The Mangekyo Sharingan is arguably one of the strongest ocular techniques in the entire Naruto series, and its overarching nature has impacted much of the series plot. Despite making the wielder nearly invincible, Naruto thoroughly balances out the overpowered state of the Mangekyo by adding a crippling drawback – blindness. With each usage of this visual jutsu, the Mangekyo loses a fraction of its light, eventually leaving the wielder blind after continuous usage.
While a few Mangekyo users such as Itachi and Sasuke have been witnessed losing their eyesight and eventually going blind, a few others such as Obito and Kakashi didn't quite seem to undergo this debilitating side-effect. While Obito's case is unique due to half his body being imbued with Hashirama's cells, Kakashi should have experienced some sort of stress of losing his vision. However, Kakashi does state that his vision seems to be unclear during the Fourth Great Ninja War, yet it doesn't quite add up that he never experienced this side-effect before despite possessing the Mangekyo for nearly two decades.
Kakashi's Unique Mangekyo Sharingan
Being in possession of a Sharingan as a non-Uchiha is a rather unique case, as Kakashi's body was never meant to withstand the strenuous nature of this ocular jutsu. Since his Sharingan remained perpetually active, it drained a massive chunk of his chakra, and even a single use would leave him bedridden for days. Despite the arduous nature of using the Sharingan, and its advanced Mangekyo version, Kakashi still utilized this technique several times throughout the series.
While the base Sharingan has no effect on eyesight, the Mangekyo is bound to leave a lasting impact in some way, yet Kakashi had never stated that his eyesight was weakening, until the Great War arc at least. The only Mangekyo ability that Kakashi utilized was the Kamui, which is a low-impact ability compared to others such as Susanoo and Amaterasu. Still, Kakashi was seen using this ability multiple times throughout Shippuden, yet he never experienced any sort of blinding effect other than the extreme chakra drainage he experienced every time.
The first time he did undergo this crippling drawback was during his fight with Madara Uchiha, the series' end-game villain. This indicates that Kakashi had never experienced the side-effects before, which is rather odd considering he has multiple instances of Mangekyo usage under his belt. However, a few plausible explanations do account for this seemingly odd discrepancy with Kakashi's Mangekyo.
Kakashi's Mangekyo Usage Was Minimal
While Kakashi's overall Mangekyo usage was quite substantial, it still pales in comparison to the likes of Itachi, Sasuke, or Madara. The three Uchihas almost spammed their Mangekyo Sharingan, using abilities such as Sausanoo and Amaterasu constantly in almost every battle. Moreover, their vision disintegrated over a matter of years, especially in the case of Itachi who had his Mangekyo for quite a few years. Sasuke and Madara seemed to have lost their vision quite early on, though that could be attributed to their back-to-back fights.
Kakashi, on the other hand, only started using his Mangekyo in the beginning of Shippuden, with his very first usage against Deidera. He had to use his Mangekyo scarcely since he hadn't mastered it yet, and every single instance of usage resulted in him severely exhausted and debilitated. This inherent limitation led to Kakashi's overall Mangekyo usage being extremely limited. His Mangekyo's only mass usage can be witnessed during the Fourth Great Ninja War, which, consequently, was his first instance of experiencing blurry vision.
Kamui Is a Low-Impact Mangekyo Ability
The Mangekyo's abilities are all highly classified and equally destructive, yet their impact on wielder's vision seems to differ. It has been implied that the Susanoo has the highest impact on the Mangekyo's light out of all the abilities, a fact that was established after Sasuke experienced considerable pain while manifesting a full-armored Susanoo. On the other hand, Kamui seems to have the lowest impact on the Mangekyo, evident from its consistent usage on both Obito and Kakashi's part without any sort of excruciating pain or bleeding eyes.
Kakashi did seem to experience heavy strain after each usage of Kamui, though that could be attributed to his general lack of tolerance to Mangekyo strain. Even the base Sharingan would evoke a similar response in Kakashi, meaning it's only natural for him to undergo debilitating stress while using the Mangekyo's abilities. Despite the physical impact on Kakashi, he was still able to use the ability multiple times in succession during the war, meaning Kamui isn’t exactly a high-intensity ability and its mass usage won’t result in crippling blindness that easily. From these events, it can be inferred that the Kamui is indeed a low-impact ability, with minimal effect on Kakashi's vision.