How do you know if the picture has been PS?

This is a big problem. Identify whether the picture has been processed by PS (or other software post-processing, hereinafter referred to as PS), and subjectively see if your visual experience is rich enough. If you observe all kinds of real scenes and photos carefully enough, know some photography and optics, and know some human structures (or have real models as a reference), then you can almost instinctively see "a picture?" What should it be like? ",and then from the details of technical analysis to confirm.

Objectively speaking, the picture must be large enough and clear enough, and not over-compressed by the software. Otherwise, it is difficult to judge whether the "strange thing" in the picture is the skillful hand of PS craftsman or the ghost of compression algorithm.

There are many kinds of PS photos, among which changing the background, changing the avatar, adding or deleting elements in the picture, and excessively adjusting the color saturation of the picture are not allowed by the publishing house, because after these changes, the content expressed in the picture is usually far from the original picture.

There are several key points in observing the relationship between all the elements in the picture:

Is the direction, intensity and hardness of light consistent?

Whether hue, saturation and color scale are consistent.

Is clarity consistent with sharpness?

Whether the perspective effect is consistent

Whether the low-quality effect caused by poor equipment or shooting techniques is consistent (such as purple edge of lens and radial blur caused by hand shake)

Is the depth of field effect "correct" (how to explain the "incorrect" effect commonly seen in false off-axis false macro photos ...)