Guide · rebuilt 24 October 2026
Straightening a horizon without guessing
A crooked horizon is the one rotation that has a right answer you can see in the picture. The work is choosing which line in the picture is the one that ought to be level, measuring it rather than eyeballing it, and knowing what the correction will cost at the edges before you commit to it.
Choose the reference before the angle
Water is the only horizon that is truly level in a photograph: the sea, a lake, a flooded field. A line of hills is not, and nor is a road, a roofline or the top of a wall seen at an angle — those follow perspective and are supposed to slope. When there is no water, the next best reference is a vertical: a door frame, a lamp post, the corner of a building near the middle of the frame. Gravity makes them plumb, and a camera held level keeps them plumb in the picture.
- Sea or lake horizon: level it.
- A building straight ahead: make its central verticals plumb; the edges of a wide-angle frame lean outward by design.
- A horizon behind hills: level the water or the distant flat line, not the ridge.
- No straight line at all: leave it. A confident-looking correction with nothing to measure it against is a guess.
Measure across the whole frame
Lay the guide along the reference from one side of the picture to the other. A short guide on one wave or one brick is thrown off by the local wobble; a long one averages it away. On the straighten page a line within 45° of level is levelled and a steeper one is made plumb, and the button says the exact correction before you press it.
The horizon finder does the same with arithmetic: it looks for long straight edges near level and near plumb, and offers up to three — the strongest level line, the strongest upright line and the one nearest your own guide — each drawn on the picture with its angle, its confidence and what it would cost. A picture with no confident line gets “No confident line found”, not a small invented angle.
What each degree costs
Turning a rectangle opens wedges at its corners. Cropping them away keeps the original shape but loses an edge strip; expanding keeps every pixel and fills the wedges instead. For a 4032 × 3024 photograph the numbers are exact, and they are computed here by the same code the tool runs:
| Correction | Crop to the original shape | Area lost | Expand instead |
|---|---|---|---|
| 0.5° | 3983 × 2987 | 2.4% | 4059 × 3060 |
| 1° | 3938 × 2953 | 4.6% | 4085 × 3094 |
| 2° | 3852 × 2889 | 8.7% | 4136 × 3163 |
| 3° | 3771 × 2828 | 12.5% | 4185 × 3231 |
| 5° | 3622 × 2716 | 19.3% | 4281 × 3364 |
| 8° | 3426 × 2570 | 27.8% | 4414 × 3556 |
A degree costs about four percent of the picture; three degrees costs over a tenth. That is why the correction is worth measuring rather than nudging until it looks right: every unnecessary tenth of a degree is pixels thrown away. If the subject sits near an edge, Expand with a colour or edge-extended fill keeps it.
Check people and verticals last
A level horizon can still feel wrong when a wide lens or a tilted camera makes the architecture in the middle lean. If the building looks like it is falling backwards, that is perspective — converging verticals — and no rotation fixes it; a rotation only trades one lean for another. Pick the line that matters most for the picture, level it, and stop.