How to straighten a crooked photo

A sea horizon that slopes by two degrees is enough to make a holiday photo feel off, even if the viewer cannot say why. Photos of documents, shelves and buildings show tilt even more clearly. The fix is a small rotation followed by a crop, and it takes under a minute.

Find a line that should be level

Before rotating, choose a reference: the horizon, the edge of a table, a window frame, a shelf, the top edge of a document. For buildings, vertical lines such as door frames are usually a better guide than horizontal ones. Your eye compares the line against the edges of the frame, so that is what you are correcting.

Straighten step by step

1. Open Rotate Image and add the photo.
2. Use the fine angle control to rotate by small steps, anywhere between −45 and +45 degrees. Most crooked photos need between half a degree and three degrees.
3. Watch your reference line against the edge of the preview. When it runs parallel, stop.
4. Choose a corner fill colour. Rotating by an angle leaves four triangular gaps at the corners, which the tool fills with that colour.
5. Download, then crop away the corner gaps.

Crop away the corners

Open the straightened photo in Crop Image and draw a crop just inside the filled corners. Choose an aspect ratio preset such as 3:2 or 4:3 if you want to keep the original shape. The larger the rotation, the more you lose from the edges: a one-degree rotation costs very little, ten degrees costs a lot.

Quality: rotation versus flipping

Rotating by exactly 90 or 180 degrees, or flipping, only moves pixels, so it is lossless. Rotating by any other angle has to calculate new pixels in between the old ones, which softens fine detail very slightly. The tool tells you which case applies. On a normal photo the softening is invisible, but it is a reason to straighten once from the original rather than nudging an already-rotated copy several times.

When straightening is not enough

• Buildings that lean inwards because the camera was tilted upwards need perspective correction, not rotation. Rotating one side straight makes the other lean more.
• Documents photographed at an angle are trapezoids, not tilted rectangles. Rotation helps a little; photographing from directly above helps much more.
• Wide-angle phone lenses curve straight lines near the edges. Use the centre line as your reference.

Prevent it next time

Most phone cameras can show a grid on the viewfinder; turn it on in the camera settings and line the horizon up with a grid line. Some also show a level indicator when the phone is held flat for overhead shots, which is useful for photographing documents and food.

If the photo is sideways rather than slightly tilted, that is a different problem; see why photos appear sideways and how to fix them.