Today we are constantly bombarded with images most of them are retouched in one way or other. Some times the retouch is really slight, brightens, contrast but with Photoshop it can reach to ridiculous heights. I'm not just talking about pictures of models, but journalism pictures that are retouched putting in question the trust of a media, and also everyday "funny" pictures that arrive to our mailboxes constantly. It's true that any digital camera have some self-retouching (white balance, brightness and JPEG compression), but programs like Photoshop or GIMP have become a everyday tools making almost impossible to avoid retouched images.
How do we recognize a retouched image?
First of all common sense. Most of the most amazing pictures today, including some from "serious" places are retouched. If something in the picture looks wrong or out of place there's a chance that it actually is.
If we wan to go a little further to reveal the authenticity of a picture there are some tools we can actually use.
TinEye is a reverse image search engine that let you look similar pictures to the one you provide. The pictures it shows are not only the same image found online but also includes modified images of the original and smaller, larger, and cropped versions of the image. In the results we can compare the image we have with other versions. If the picture we have is photoshopped based in an online picture there's a big chance that the original is there as well.
Take the above picture for example. The common sense says that something wrong with it. Tineye comes with this results. Is fairly obvious which one is the original one.
We can also check for the Exif information of an image. Exif data (Exchangeable image file format) is the metadata that a digital camera embeds in the image. This data talks about the camera and how the picture was taken. If the picture doesn't have this information there's a chance that it go trough some editing software (although this is not a rule). There's a lot of tools to get this information: This Exif Viewer its an online tools that comes up with a lot of information. Photome is a small windows application that lets you see and edit the metadata of a image.
Error level analysis (ELA)
The Error level analysis (ELA) its a method that re-saves the image to a known error rate, and then compare the difference. If the pixels in a image have been saves multiple times, for example when they are retouched, there won't be any difference. Images that have not been saved to many times will show a large amount of change.
This is a tool a little more professional and hard to understand but with a lot of possibilities. The are several examples really interesting to follow. Here's an analysis of a Victoria's Secret picture with a in deep study.
The same tool can also be used to check for false blurring and other kind of retouching.
Error Level Comparer is a small tool for windows that let us try this in our computer
Other forensics methods
There are other methods and tools to study images to see if there some kind of tampering. Some of the methods used for professionals are (source):
- Lighting sources: Lights and shadows in all the picture should be coherent with the setting. Shadows or bright sections in different positions can revealed a picture retouched or constructed from different pictures.
- Clone tool: One of the most used tools in Photoshop can be spotted easily if we look for cloned sections of an image. There are algorithms to do that.
- Light reflection in the pupils: All the pupils in the people of a image that wasn't retouched should have similar reflections showing were the lights were located. Different reflections reveals that the picture was constructed with different sources.
- Camera Fingerprints: To check if the image comes from the alleged camera that was used to take the picture.
There are several ways to reveal a retouched photograph, but probably the best rule is not to believe everything one see online. And some times we can also being surprised with images that look photoshopped but aren't.