It used to be different than that, Unsure on when it changed it seems, or maybe the Wiki realized later that the calculation was different. It seems that it actually is a 50/50 split now for weapon kills.

Do note though, kills done by your companion reward you no affinity for your frame or weapons. You did not make the kill, so you do not learn from it nor was knowledge from the kill shared with you by a Tenno. Affinity from other player's kills however is shared to the companion and it's weapon.

So when you say 50/50, does that mean my frame gets 50% and the weapons I killed with gets the other 50% and my other two equipped weapons get 0%? Or does my frame get 50% and my weapons get the other 50% split three ways between them?

As was said on the Affinity page, which I'd recommend you go and read up for more info:

It seems it used to be a misconception that kills granted the exp in the 25/75 split, but kills specificly are instead 50/50 with the frame and the killing weapon. Your other weapons get nothing. Affinity gained from most other actions like completing objectives and ally kills is split in the 25/75 amount. So if you want to powerlevel a weapon and can manage it, do so by using the weapon exclusively with a team of other players, otherwise bring a complementary weapon along to help in a pinch. Missions like Spy offer quite a lot of bonus exp for mission objective completion, and stealth is a nice additive for exp bonus. Others like Interception offer 500 affinity for every time you cap the 4th tower, so you can keep letting the enemy take one tower to maximize exp gain.

There is still a lot of confusion on this topic despite the posts and info listed here when it comes to the other player getting kills, how it is done, and what you get from it.  From what I have seen it doesn't matter how the other player gets kills.  You still get the same afinity as if you got the kill, then you have that split 25% per item unless you remove items from your build? 

okay so I was here yesterday and it was not usefull since there is still some confusion left. So I tested it all in solo first last night and today with a friend for that last part which is tenno affinity

Okay let's start with kills from warframe since it is the fastest to say. The warframe gets 100% of the affinity gained

Kills with a weapon. This is split between the warframe and the weapon that was out. Both getting equal amounts (50/50)

Xp outside kills done by you (hacking, completing an objective, team mate killing an enemy, etc..) gives the warframe 25% of the affinity and the rest is divided among the weapons. If you have only one weapon then it will get the full 75%. if you have 2 weapons it will be 37.5% for both weapons. 25% for each weapon if you have 3 weapons equiped.

Credit: AegisAmaranth, for helping me test this and for clearing typo's (darn english xD)

You could've just read through Affinity#Acquisition instead of a two year old forum thread.

TBH that section might need at least a little work. Clearly there's something wrong with its presentation if it's still causing confusion. The formatting does appear to be screwed up in at least one place: the bullet point for Affinity gain from allies' kills seems to be misaligned. Unless I'm missing something myself, it has no relation to the point above it. The wording of the section might need some clarification as well, but I'm not an expert on Warframe's game mechanics so I can't say for sure.

So I don't know. Maybe read through it yourself to find where the confusion could have come from, or ask what made it unclear to someone before you make a snide remark at them for presenting the information they found out of their own desire to help others.

I know this is a huge necro, but seeign as this is the firt google link that pops up, I had to clarify this.

If you have 1 weapon equipped, you DO get more xp for that weapon. Not sure why he would say the ratio doesn't change. The frame gets 25% and the weapons get 75% across all 3, so if you have one equipped, that weapon will get the 75%; Seeing as though the XP has nowhere to go.

Technically having a sentinel with a weapon does affect your affinity gain because you don’t receive affinity from their kills. In turn, if they get a kill, there is less enemies for you to kill, which means less affinity to gain. I equip carrier prime for the amo recycle and vacuum, but I never put a weapon on it. That way it’s not stealing any kills.

No such thing as a "necro" in a wiki that's solely discussing the merits of how affinity is split and how to take advantage of it.

If you want all the 75% weapon affinity to go into 1 weapon, remove all other weapons. Therefore you will level THAT weapon 3x faster from ally kills which is how most people farm their affinity for weapons, in groups, in endless modes.

Weapon kills itself splits 50/50 with warframe. So use a weapon to kill things if you want to get a little more xp faster that isn't bonus XP from allies.