For those who worry about the 1:2 magnification limitation, add Kenko or Canon extension tubes and you´re OK to go a lot closer.
Then you loose a lot of light. Why do you buy a makro anyway when you can get the same effect with extension tubes? And if it's so easy and without backdraws, then please write Zeiss to get a longer back between bayonett and lense, so the extension is already attached and they can claim real 1:1.
You could work for marketing.
Hehe, Yes, I can work for marketing and occasionally I do
But in this case I think I can play the role of the engineer.
In order for a camera lens to focus progressively closer, which is rather essential for a macro lens, the lens apparatus has to move further from the camera's sensor (called "extension"). For low magnifications, the extension is tiny, so the lens is always at the expected distance of roughly one focal length away from the sensor. However, once one approaches 0.25-0.5X or greater magnifications, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor.
So in principle you can say that an extension tube increases your effective f-stop with loss of light as a consequence. But that is because of the increased magnification, not the extension tube per se.
There are lots of stuff on the internet on the subject.