FOV Calculator

This converts a field of view between the three axes it can be quoted on and between two aspect ratios. Enter the number, say whether it is horizontal, vertical or diagonal, give the aspect ratio it belongs to, and it returns all three angles, the size of the picture at unit distance and the 35 mm lens that frames the same view. The second mode takes the base aspect a game defines its setting at and your own screen, and applies either Hor+ or Vert- scaling. The geometry is exact; which convention a game uses is something you have to supply.

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Source engine games, Quake and most shooters use a horizontal figure. Unreal Engine, Unity and most console games use a vertical one. Camera lenses are quoted diagonally.
16 and 9 for a normal widescreen, 21 and 9 for ultrawide, 32 and 9 for super ultrawide, 4 and 3 for an old 4:3 screen.
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Hor+ is what almost every PC game does and what you want. Vert- turns up in console ports and in a few older engines, and it is why a wider monitor can leave you seeing less.
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Related Tools
Camera Exposure Calculator FPS Calculator (Frames Per Second) Aspect Ratio Calculator eDPI Calculator