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So, to make it normal for a human to have tetrachromacy, having four color receptors, would make our night vision nonexistent.

In an answer in the question linked above, someone brought up the tapetum lucidum, a layer of tissue behind the retina that reflects light, increasing the availability to photoreceptors, though at the risk of losing detail. This mirror alone explains why animals look creepy when we expose them to flashlights or night cameras.

So would night-vision improve if a human has both tetrachromacy AND a tapetum lucidum?

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    $\begingroup$ Could the downvoter give John a bit of feedback? By not doing so you're basically saying "your question was not worth my time but you don't deserve to know why" which is quite rude - regardless of if the vote itself is warranted or not. $\endgroup$ – Zxyrra Dec 30 '16 at 23:11
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    $\begingroup$ Yeah the eye is one of those organs that has been pushed to the point you can't improve one aspect without costing another. If you really want to improve the eye all around you need to go more basic, build the retina the right way around like it is in squid, insteads of inside out as in vertebrates, you would see an improvement in all parts of vision, it would even get rid of the blindspot. you get better day and night vision, and less risk of detached retina to boot. $\endgroup$ – John Jan 1 '17 at 16:10
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Yes, tapetum lucidum simply gives the photons a second chance to hit the receptors.

No, additional receptors for color would probably take place from rod cells, that are specialiced for night-vision at the cost of not seeing colors.

You have an equation between these two. Tetrachromacy is probably less worth for survival than night-vision, thus I would assume that the number of rod cells would not be compromised.

What do you mean by improve? Tapetum lucidum decreases the long distance sight, but makes it possible to see in darker situation. As an example here is how cats see: http://nickolaylamm.com/art-for-clients/what-do-cats-see/ You cannot really say that it is worse or better; it is different and better for some things while worse for some another.

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