Download the MP3 audio version of this story here, or sign up for The Explainer’s free daily podcast on iTunes.
Lawyers released digital photos of a Duke lacrosse team party to the media this week, in an attempt to discredit the exotic dancer who has accused several players of rape. Time-stamped images show the woman performing at the party and then smiling on the back porch at 12:30 a.m. Can you trust the time stamp on a digital image?
Nope. When you take a picture with a digital camera, the exact date and time of the shutter-release are recorded to your image file, along with many other bits of “metadata.” Every time you take a picture, your camera will also save information about your exposure time, f-stop setting, ISO, focal length, and so on. (It will even store a second time stamp for the moment the image file gets written to your memory card—which usually happens just a few moments after you take the picture.) But the time stamps are only as accurate as the clock in your camera; if you forgot to set it an hour ahead for daylight savings, your metadata would be an hour off. You can also modify any of the metadata with a simple computer command if you have the right software.
Digital cameras append metadata to an image file according to a standard format called EXIF, short for “Exchangeable Image File.” You won’t see the information when you load up the picture (and the date and time won’t appear in the corner of your image), but you can call up the details with a few clicks of the mouse. In Photoshop all you have to do is go to “File: File Info”; camera-specific image browsers make it just as easy.
Photoshop and many other software packages let you edit certain parts of the EXIF data. You can add a caption, for example, or copyright information if you’re a professional photographer. Other metadata—like the original time stamp, for example—is “read-only,” which means you can’t modify it with standard software. Even if you try to fiddle with the binary code using a hex editor, it may not be obvious how to change the date and time without corrupting the file.
That doesn’t mean it takes a serious computer hacker to adjust the numbers. Within a span of about 20 minutes, the Explainer was able to download a utility called ExifTool from the Web and edit the time stamps on all his photos: Each and every one now appears to have been taken at 12:30 a.m. on the night of the Duke lacrosse team party.
If a lawyer wants to use time-stamp metadata in court, he’ll be much better off if the police seized the digital camera that took the pictures. That way he can argue that the pictures on the camera couldn’t have been modified, and he can also demonstrate that the clock inside the camera wasn’t improperly set. He can also try to corroborate the time stamps with the images themselves: One of the photos in the Duke case purportedly shows a player’s wristwatch with the time matching that given in the metadata.
Got a question about today’s news? Ask the Explainer.
Explainer thanks George Reis of Imaging Forensics.