A sufficiently-careful edit would be undetectable. Slightly adjusting the color value of a single pixel, for example, could probably never be identified as an alteration, because there’s no way to determine that it’s not simply how the original situation was actually photographed.
But in practice, it’s very, *very* difficult to make meaningful changes while perfectly accounting for all the changes necessary to make the new image completely consistent.
Borders can be too sharp, or fail to match up. The gradations of color that result from light and shadow can be inconsistent. One part of the image might no longer seem compatible with other parts — if one eye has a reflection of a light source and the other doesn’t, that’s not really plausible.
All these sorts of subtle incongruities can be sought out and used to make highly educated guesses. It’s not theoretically possible to be truly certain in all cases, but it’s often possible to identify that editing has taken place.
There are a lot of ways to do this! AI still isn’t perfect, and careful scrutiny will often reveal that the subtle details of an image don’t quite make sense, like shadows going in the wrong direction or patterns repeating in a strange way, or objects that are the right shape, but don’t actually seem to be functional. When analyzing a photograph, there are numerous techniques to determine if it has been altered in some way. For instance, if you composite two images together to, say, put someone’s head on another person’s body, you might be able to tell that the light casting shadows on the head isn’t coming from the same direction as the rest of the body, or that the person’s face is not actually at the same focus depth as other things in the image that seem to be at the same spot (imagine a photograph shot down a long street such that the cars far away are in focus, and then also, strangely, a person walking close to the camera is also in focus).
It depends what you mean by ‘edited’. Look at some iconic analogue (monochrome) photos taken by Ansel Adams, he would spend hours in the darkroom dodging and burning a print and produce amazing images.
Almost all images you see have been edited, either by cropping the image or by dodging (darkening part of the image) or burning (lightning part of the image). Photographers get into trouble when they move from enhancing the image (crop, dodge or burn) to image manipulation by adding/removing elements or moving parts of the image around.
Photo competitions rely on honesty and other photographers examining the images, there have been a number of quite famous photographers who have been caught manipulating images in photo competitions and disqualified.
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