Magnification and field of view

[MacGee]

Two hypothetical 8x40 roof bins. One has a FOV of 5°, the other has a FOV of 10°. So the image from the 10° bin has twice as much stuff in it as the image from the 5° bin. Meaning the objects in the 5° bin are twice as big, right? And the size of the objects you see through a bin are decided by a combination of magnification, exit pupil and FOV, right? So under what what circumstances would the image that you see through an 8x bin actually be 8x?

Michael

[Tero]

Meaning the objects in the 5° bin are twice as big, right
no
see
http://www.birdwatching.com/optics/binoculars4_fov.html

this does not explain the two models, but you need to think in angles
http://www.sherwoods-photo.com/opticron_binos/Field-of-View.jpg
different models show different apparent angles. Think of the tree staying the same size but you see a wider field around the tree on all sides.

[ThoLa]

Two hypothetical 8x40 roof bins. One has a FOV of 5°, the other has a FOV of 10°. So the image from the 10° bin has twice as much stuff in it as the image from the 5° bin. Meaning the objects in the 5° bin are twice as big, right? And the size of the objects you see through a bin are decided by a combination of magnification, exit pupil and FOV, right? So under what what circumstances would the image that you see through an 8x bin actually be 8x?
Michael

Hello!
The real magnification will not be affected by the field of view. One field of view will just be wider, twice as much in terms of diameter. The area of the field of view is given (provided it is circular) by A = (radius)squared times Pi.

Things may apparently (!) seem to have different magnifications because of different fovs but this is a nice example of an optical illusion.

So the answer to your question is: No is strictly physical terms, .... somewhat yes in psycho-physiological terms.

Does it help or only confuse?

Tom

[Kevin Purcell]

Perhaps it helps to think of the magnification as a "zoom" and the FOV as an aperture (the field stop) which determines how much of the scene you see at that magnification.

For example, using your normal 1x eye ;)

If you omit any aperture you "see" (perceive) about a 70 degree FOV at 1x which is very sharp in the center and gets less sharp quite rapidly out to the edges. It has a small sweet spot (which your brain makes look larger with some clever processing).

Now pick up a toilet roll tube (40mm dia and 110mm long, in my case) and look down it. Your field of view is now about 20 degrees. Same 1x view but you see only 20 degrees of the field.

The same applies if you make the toilet tube out of plastic, Al or Mg and fill it full of interestingly cut bits of glass.

[MacGee]

Things may apparently (!) seem to have different magnifications because of different fovs but this is a nice example of an optical illusion.
Thanks, Tom; I suspected it was going to be yet another optical illusion. This presumably means that the 5° view, though twice the size of the 10° view (apparently) doesn't contain any more information?

Michael

[Tero]

Michael, you did not look at the linked picture or the other replies. YES it contains more infrormation. Looking at a toilet paper roll twice the diameter of the normal, Kevin's answer, you will see TWO BIRDS in your view instead of ONE BIRD. Is two birds not more information than one bird? ;) Figure it out. Use props if necessary.

[Sancho]

The same applies if you make the toilet tube out of plastic, Al or Mg and fill it full of interestingly cut bits of glass.
.....and tease the bejasus out of gullible morons like me with seductive and tantalising ads for new tubes with new bits inside. Costing more money than the average Ugandan sees in a decade. But hey, Macgee, thanks for the question, and I´m real proud of myself ´cos I copped the answer instantly even though I don´t understand the science bits....the way I guessed it, it´s like this: Same Magnification, Different FOV´s, means the wider-FOV model has More Stuff, not Bigger Stuff in the FOV. (Is that right? I know this from experience ´cos I used to have a very nice pair of Avian 8x42 that had FOV of only 6.5 degrees. I sold them ´cos I couldn´t see enough stuff, even though it was always magnified by eight).

[ThoLa]

..... sold them ´cos I couldn´t see enough stuff, even though it was always magnified by eight).

See! That's why you NEED an 8+12 Duo....;)

Tom

[ThoLa]

Thanks, Tom; I suspected it was going to be yet another optical illusion. This presumably means that the 5° view, though twice the size of the 10° view (apparently) doesn't contain any more information?

Michael

Hello Michael!
Maybe the numbers have to be the other way around (5/10; 10/5)?

A wider field will contain more information in absolute terms. Maybe it becomes easier to grasp if you think of taking a picture with a digital camera. It will take more pixels to store the information of the wider fov than that in a narrower fov. And if you pump up the magnification but keep the fov constant the amount of information will stay the same! It will just be more "concentrated" (on a bird, say, while ignoring the surroundings).

If additional magnification yields more information depends on the resolution of the system (It's a widely held misunderstanding that higher magnification AUTOMATICALLY means better/higher resolution).

A wider field of view is simply more pleasant than looking down a barrel (irrespective of the question whether it's made from a prestigious kitchen roll or a shabby high-tech alloy).

Happy watching (whatever you'll be watching through),
Tom