The king, the ecclesiastics, and the lords levied regular tributes, each on the serfs of their own domains. I can prove this with respect to the king with the capitulary De Villis ; with respect to ecclesiastics by the law codes  of the barbarians ; and with respect to the lords by the statutes that Charlemagne issued on that subject. 
These tributes were called census ; they were economic and not fiscal duties, private fees solely and not public impositions.
I say that what was called census was a tribute levied on serfs. I can prove this with a formula of Marculfus, which includes a permission from the king to become a priest, provided one is freeborn and not listed on the register of the cens.  I can further prove it by a commission which Charlemagne gave to a count whom he sent into the Saxon regions  ; it includes the emancipation of the Saxons for having embraced Christianity, and it is properly a charter of free birth.  This prince restores to them their original civil liberty, and exempts them from paying the cens.  It was therefore the same thing to be a serf and to pay the cens ; to be free and not to pay it.
By a sort of letters-patent from the same prince in favor of the Spaniards who had been received into the monarchy,  the counts are prohibited from requiring of them any cens at all, and from taking their lands. We know that foreigners who arrived in France were treated as serfs ; and Charlemagne, wanting them to be regarded as free men, since he wanted them to own their own land, prohibited requiring the cens of them.
A capitulary of Charles the Bald issued in favor of the same Spaniards, would have them treated as other Franks were treated, and forbids requiring the cens of them  : free men therefore did not pay it.
Article 30 of the Edict of Pistres reforms the abuse by which some colonists of the king or the Church were selling the lands attached to their manors to ecclesiastics or to men of their condition, and reserving for themselves nothing but a little house, in such a way that the cens could no longer be collected on it ; and he orders that things be restored to their original state : the cens was therefore a tribute of slaves.
It further results from this that there was no general cens in the monarchy, and this is clear from a large number of texts. For what would this capitulary mean  : “It is our will that the royal cens be required in all places where it was once legitimately required” ?  What would be the meaning of the one  where Charlemagne orders his envoys in the provinces to make an exact inquiry into all the cens that had once existed in the king’s domain ?  And the one  where he disposes of the cens paid by those from whom they are being required ?  What meaning can we give to this other one  where we read : “If someone  has acquired a tributary estate on which we were accustomed to raising the cens” ? Or this one,  finally, where Charles the Bald speaks of censual lands, of which the cens had from all time belonged to the king ? 
Note that there are some texts which at first seem to refute what I have said, and which nonetheless confirm it. We have seen above that free men under the monarchy were obliged only to furnish certain conveyances ; the capitulary which I have just cited calls that census, and contrasts it to the cens which was paid by serfs. 
Moreover, the Edict of Pistres  speaks of those free men who had to pay the royal cens for themselves and for their houses,  and who had sold themselves during the famine. The king wants them to be redeemed. The reason is  that those who were freed by royal letters did not normally acquire a full and entire freedom,  but paid censum in capite, and that is the sort of persons who are spoken of here.
We must therefore discard the idea of a general and universal cens derived from the Romans’ administration, from which we suppose that the duties of lords have similarly derived by usurpations. What was called cens in the French monarchy, independently of the abuse that has been made of this word, was an individual duty levied on serfs by masters.
I beg the reader’s forgiveness for the mortal tedium which so many quotations must give him ; I would be more brief if I did not still find before me the book on The Establishment of the French Monarchy in the Gauls by the abbé Dubos. Nothing sets back the progress of knowledge more than a bad book by a well-known author, because before you can instruct, you must first disabuse.