Perspective Crop Online
Four corners in, a straight rectangle out, at the size you choose. Photoshop's Perspective Crop without Photoshop.
Crop and straighten in one move
A photo of a document on a desk, a painting in a gallery, a slide on a projector screen or a shop sign is a photo of a rectangle seen from an angle. A normal crop can only cut a rectangle out of the photo, which leaves the object as a trapezoid. A perspective crop draws the four corners of the object instead, and maps what is inside them to a true rectangle: cropped to the object and straightened, in one step, at full resolution.
Open your photo, drag the four handles onto the corners (zoom the browser if the corners are small), pick the shape and the size, and download.
Getting the proportions right
Auto works out the rectangle's real proportions from how the four corners sit in perspective, and is right for most moderate angles. When you know the proportions, set them: A4 or Letter for pages, 16:9 for screens, square for record sleeves and tiles. A wrong proportion is the most common flaw in a perspective crop: the page looks straight but the text is subtly wide or narrow.
Output size
The output width defaults to the width the object had in the photo, which is the honest size: no detail is invented. Set it explicitly when the result has to fit something, for example 2480 px for an A4 page at 300 dpi or 1600 px for a slide. Enlarging past the original pixel count softens the result; the far side of a steeply photographed object is where it shows first.
The same thing inside your app
Photoshop: the Perspective Crop tool (under the Crop tool, shortcut C). Affinity Photo: the Perspective tool in Source mode then Destination, followed by a crop (lesson). Lightroom: Transform → Guided, then Constrain Crop (lesson). GIMP: the Perspective tool in Corrective mode, then Crop to Content (lesson). This page does the same with nothing installed.
Related
The perspective correction tool is the same transform framed for fixing a whole photo; perspective warp does the opposite, putting a flat image onto a surface in a photo; the vanishing point finder reads the lens and horizon from a photo when you want to draw inside it rather than crop out of it.
Questions
What is a perspective crop?
A crop and a perspective correction in one step. Instead of a rectangle, you draw a four-sided shape around an object seen at an angle; the tool maps that shape to a rectangle, so the result is cropped to the object and straightened at the same time. It is what Photoshop's Perspective Crop tool does.
How is this different from the perspective correction tool?
It is the same transform, with output sizes added. Perspective correction is written around fixing a photo; perspective crop is written around extracting a rectangle (a document, a screen, a picture in a frame) at a chosen size. Use whichever page you landed on.
How do I get a scanned document at 300 dpi?
Set the shape to A4 or Letter and the output width to 2480 px (A4 at 300 dpi) or 2550 px (Letter at 300 dpi). The result is a page-shaped image at print resolution, provided the photo had enough pixels across the page to begin with.
Why is the result blurry on one side?
That side was small in the photo and has been stretched. A page photographed from a steep angle has far fewer pixels on its far edge; no crop can invent them. Shoot squarer, or closer, and use the crop for the last few degrees.
Can I crop a phone screen or a TV out of a photo?
Yes: put the corners on the screen's corners and set the shape to 16:9 (or 9:16 for a phone held upright). The result is the screen as if photographed straight on, which is also the way to grab a slide from a photo of a projector.
Is there a size limit?
No fixed limit on the input; 12–48 MP phone photos work. The output width is capped at 4,000 px.