.3 ft at 1000 m
118*3.3 ft at 1000*3.3 ft
118*3.3 ft at 1000*3.3/3 yds

We need at 1000 yds

118*3.3 ft at 1000*3.3/3 yds
118 ft at 1000/3 yds
118*3 ft at 1000 yds
514 ft at 1000 yds

Note
1yd = 3ft
1m = 3.3ft (almost)

 

[MiddleRiver]

Yep, I just drew it out just now (CAD). Numbers are correct.

 

[Maljunulo]

Many seem unable to grasp the concept of angular field of view, so they have erected a clumsy and confusing substitute, involving both metric and imperial measure.

Unfortunate.

 

[Holger Merlitz]

Well, even the NASA got these things messed up and failed to send one of their space crafts to Mars after having confused yards and meter in their navigation software.

Cheers,
Holger

 

[Binastro]

Aircraft have crashed because pounds and litres of fuel were mixed up.

The Hubble telescope was an initial disaster as the mirror was accurate but with the wrong curve.
This would immediately have been spotted by a talented amateur telescope maker.
I think the makers hoped the launch would fail, so no one would ever find out.

A fist at arm's length is 8 to 10 degrees across.
When measured it gives quite accurate results.
This is roughly a binocular field size.

An outstretched hand at arm's length is about 22 to 25 degrees.

A forefinger 1.5 degrees.

The distance between the two big knuckles is 3 degrees.

People reporting so called UFOs say it was as big as an orange or other such statements.
The question is, how far away is the orange?

The only way to describe such unknown objects is in degrees and degrees of travel per second and estimate the direction of travel.

Even many pilots can't estimate angular distance.

Regards,
B.

 

[Maljunulo]

Field of view is not a “red herring” it is a vital part of what you see.

Remember that in order to appreciate the field of view, you have to actually look through the binocular.

 

[Maljunulo]

People reporting so called UFOs say it was as big as an orange or other such statements.
The question is, how far away is the orange?

The only way to describe such unknown objects is in degrees and degrees of travel per second and estimate the direction of travel.

Regards,
B.

If you do not know size, you cannot estimate distance, and if you do not know distance, you cannot estimate size.

Knowing only one makes it impossible to know both. (except angular size, in which case you know neither)

 

[Binastro]

If you know angular size and movement in degrees per second you have a good idea what it isn't.

I worked out I was probably seeing very faint buzzards at night, not helicopters, or faintly lit aircraft cockpits.
Magnitude 7 in 18x50 IS binocular.

Nowadays with drones, this might be more difficult.

To say the object is the size of an orange as a guess means you know nothing. Either what it is or what it isn't.

If you are in an aircraft and the object is stationary, if you are moving relative to the object you can estimate distance from your speed and the angular motion.

Regards,
B.