How ShutterOdo reads a shutter count

From a photo

Besides the standard EXIF fields, cameras write a private block called maker notes. Its layout is different for every brand and often every model, and Sony scrambles it. Nikon, Sony, Pentax, Fujifilm and some Canon bodies put the shutter count in there.

ShutterOdo reads maker notes with ExifTool 13.42, the reference metadata reader by Phil Harvey, compiled to WebAssembly. It runs inside the page. The photo is read from your own disk and is never sent anywhere. The reader is about 8 MB and downloads once, the first time you check a photo.

Editing apps, exports and messaging apps usually strip maker notes, which is why the original file from the card matters.

Over USB

Cameras speak PTP (Picture Transfer Protocol) to computers. Chrome and Edge can open a PTP session through WebUSB after you pick the camera in the browser's own permission prompt.

For Canon EOS bodies, ShutterOdo opens a session, reads the device info, switches on Canon's event reporting and waits for property 0xD1AC, the shutter counter Canon sends to tethering software. For Fujifilm it asks for property 0xD310. Then it closes the session. It doesn't change settings or touch the memory card. The method follows libgphoto2.

How accurate is it?

The number is the camera's own record, so it's as accurate as the camera. Three things to keep in mind:

• A count read from a photo can be edited by someone determined to do it. Ask a seller for a new photo of something you choose.
• Electronic shutter shots count differently by brand. Canon's R5 and R6 include them, some Nikon Z bodies split them out, and some Sony bodies skip silent shots.
• A replaced shutter starts again from 0.

To confirm a reading, take one more photo and check again. It should go up by 1.

Who makes ShutterOdo

ShutterOdo is built by Owlgo LLC, an independent software studio in the Boston area. It's free to use. If a camera doesn't work, copy the technical log from the checker and email it to support@owlgo.io.