One of the questions I am asked most often is: How can you spot a Shop? Usually, my baseline rule for an image is: If something looks ”not quite right” about a photo, that usually means there is something not quite right about the photo.
Digital forensics expert Hany Farid wrote a couple articles on this subject for the Scientific American which I highly recommend. He doesn’t get too technical, either – they are good reads even for the novice Shopper. The first was titled Digital Forensics: How Experts Uncover Doctored Images, and the second is a much more detailed supplement to that article titled Digital Forensics: 5 Ways to Spot a Fake Photo. The supplement article goes into greater detail and shows lots of visual examples of how lighting, reflections and shadowing play a part in uncovering Shops. These articles will reinforce the old adage: Don’t always believe what you see.
If you are a more advanced Photoshop user and are interested in the real nitty gritty of digital forensics, check out Hany Faird’s website. There you’ll find links to most of the research papers he has written on the topic of digital manipulation and image analysis.