Imagine a rectangular pyramid projected from your camera’s lens, with the point just behind the imaging area. This pyramid expands from this point to enclose the imaging area and then out through the lens to encompass the area that the camera can “see.” The imaging area’s aspect ratio forms the shape of the pyramid, and the lens’s focal length dictates how that pyramid spreads from the camera outwards. A shorter focal length results in a flatter pyramid, and the wider the imaging area’s aspect ratio, the less symmetrical the pyramid’s base is.
This tab calculates three angles on this pyramid, called the camera’s angles of view. It’s easiest to understand this in terms of what problems you can solve when you know the angles of view:
What focal length lens on a 4×5 view camera would be equivalent to a 35 mm camera’s 50 mm lens? Answer: About 180 mm.
Given that the full moon is half a degree wide as seen from the Earth, how long a lens will I have to use to capture an image of the moon that fills half the width of the frame, using a 35 mm camera? Answer: About 2000 mm.
If I place my 6×7 medium format camera in the corner of a room, how wide a lens will I need to capture the entire room wall-to-wall, in landscape orientation? Answer: 30 mm or wider.
In this formula, d is the length of the imaging area in the direction for which you want the angle measurement, and f is the focal length of the lens. ƒ/Calc just runs through this formula three times, using the imaging area’s horizontal, vertical, and diagonal dimensions. ƒ/Calc only knows the first two dimensions for each frame size; it calculates the diagonal length using the Pythagorean theorem.