Photoshop is an amazing tool for altering reality, but it's only really great when you're aware of its effects. Here are several tips for detecting a Photoshopped images and earning your digital forensics merit badge.
Using Your Eyes
When it comes to detecting anomalies in a photo, your eyes are really your best asset. Without practice, they'll fool you, but you can train yourself to start noticing the little imperfections and oddities that'll point right to a manipulated photograph. For solely educational purposes, I gave Mark Zuckerberg a new face to illustrate a few fake detection points. You can see both images above — the original is on the left and the altered version is on the right (if that isn't very obvious already).
I decided to give Zuckerberg the dreamy eyes of Zac Efron (which, I think, were already pretty Photoshopped to begin with). Notice how white they are. Movie-star looks or not, all eyes have veins and other imperfections in the sclera (the white part of the eye). I created some grey texture in the scleras to try to make them a little less fake-perfect, but nothing too excessive. Nonetheless, if you see really white eyes, you can assume someone's been Photoshopping the image.
Most faces are pretty dynamic, topographically speaking, and the eyes are a great example. Although eyes can sometimes look a little flat in a photo (thanks to good lighting and probably some help from Photoshop in post), you'll generally find a few areas in a photo that picked up shadows. If this is the case, try to determine if the shadows are realistic for the face. If you look at the right eye in both of the Zuckerberg images, you'll see that the shadow to the right of it has changed quite a bit. The original shadow did not work with the original eye, so I tried to correct it. Nonetheless, you can see that they eye isn't sinking in as much as the shadow. This could've been helped with a little warping and painting by hand, but there's only so much you can do. Poor perspective is always a good indicator of a fake.
The nose in this photo is probably the most unrealistic part of it, and that's not just because it belongs to Taylor Swift. Noses are particularly tough because they need to have both the texture and proper colour of the rest of the face. If a new nose is being transplanted or altered, it's a bit of a challenge to keep those things in place. In this particular nose, there are a few things to notice. One, despite my efforts to retexture the nose with Zuckerberg's freckled skin, you can still see Swift's smooth and freckle-free skin underneath it. It may not be obvious at first glance, but if you look at the nose closely, you'll start to see it. Part of this is due to the difference in lighting as well, as the transitions between topographical stages of the nose are much smoother in Swift's nose than they are in Zuckerberg's. Finally, take a look at the nostrils and notice how they're a bit red. This is where I was lazy with blending. Because it's skin, you're going to have red in the shadows, but shadows aren't going to be that red. More highly saturated and lighter red shadows in areas of the face can often point to lazy blending of a fake element.
I borrowed Jimmy Fallon's petite mouth to finish the face. The edges are key here. Most lips catch a little light at the bottom since they stick out a bit more. Notice how that seems to happen twice with these particular lips. You see it first in the reddish-pink part and again underneath the bottom of the lips. This is a poorly blended edge between Zuckerberg's face and Fallon's lips. It's not hugely noticeable from afar, but it's these rough edges that are some of the biggest indicators of a lazy fake.
When you're not dealing with human faces, edges and poor blending are still your best friend in detecting fakes. Photoshopping images really means combining a lot of layers and redrawing objects to look like they were really there. Unless you have objects shot in nearly the same lighting, it can be tough to blend them together realistically. Bad blending is usually easy to detect, but in all cases you'll find evidence around the edges. Look for lighting discrepancies and edges that just don't seem to make sense. If you're not sure about one object, see if you can find some inconsistencies in another. If the lighting doesn't match between two objects, those possibly fake edges most likely are fake edges.
Digital forensics expert Hany Fried points out how subtle these differences can be but how lighting plays such a big role in detecting fakes. Fried has posted lots of information on image analysis that's worth a read if you're looking to get a little more serious about detecting Photoshop forgeries.
Image analysis software can help you detect unrealistic characteristics of an image, but this software is generally pretty expensive. Instead, here are two tips for fake detection with a free online tool and Photoshop. (Yes, Photoshop is expensive too, but you probably already have it and you can use other image editing apps instead.)
Image Error Level Analysis is a tool that creates a heat map-like image out of a JPEG that shows you the highest points of compression. Excessively Photoshopped images may contain JPEG parts that have been saved over and over again. This little tool will, in theory, detect those parts and highlight them for you. Personally, I haven't had much luck, and it seemed to think my Zuckerberg makeover was generally less altered (as best as I can interpret the results), but your mileage may vary.
Using Photoshop, you have the ability to switch between different colour layers in your photo. The Zuckerberg face replacement is in RGB, so I was able to take a look at the red, green and blue layers of the photo. Switching is pretty easy. You can either do it by going into the Channels panel and doing it manually or via the keyboard using Control/Command + 2, 3 or 4. In the latest version of Photoshop (CS5), 2 is red, 3 is green and 4 is blue. Looking at the blue layer (depicted above), it's exceptionally obvious that the nose is fake. You can see a huge difference in lighting. This is harder to spot in the regular photo, but when you extra the blue layer it's much easier to see.
Alternatively, another way to catch this is boosting the saturation like crazy. Not only does this start to show areas of JPEG compression, but it'll accentuate the tonal and colour differences in the photo and make them easier to detect.
There are still tons of other ways to notice imperfections. If you've got any good tips on detecting a Photoshop fake, you know where to post them (hint: the comments).