How does it find the angle?

The level fix looks for the strongest near-upright and near-flat lines in the picture — a horizon, a door frame, a table edge — and turns the frame so they sit true. It reads up to 8° either side. When nothing reads as off level, it says so and changes nothing.

Turning a rectangle leaves four empty triangles at the corners. The crop shrinks inward just far enough that none of them is saved, so the output is a little smaller than the input.

What is the largest angle?

The automatic reading goes up to 8°. The Straighten dial in Finish goes to 15° either way, in tenths of a degree. A photo more than 15° off is usually meant to be.

Every fix is made on this device.

Straighten a photo

The level fix is armed. Tap anywhere and it reads the tilt, or use Finish → Straighten.

Drop a photo in. Tap the spot you want gone.Click to browse. A copied picture pastes straight in. Nothing is sent.Spot · Heal stroke · Edge dab · Lift · Red-eye · Level — 6 marks, then it stops

Nothing to hand?

A drawing, not a photograph. Drag the seam: a speck of sensor dust in the sky, something dark pressed against the top border, the lower left corner sitting in shade — one tap each, and the one at the border only shows with the seam pushed right.

What is wrong with it?

Optional — the uploader works without it. It takes about five seconds and can save you a tap you would have undone.

Two ways to level a frame

Tap anywhere with the level fix (key 6). It reads the frame, turns it once, and lists the result as a numbered fix you can remove.
Or go to Finish and drag the Straighten dial yourself, from −15° to 15°. The crop follows so no blank corner survives.

A frame that is already level is told so: the tap is refused with nothing changed. Related: Old photo touch up for a crooked scan; Profile picture editor for a square crop after levelling.

Questions about straightening

How does it find the angle?: The level fix looks for the strongest near-upright and near-flat lines in the picture — a horizon, a door frame, a table edge — and turns the frame so they sit true. It reads up to 8° either side. When nothing reads as off level, it says so and changes nothing.
Why is it cropped?: Turning a rectangle leaves four empty triangles at the corners. The crop shrinks inward just far enough that none of them is saved, so the output is a little smaller than the input.
What is the largest angle?: The automatic reading goes up to 8°. The Straighten dial in Finish goes to 15° either way, in tenths of a degree. A photo more than 15° off is usually meant to be.

Photographic visual for Straighten Photo — Straighten Photo