Binoculars Field of View
The wide vision
You usually get a wider field of view with an 8-power binocular than you do with the same model in 10 power.
For spotting a bird through binoculars, all you have to do is get the bird anywhere in the image. A wider field of view makes it easier to spot a songbird among the branches (or a seabird on the waves).
In the two views of tree branches, the magification is greater on the left, with 10-power binoculars. There's a cardinal there, too. Well, almost. The tip of its left wing barely shows, at the edge of the circle, just above 3 o'clock.
In the right-hand image, through 8-power binoculars, the berries and branches are 20% smaller. But in the 8-power view, which is aimed at the same spot, you get to see the entire cardinal.
If you were having trouble finding the bird in your field of view, it would be easier to find it using the 8-power binocular.
Field of view example
Not all 8x binoculars have the same field of view. Nor all 10x binoculars. But if the two binoculars have the same name and are from the same manufacturer, and they both have the same size objective lens, the one with lower magnification will have a wider field of view.
For example, compare the Vortex Diamondback HD 8x42 to the 10x42 model. They look the same. They're the same size and almost exactly the same weight. The 10x42 makes everything look slightly bigger. But while the 10x42 has 330-foot-wide field of view at 1000 yards distance, the 8x42 has a 393-foot field of view. About 20% wider.
This is why in general, it's easier to locate a bird with lower-power binocular. And it's why 8-power binoculars are popular among birders. Especially if they sometimes have trouble finding a bird in the binoculars.
A suprising twist
The field of view is determined by the design of the eyepiece. You can't guess the field of view just by knowing by the size of the objective lens. You have to look it up in the specifications of the binocular But here's a surprising twist.
For example, the Vortex Diamondback HD 8x32 is more than an inch shorter than the 8x42. And the 8x32 weighs about 5 ounces less. Yet the 8x32 has a 426-foot field of view, compared to the 8x42's 393-foot field of view.
This is almost hard to believe, but it holds true for virtually all binoculars. It's something to consider when you're choosing a new binocular.