Field width
How many feet of ground fill the screen at 100 yards. Wider makes scanning faster; narrower usually means more detail on the animal.
What it means
Field width is the field of view turned into something you can picture: the width of the strip of ground, in feet, that fills the screen at 100 yards at base magnification.
How we calculate it
The sensor’s physical width (pixels across × pixel pitch) and the focal length set the horizontal angle of view. Field width is twice the distance times the tangent of half that angle, converted to feet.
A shortcut: at 100 yards, feet across is about 300 × sensor width in millimeters ÷ focal length in millimeters. A 384-wide, 12 µm sensor is 4.6 mm across; behind a 35 mm lens it shows about 39 feet at 100 yards. A 640-wide sensor with the same pitch and lens is 7.7 mm across and shows about 66 feet, with the same detail on the animal. Another shortcut, from degrees: feet at 100 yards is about 5.2 × the horizontal field of view.
Field width grows in step with distance. Double the distance and the strip doubles: that 39-foot view is about 79 feet at 200 yards.
When a maker does not publish the focal length, we work it out from the published field of view, so the result still rests on the maker’s own figure.
What it changes in the field
Field width tells you how a unit will feel when you scan. A 40-foot view at 100 yards takes many sweeps to cover a quarter section; a 90-foot view takes far fewer. It also tells you how much room you have to follow a coyote that circles to get downwind.
What to look for
Pair it with pixels on target. The two trade against each other on the same sensor: more width, less detail. A higher-resolution sensor is the way to get more of both.
Night vision tubes have no pixels, but field width works the same from the published field of view: a 40° tube shows about 218 feet across at 100 yards.