FOV and magnification

[labougie]

I only have a little brain and it's currently slightly confused between FOV and magnification. Given two pairs of binoculars, each with the same magnification but one with an 8deg and the other a 6 deg FOV, will a given object fill a smaller percentage of the FOV in the wider angle pair?

[GazOC]

Yes, the object will be the same size in both binos but more surrounding sky will be visible in the wide angle set.

[labougie]

Yes, that makes sense. Could I check the magnification of a pair of bins by estimating (say) how many moon diameters would fit across the (one eyed) FOV of a given pair of bins (plus knowing what the manufacturers figure for FOV was) and working backwards from there?

[labougie]

Garbage!! Deleted

[Kaptain Klevtsov]

It works like this. You get the magnification bit, thats works as it says, stuff looks bigger.

The next bit is like the size of the window that you are looking through, so a bigger FOV gets you more black around the moon (which is the same size in your example). Posh bins are bigger windows, cheaper ones are like looking through a porthole, usually.

Kaptain klevtsov

[GazOC]

Yes, that makes sense. Could I check the magnification of a pair of bins by estimating (say) how many moon diameters would fit across the (one eyed) FOV of a given pair of bins (plus knowing what the manufacturers figure for FOV was) and working backwards from there?

Or you could measure the front lens and the exit pupil and divide the former by the latter to get the magnification. Do you have a reason for doubting the manufacturers specs?

[labougie]

Yes, I'm dubious about the magnification and just fancied to do the maths to check it out. See if this works - 8x40 bins with an 8.2deg FOV. The moon's diameter is .259deg from here, so .259 as a percentage of 8.2 is 3.158 odd%, times the 8 magnification is 25 odd%, so I should be able to see 4 moon diameters in the FOV through those bins. Is that a reasonable way to work it out? I'm actually estimating about 5 diameters, so either the FOV is larger or the magnification smaller than what it says on the tin - not surprising - they're serious cheapies.

[GazOC]

The moons diameter is just over 0.5 deg IIRC. Try my method, point the binos at a light source and measure the beam of light coming out of the eyepieces and divide 40 by that number to get the magnification.

[John]

The exact focal length, magnification and field of view of optical equipment can often differ a bit in reality from the claims made by the manufacturers - even with expensive brands.

John

[labougie]

The moons diameter is just over 0.5 deg IIRC. Try my method, point the binos at a light source and measure the beam of light coming out of the eyepieces and divide 40 by that number to get the magnification.

Thanks for that - doing it your way gives me 5.6mm at the eyepiece, a magnification of about 7. You're quite right about the moon's angle of arc - I did the right angled triangle solution for 384403km and 1737km radius which comes to .259deg then promptly forgot to double it! My logic regarding the angle of arc and FOV must have been spurious as well. Lidl will have the Bressers in on the 15th so I'll grab a pair and get an idea of what a decent view is like.

[TheThing]

The Lidl Bressers are more like 10x42 than 10x50 as there appears to be some stopping down inside.

That said, they are excellent bins (I use my pair for birding mostly) and a brilliant price!