How to straighten a crooked photo

A horizon that leans two degrees is enough to make a photo feel wrong without anyone being able to say why. Straightening it takes ten seconds. The only skill is knowing what to line up against, and knowing that every degree of rotation trims a little off the edges.

Straighten, rotate, and what the difference is

Rotate turns the photo in 90 degree steps and is for a photo that came in sideways. Straighten turns it by a fraction of a degree to a few degrees, and is for a photo that is nearly level. In Raspy both live in the Crop tool: Rotate is a button, Straighten is a dial that runs from -45 to +45 degrees in half degree steps.

Steps

1. Open the photo and switch to Crop.
2. Choose Straighten under the frame. The dial appears with 0 in the centre.
3. Find a reference. A horizon, the edge of a building, a table top, a doorframe. Pick one that should be exactly horizontal or exactly vertical, and prefer one near the middle of the frame, since lens distortion bends lines near the edges.
4. Drag the dial until the reference is level. The frame rotates live and the grid lines over the photo give you something to compare against.
5. Look at the whole photo, not just the reference line. A leaning person or a sloped hill can fool the eye; the grid does not lie, but the photo should also feel right.

What straightening costs

A rotated rectangle no longer fills its original frame; the corners would be empty. Raspy scales the photo up just enough to cover the frame, so there are never blank corners to crop out by hand. The price is that the edges of the photo are trimmed, by roughly 2.5% of the frame for each degree on a 3:2 photo. Two degrees costs a sliver. Ten degrees costs a noticeable strip all round, and anything close to the edge of the original, such as a head near the top, can be lost. If the subject is tight to an edge, either accept a slight tilt or crop deliberately so you choose what goes.

When to leave it

Not every tilt is a mistake. A deliberate angle on a street shot or a photo taken looking up at a building has lines that are not meant to be level, and forcing one of them straight only makes the others look worse. Straighten is for photos where a level line was intended and missed. When in doubt, straighten, hold the photo to compare with the original, and keep whichever one your eye settles on.

Undoing it

Double-click the dial to return it to zero. A drag counts as one step in history, so Cmd+Z (or Ctrl+Z) undoes the whole straighten rather than one tick of it. The original file is never changed; a save always writes a new copy.