Troubleshooting
How to Straighten a Crooked Scan or Photo of a Document
Straighten a crooked scan or photo of a document for free — auto-detected edges and perspective correction flatten angled pages into clean, square PDFs.
Hamza M.
PDF Workflow Editor
You photographed a document at an angle because desks and hands are rarely square. The photo shows a trapezoid — edges lean, text tapers, and it does not read like a document. A scanner that detects the page edges and applies a perspective correction turns that slanted photo into a flat, square page automatically.
Here is how to straighten a crooked scan or photo for free, and how to help the detector when it struggles.
Let edge detection do the straightening
- 1Open the Scan PDF tool and point the camera at the angled page.
- 2Wait for the detector to draw a quadrilateral around the page — it follows the true paper edges, not the frame.
- 3Hold still until the outline locks (green), and capture.
- 4Check the review screen: the page is now cropped to those edges and perspective-corrected to a flat rectangle.
- 5Rotate if it needs a final nudge, then save.
The warp is not just a crop. The scanner remaps the page using the four detected corners, so text that leans away from you is rebuilt as if the page were flat on a scanner bed.
When detection fails, do this
| Problem | Cause | Fix |
|---|---|---|
| No outline appears | Low light hides the page edges | Add light from the side so the page edges contrast with the desk |
| Outline grabs the table too | Page blends into a white background | Slip a dark sheet behind the page before scanning |
| Outline keeps wobbling | Hand-held camera shake | Lay the phone down, or lean it against something and use the shutter |
| Corners are cut off | Page extends past the frame | Pull back until a clear margin surrounds all four corners |
| The result looks skewed anyway | Page was very angled | Move closer and shoot from above the page rather than from the side |
Straighten photos you already took
If the crooked photo is already in your camera roll, you can still fix it: add the photo into the Scan tool as a page and the same edge detection and perspective correction applies. Pages taken with decent lighting and a clear margin around the paper warp cleanly even when they were never "scanned".
Why steady beats clever
The correction is dramatic when the page is close and only slightly tilted, and weakest when the photo is taken from a grazing angle. A phone held roughly parallel to the page almost always produces a PDF that looks like it came off a flatbed.
Do it now
Scan PDF
Scan documents with your camera and save as PDF. It runs in this browser tab — your file is not uploaded anywhere.
Open Scan PDFFrequently asked questions
How do I straighten a scanned or photographed page for free?+
Use a scanner that detects the page edges and applies perspective correction. It crops to the detected quadrilateral and warps it into a flat rectangle, all in the browser with no app.
What does perspective correction actually do?+
It remaps the page from the four detected corners so the angled photo is rebuilt as if the document were facing the camera straight on — text and geometry stop leaning.
Why does the detector sometimes not find the page?+
Low light, low contrast against the desk, glare, or corners outside the frame. Even light, a contrasting background, and a clear margin around the page fix most cases.
Can I straighten a photo I already took?+
Yes. Add the existing photo as a page in the scanner and it runs the same edge detection and perspective correction over it.
Does the B&W filter straighten my page?+
No. Filters change tone and contrast only; straightening comes from the detected edges. Apply filters after the page is warped.