画像のサイズを変更するとメタデータが削除されますか? (驚くべき真実)

I tested this last week. Snapped a photo on my phone, resized it to 200 pixels wide, then ran both copies through an EXIF viewer. The tiny thumbnail still carried my GPS coordinates, phone model, and the exact second I pressed the shutter.

Does resizing an image remove its metadata? Almost never. A few tools strip EXIF data by accident, but most of them keep it — and they keep it by design.

Here’s why. When you resize a photo, you’re tossing out pixels. The metadata lives in a separate chunk of the file called the EXIF header. Cutting the image down to 100×100 pixels doesn’t touch that header. It’s like redacting a document by making the font smaller — the sensitive parts are still there, just harder to notice.

What happens depends entirely on which tool you used.

Photoshop and GIMP give you a choice, but it’s easy to miss. “Save As” preserves metadata. “Export for Web” has a checkbox — forget to uncheck it and your location data ships with the thumbnail. I’ve done it.

Your phone is the worst offender. iOS and Android both keep photo metadata when you crop or resize in the built-in Photos app. Apple assumes you want to preserve your memories; privacy isn’t the default. Every resized version keeps the original GPS tag, timestamp, and device ID.

Online image resizers are a gamble. Some strip metadata to save server space. Others pass the EXIF header straight through. You won’t know unless you check afterward — and most people never do.

The JPEG format makes this clear. Metadata sits in what’s called an APP1 segment, which is separate from the image data. Resizing algorithms only touch the image pixels. Unless the software rebuilds the whole file without the APP1 block, your EXIF data stays intact. Even if you shrink the image to a single pixel.

So how do you actually remove photo metadata?

If you’re comfortable with command line: ExifTool, by Phil Harvey, strips everything in one command. 無料, open source, handles every format.

For everyone else: exifaudit.com. ファイルをアップロードします, it shows every metadata field — GPS coordinates, camera serial number, software version, embedded thumbnail — and then wipes them all. The audit-first approach matters. Seeing what your files leak, before you scrub them, teaches you what to watch for. I’ve found location data in screenshots I thought were clean.

Check before you upload. Craigslist doesn’t strip EXIF data. Most forums don’t. Facebook claims they do but I wouldn’t count on it. Three seconds of verification is better than six months of wondering who has your address.