There are a lot of complex questions about the many eye techniques of this series and how they all fit together/compare and I am interested to hear peoples thoughts on them and pose some questions of my own.
Lately I have been wondering about Obito's Mangekyou Sharingan and what makes it different from the Eternal Mangekyou and whether it is different from the normal Mangekyou. There are a few oddities about Obito's Mangekyou that I can't really figure out. As we have seen, Obito seems to use his Mangekyou without any apparent consequences to his eyes or health. Every character who has awakened the Eternal Mangekyou suffered severely from its over use. Itachi, Madara, and Sasuke all seem to suffer from blindness and both Itachi and Sasuke showed other physical damage from using the Mangekyou. Obito on the other hand has had his Mangekyou for well over 16 years and he has used it constantly but his eyesight remains completely unchanged and he can use it with no signs of fatigue. The only time we saw any physical limits of his Mangekyou was when Konan pointed out that he can only use it for 5 minutes straight.
We have been told, and shown on multiple occasions, that having Hashirama's DNA strengthens the sharingan. We saw it with Danzou and it was mentioned by Itachi as well. Yet Obito's sharingan seems more than just strengthened, he also doesn't appear to suffer any of the negative side effects of the normal Mangekyou. So far I don't really see any significant differences between the Eternal Mangekyou and the regular Mangekyou besides it having no apparent negative side effects. Even with the Eternal Mangekyou Sasuke hasn't really shown any addition abilities. So is Obito's Sharingan + Hashirama's DNA just as good as the Eternal Mangekyou? Also, if Obito were able to recover his other Eye from Kakashi would he be able to awaken Susanoo or would he have a different power? We have already seen that there are several different Mangekyou abilities but Itachi, Sasuke, and Madara all possess a form of Susanoo. So I am wondering if Susanoo is always the 3rd Mangekyou ability.
So far, the only major difference I can see between the Eternal Mangekyou and the Mangekyou+Hashirama cells, is that the Eternal Mangekyou seems to be a requirement to awaken the Rinnegan. Yet those advantages still seem to be irrelevant for Obito because he already has the Rinnegan.
Another weird thing is that Kakashi really doesn't seem to have suffered much of a loss of eye sight. He has used his Mangekyou quite a bit (especially for a non-Uchiha) and while he claimed to know about the side effects(implying he might have noticed his eyesight failing) that was a long time ago and since then he hasn't really displayed any noticeable impairments besides fatigue.
So what really is the difference between a regular Mangekyou+hashirama DNA and the Eternal Mangekyou? Kishimoto keeps making the eyes in this series more complicated but every time there are more questions that aren't answered.
Another thing I am wondering is, does having only one Rinnegan mean that the user can only use half of the Rinnegan's techniques? As we have seen with the Mangekyou, if the user only has one Mangekyou they can only use one Technique. Each eye has a different ability and having both eyes seems to allow the user to use Susanoo. Obito only has one Rinnegan and so far we have only seen him use a few Rinnegan techniques but is that because he only has one or is it merely because he doesn't choose to use them all. Madara has also only used a few of the Rinnegan's techniques but since he awakened the Rinnegan I always assumed that he could use them all . Nagato was the only one who used all the Path's but it seems like he had to learn them gradually, since he apparently didn't have most of them when he was a student of Jiraiya.
Hopefully we will get more clarification about the eye techniques soon but I am curious to hear theories that people might have on the subject.