Keystone correction for gobos

Updated 8 October 2026

A gobo projected straight at a surface lands as drawn. Hang the fixture high, low or off to one side and the image spreads unevenly: text that should be square comes out as a trapezoid, wider and dimmer at the far edge. Correcting that in the artwork is called keystone correction.

Why it happens

A profile's beam widens with distance. When the fixture is square on to the surface, every part of the image travels the same distance and grows by the same amount. At an angle, the far side of the image travels further than the near side, so it comes out larger, and because the same light is spread over more area, dimmer. Apollo describes the result as a trapezoid shaped like an architectural keystone, which is where the name comes from.

The fix is to draw the gobo already distorted the opposite way, squeezed where the projection will stretch, so that it lands square. A focus can help a little, but only the artwork can undo the shape.

When to correct it

Apollo and GoboSource both give the same rule of thumb: consider keystone correction once the projection angle goes past about 30 degrees. Below that, many designers accept the distortion. Keystoning only works for one position, so a gobo that tours to venues with different rigging angles usually cannot be corrected for all of them.

What the gobo maker asks for

If the maker is doing the correction, they need the geometry. Rosco's keystone sheet asks for six things:

1. The fixture, make and model.
2. The lens being used.
3. The lens focal length in millimetres, which Rosco prefers to a beam angle.
4. The image size you want, as height and width or a diameter.
5. The perpendicular throw: the distance from the fixture to the surface, measured at a right angle to it. Rosco also calls this the Z offset.
6. Where the fixture is relative to the centre of the image: the X offset across (negative if the fixture is to the left, positive to the right) and the Y offset (negative below the image centre, positive above), as seen standing behind the fixture looking at the surface.

Apollo's keystone form asks for similar data in feet: the lens degree, the distance from the fixture to the surface, the size you want the image to be, and how far the fixture is above, below, left or right of the image centre. Apollo takes custom orders through its dealers. Projected Image says its team corrects distortion during manufacture but does not publish a data list.

Doing it yourself

Gobo Designer works out the correction in your browser from where the fixture hangs and where it points, and shows the result on the surface and in the gate before you order. It also shows the numbers in Rosco's format (projected image size, perpendicular throw and X and Y offsets) under "For the gobo maker", so you can send corrected artwork, or send the data and let the maker correct it, and check one against the other.