Cropping looks like the one edit that cannot go wrong: you pick a rectangle and throw away the rest. Yet the result can come out too soft to print, a JPEG can lose quality in parts of the picture nobody touched, and in 2023 cropped screenshots from Pixel phones turned out to still contain the parts their owners had cut. Each of those has a specific cause, and each one is avoidable.
A Crop Costs Area, and Area Falls Faster Than You Think
Resolution is counted in area, so the loss from a crop grows with the square of how far you zoom. Cut a 4000×3000 photo down to its central 2000×1500 and the framing looks twice as close, but only a quarter of the pixels remain. That 12-megapixel picture is now a 3-megapixel one.
This is the same arithmetic as digital zoom on a phone, which is a crop with a marketing name. A 3x digital zoom keeps one ninth of the sensor. Newer phones soften the blow with computational sharpening, but a crop in an editor has only the pixels already recorded.
The number that decides whether a crop is acceptable is the size it will be shown at. For the web, compare the crop's width with the width it will display at, doubled for high-density screens. For print, divide the pixels by the inches: an 8×10 inch print at 300 pixels per inch needs 2400×3000, and a 1200×1500 crop gives you 150 pixels per inch, which looks fine at arm's length and soft up close.
What Each Common Shape Takes From a 4000×3000 Photo
The largest centred crop of each shape from a typical 4:3 phone photo. The further a ratio sits from 4:3, the more it costs, before you have zoomed in at all.
| Ratio | Used for | Crop size | Pixels kept |
|---|---|---|---|
| 3:2 | Prints, most camera sensors | 4000 × 2667 | 89% |
| 1:1 | Profile pictures | 3000 × 3000 | 75% |
| 16:9 | YouTube thumbnails, slides | 4000 × 2250 | 75% |
| 1.91:1 | Link previews, 1200 × 630 | 4000 × 2100 | 70% |
| 4:5 | Instagram feed, 1080 × 1350 | 2400 × 3000 | 60% |
| 9:16 | Stories and Reels, 1080 × 1920 | 1688 × 3000 | 42% |
Ratios Rarely Land on Whole Pixels
A file holds whole pixels only, so most ratios have to round. 16:9 from a width of 1000 asks for a height of 562.5, and something has to give. The row above shows it too: 9:16 from a height of 3000 is 1687.5 wide, which becomes 1688.
One pixel in a thousand does not show, but it can fail an upload form that checks for an exact shape. If the number matters, work from a multiple of the ratio's own terms. Widths divisible by 16 give exact 16:9 heights, so 1280×720 and 1920×1080 come out clean, while 1000 wide never can.
A rotated photo often is not rotated
Phones usually store the sensor's pixels in one fixed direction and write an Orientation value, EXIF tag 274, telling viewers how to turn them. A tool that reads the raw pixels and ignores the tag places the crop on a sideways picture, so the rectangle you drew ends up over the wrong part of the scene. The orientation has to be applied before any coordinates are measured.
Why a JPEG Can Only Be Cropped Losslessly on Block Lines
JPEG, standardised as ITU T.81, does not store pixels. It stores 8×8 blocks of frequency coefficients, and with the 4:2:0 chroma subsampling most cameras use, those blocks are grouped into units 16 pixels square, called MCUs. Removing whole units means rewriting only the file's bookkeeping, with every remaining coefficient untouched. That is a true lossless crop, and it is what jpegtran -crop from libjpeg does.
The catch is the top left corner. A lossless crop has to start on a block boundary, so jpegtran quietly moves your corner up and left to the nearest one, which can add up to 15 extra pixels. The right and bottom edges are free. Hence the crop you get may be slightly larger than the crop you asked for.
A browser has no such route. A canvas decodes the JPEG to plain pixels, and writing it back means a full new encode, so a JPEG cropped on a web page goes through compression a second time, including the areas you kept exactly as they were. At quality 90 this is hard to see once. It adds up if the same file is cropped and saved again and again, so always crop from the original rather than from the previous crop. For a file where any loss is unacceptable, use jpegtran, or save the crop as PNG, which adds no further loss at the cost of a much larger file.
Cropped Does Not Always Mean Gone
In March 2023, researchers including Simon Aarons and David Buchanan disclosed aCropalypse, tracked as CVE-2023-21036. The Markup editor on Google Pixel phones wrote a cropped screenshot over the original file without shortening it. The new, smaller PNG sat at the start of the file, and the leftover bytes of the original stayed after its end marker. Viewers stop reading at that marker, so everything looked normal, but the tail of the file could be decoded to recover much of what had been cropped out. Their demonstration restored a card number that had been blacked out in a photo shared on Discord, and patching the phones did nothing for images already posted. A similar flaw in the Windows Snipping Tool followed, as CVE-2023-28303.
An older leak works differently. The EXIF block in a JPEG can hold a small thumbnail of the picture, and an editor that updates the main image without regenerating that thumbnail leaves a tiny copy of the uncropped scene inside the file.
Both failures have the same cause: the tool edited the existing file instead of writing a clean one. A crop that goes through a canvas is safe from both, because the output is built from nothing but the pixels in the rectangle. PDF needs particular suspicion here, since cropping a page in most PDF software only sets a CropBox, a display window, and leaves the full page in the file. To remove something sensitive from a PDF, use a redaction feature or export the page as an image.
Crop to a ratio or an exact pixel size
Drag or type the crop, see the share of pixels you are keeping, and save it in the format it came in.
Getting the Crop Right
A few habits save a second attempt.
Crop first, then resize
Doing it in this order means the scaler only has to deal with the pixels you are keeping, and the final dimensions are the ones you asked for rather than the ones left over after trimming.
Leave room for another platform's crop
Feeds and search results trim images on their own, and a link preview may show differently in each app. Keep the subject away from the outer edge so a further trim does not cut it.
Check the current size before posting
The sizes platforms recommend do change. Use the table above as a guide to shape, and look at the platform's help pages when a specific upload depends on it.
Keep the uncropped original
A crop cannot be undone once it has been saved over the source. A separate copy lets you recrop later for a shape you did not plan for.
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