Superman's long list of superpowers makes him one of the strongest heroes in comic book history. Super strength, flight and heat vision are just a few of his most noteworthy powers, but his X-Ray vision is regularly overlooked. Giving him the ability to see through wood, cloth, metal and plastic, it also introduces Superman's other weakness: lead.
This surprising limitation has led to several lead-coated traps and contraptions throughout DC Comics history, but it also means fans are likely to never actually stop and ask: just why can't Superman see through lead, anyway? The answer won't be what fans expect, but let's get to the answer all the same.
Needless to say, no superhero or supervillain is going to complain about Superman having another limitation or weakness. He may be vulnerable to Kryptonite, but beyond that rare mineral, he's one of the most powerful fictional characters ever created. X-Ray vision is just the cherry on top of an already overpowered sundae that includes freeze breath and heat vision! Kal-El's X-Ray vision actually predates that heat vision, as it was his "X-Rays" that he used to burn through metal and foes in his comic book beginnings. It wasn't until the 1960's that writers started to differentiate between the two powers.
Since then, heat vision has become one of Superman's foremost powers, while X-Ray vision is more of a joke. In fact, the heat vision was even upgraded when Geoff Johns created the "Super Flare" superpower in his late-New 52 Superman run in 2015. In the Men of Tomorrow arc, the Super Flare created a mushroom-cloud of chaos, demonstrating one of the most destructive uses of Superman's powers in comic book history.
So if Superman's heat vision can blast through lead, why can't his X-Ray vision see through it? Because actual X-Rays can't either! Superman might be able to see color, materials, and other fine details through his X-Ray vision, but actually X-Rays only discern the denseness of the material of what they're looking at. Lead is much too dense for X-Rays to breach, which is why patients around the world wear lead vests to protect themselves when getting their own X-Rays. Much like Kryptonite itself, this limitation was created to keep Superman from being all-powerful. If he were able to see through anything and determine any detail from any distance, how are any of his villains going to get the upper-hand every now and then?
While it might not be one of his go-to superpowers, Superman still puts his X-Rays to good use now and then. It was featured in Zack Snyder's Man of Steel and current Superman comic series writer Brian Michael Bendis featured the power in his Superman: Leviathon Rising #1 title released in March. In that issue, he was able to see basically every single detail of a battle when looking through the ceiling of a building. Not exactly what X-Rays are capable of, but comic books aren't exactly known for scientific accuracy.
Were Superman to go around using his X-Ray vision on anything and anyone around him, he'd be spreading around quite a lot of radiation. More than enough radiation to cause horrifying ecological and biological damage. So why isn't there a comic in which Superman's powers are accused of exposing people to radiation? Technically, Watchmen already covered that controversy. But mainly, because comic book physic don't have to play by the same rules as the real world if they don't wish to.. People fly, teleport, throw fire and magic--and yes, see through nearly every material in the world with their X-Ray vision. Every material except lead.