I have some photos from Canon camera. I opened them in "Preview" in MacBookPro, changed some of the contrast and colors, and saved as. I am sending these altered photos to people, but wondered, is it possible to determine that the photos have been altered?
Yes if you are saving the file or exporting it the EXIF will reflect that. EXIF changes are easy to detect if one is comparing a file to the original, but keep in mind this can typically be easily faked or manipulated if desired.
Open up a file before and after with ExifTool and you'll be able to tell exactly what the Preview app changes.
We have lots of info already on this site about EXIF, changing it, viewing it, and the validity of it.
Yes and No.
I just tried opening a jpeg in Preview, cropped it, messed with the colour, saved. There was nothing in the metadata to indicate Preview had done anything, but that's just because Preview didn't add those notes to my edits.
Lets assume editing software does add notes. If I run mogrify with the -strip option on the image file it will delete every non-critical bit of metadata in the file along with any previews, icons and revisions.
The end user might be suspicious about a completely data-free image, but they have no way to know why it's data-free.
It's also stone-simple to create your own metadata, so I wouldn't put any faith into what it says. Unless the files are in traceable possession, and preferably checksummed, any edits are easy to hide.
It's worth pointing out that you don't need metadata to know if a photo has been altered. For example, if you push the saturation and contrast very high you'll get an unnatural look that I doubt cameras can produce. Or, crop the photo to an odd aspect ratio or resize it to an uncommon size, and those are also going to be two indicators that the photo has been altered.