Binoculars: Field of View and Magnification

[syousef]

I'm no newbie but this is sure going to sound like a newbie question.

What is the relationship between field of view/angle of view and magnification.

I always thought that angle of view and magnification went hand in had - a change in the angle the optics are taking in affects the magnification and vice versa. However the apparent field of view would depend on how much of the edge of the optics was cut off/vignetted, and how those optics distort the image over the field (curvature etc.)

Lately I've been on a cheap binocular buying kick. I was comparing the following mix of old and new purchases.
1) 10x50 Dick Smith "Digitor" branded binocs
2) 10x50 WA Andrews (clones of Saxon BFWA from what I can tell)
3) 20x50 Bushnell Powerview

To my complete surprise, Crux fit the field of view of 2 and 3. The field is just a bit too narrow to fit Crux in for 1. 3 has the LOWEST magnification from what I can tell, for both near objects and sky at infninity.

I'm trying to work out if 3 are just mislabelled or if I have a fundamental hole in my understanding of how binocualrs work. (Though I'm no specialist in optics or optical design, that would be embarassing given my education).

Thanks.

[chris lewis]

Hi

The Crux is about 6 degrees. 'Most' 10x50 binoculars will have around approx. 6 degree TFOV. [True FOV].
Field of view is related to magnification, as the magnification increases you will get a corresponding smaller field of view, [in general]. Hence the 20x50 will give you a 'narrow' FOV like around 3 degrees but with twice the magnification.
The greater the magnification the smaller the FOV, hence you inability to see Crux in the full with a 20x50 bino.
Field-of-view is the size of an area that can be viewed from edge to edge. If two people were standing 1000 yards away from you, one was to your left and one to your right and the distance between the two people was 300 feet, then your field-of-view would be 300ft at 1000 yards while looking through the binoculars.
Most binoculars will have this sort of measurement on them. [or in Mtrs.]
310 ft. at 1000 yds. is approx 6 degrees FOV.
A large field of view is especially desirable in situations where the object viewed is likely to move, or when the user is moving. A 7x50 or a 10x50 binocular will give you a wider image and these powers are good for hand holding and general dark sky sweeping. A 20x50 will need to be tripod mounted and will give a more close up view but will show much you a smaller amount of FOV. [A 20x50 will also give you a dim image - a 20x70 will give you twice the brightness.]
Power affects brightness. Other things being equal, the higher the power, the dimmer the view.
An average 7x50 has a 7.1 degree Tfov, a large 25x100 has a 2.4 degree Tfov.
Don't forget a 20x50 bino is twice the power of a 10x50 bino. The 20x50 Bushnell has the highest power then the other two you mention.
You may have heard of 'Apparent' FOV [Afov] this the 'breadth' of the actual image you get in the eyepieces. Afov is the power of the binocular times the magnification, so a 10x50 binocular will give a 50 degree Afov.
40 degrees Afov is very narrow, [like looking down a toilet tube roll] 50 degrees is acceptable, 60 is desirable and more then 65 degrees is considered a 'Wide angle' FOV. [Some say 60 degrees.]
The wider the FOV will usually give you more edge distortion so there is a trade off. The better quality binoculars will usually give you less edge distortion and a better quality image.
It can get complicated and I may have got things wrong but read up more esp. on this subject the info here and on 'Cloudy Nights'.

Chris

[dannat]

terrific answer chris

the crux should most definitely not fit in the fov of bino 3 -20x50, most likely it has a fov of 3 deg or less

it must be mislabelled,either that or someone found the eyepiece so bad in that bino they switched out the eyepieces for something diff, perhaps mailing a 7x50; you can time a star drifting across the whole fov

[syousef]

Thank you both for your answers.

yeah the 20x50s have 56m@1000m marked, and both 10x20s have 122m@1000m.

I was second guessing myself. I'm now quite certain the 20x50s are mislabelled. I've dropped an email to Bushnell. If I get an answer I'll post here.

[syousef]

In case anyone stumbles on the thread, here's an explanation I found with images that is quite good.

http://www.nikon.com/products/sportoptics/how_to/guide/binoculars/basic/basic_08.htm

[syousef]

Chris I've re-read your post about 4 times now. I think everything else you've written makes sense but I think you've not got this quite right. Minor point though. Your post was very useful to me.

According to the manufactuer's website
The Orion Scenix 10x50 has an AFOV of 70 degrees, not 50.
The Orion Scenix 7x50 of the same range has an AFOV of 49 degrees.

As I said I'm no expert in optics but in an over simplified view I would think the objective's curvature would be the determiner of power/magnification, while the curvature of the eyepiece would determine how that light is spread and thus AFOV.

[rmcconachy]

G'day syousef,

Magnification = focal length of objective lens / focal length of eyepiece

Altering the focal length of the objective lens (which changes its curvature) is one way to change the magnification of a binocular. The Nikon SE series of binoculars follow this pattern. They come in 8x32, 10x42 and 12x50 versions which all use the same eyepieces and prisms, just the objective lenses (and the portion of the binocular holding them) differ. However, many manufacturers offer series of binoculars that use common objective lenses and prisms but different focal length eyepieces to give different magnifications, e.g., the Fujinon FMT-SX 7x50 and 10x50. Any combination of the two approaches is possible but manufacturers want to use as many common parts across a range of binoculars as they can to keep costs down.

The apparent field of view (AFOV) of an eyepiece depends on several things, e.g., the curvature of the lenses within the eyepiece, their spacing, the glass types used and the aperture of the field stop.

Some of these parameters are linked by the formula Chris gave you:

AFOV ~= True field of view (TFOV) x Magnification

All the best and happy observing!

[syousef]

You entire post was fantastic but those 2 lines were gold - exactly what I needed for clarification.

Thanks,

Sammy

[anj026]

There's a lot of useful information on Cloudy Nights binocular forum best of. (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/44730/page/0/view/collapsed/sb/5/o/all/fpart/1)

[syousef]

Thanks Andy.