Formal Equivalence and Dynamic equivalence caused heated controversy. The concept of equivalence has been one of the key words in translation studies. Equivalence can be said to be the central issue in translation although its definition, relevance, and applicability within the field of translation theory have caused heated controversy, and many different theories of the concept of equivalence have been elaborated within this field in the past fifty years. The comparison of texts in different languages inevitably involves a theory of equivalence.

Eugene Nida, in consultation with other pioneers in the field, developed the theory of "dynamic equivalence" or "functional equivalence," which stressed the importance of transferring meaning, not grammatical form. Nida discussed various kinds of complexity in meaning even at a comparatively early date, beginning with his 1947 publication of Bible Translating. He explicitly spoke about translating "fullest meaning" instead of a bare minimum.

In the book ‘The Theory and Practice of Translation’, Nida (2003:1) [1] indicates that translators were not able to convey the message of the Bible: “Unfortunately translators of religious materials have sometimes not been promoted by the same feeling of urgency to make sense.” Nida reveals the cover about the methods adopted in translating the Bible , the argument shows that there are two main focuses while translating the Bible ; “the older focus in translation was the form of the message ; translators were delighted to reproduce stylistic specialties , plays on words , parallelism, rhymes, rhythms , and new grammatical structures , while the new focus shifted from the form of the message to the response of the receptor. Therefore, what the translator must determine is the response of the receptor.” (ibid: 1).

There are problems, however, with dynamic equivalence translations. Since the translator is "freer" from the grammatical forms of the original language s/he is more likely to exceed the bounds of an accurate translation, in an effort to speak naturally in the native language. That is, the dynamic equivalence translations are capable of being more natural and more precise than are formal equivalence translations, but they are also more capable of being precisely wrong.

There are some scholars of translation who opposed the theory of Dynamic equivalence such as Eco (2001:5) [2] who argues against Equivalence in meaning; “then translation scholars should have had, at least once in their life, both the experience of translating and that of being translated.” Moreover he sees that ( ibid:9) “Equivalence in meaning cannot be taken as a satisfactory criterion for a correct translation, …… We cannot even accept the naïve idea that equivalence in meaning is provided by synonym, since it is commonly accepted that there are no complete synonym in language. Father is not a synonym for daddy, daddy is not a synonym for papa, and père is not a synonym for padre.”

Eco believes that those who have been involved in the art and craft of translation are definitely in a better position to formulate theoretical reflections on the subject. Moreover, it is perfectly clear to Eco that a successful translation cannot be anchored in the notion of word equivalences.

Eco (ibid:14) sees that “The translator does not translate a text on the basis of the dictionary, but rather “on the basis of the whole history of two literatures. Therefore translating is not only connected with linguistic competence, but with intertextual, psychological, and narrative competence. Thus, the translator is forced at all times to go beyond linguistic competence to the cultural spectrum. Consequently, translations do not constitute a comparison between two languages but the interpretation of two texts in two different languages.” In order for a translation to come to life, “a good translation must generate the same effect aimed at by the original.” Yet all translations are preceded by the interpretive perspective that the translator brings to the text, which means that the translator as interpreter must become visible in the translation.

However, all of his explanations and examples reconfirm his major conviction that the goal of all translations is “to produce in a different language the same effect as the source discourse, and poetic discourse is said to aim at producing an aesthetic effect.” (ibid:93) the discussion of equivalence shows the refined thinking that Eco brings to the analysis of all the other Practical aspects of translation presented in the section “Translating and Being Translated.” (Eco 2001-5)

Basically there are two competing theories of translation. In one, the predominant purpose is to express as exactly as possible the full force and meaning of every word and turn of phrase in the original, and in the other the predominant purpose is to produce a result that does not read like a translation at all, but rather moves in its new dress with the same ease as in its native rendering. In the hands of a good translator neither of these two approaches can ever be entirely ignored.

As a matter of fact, a perfect theory of translation should be an overall concern of all theories and should meet the functional requirements of an accepted and adequate translation theory, that provides some guidelines for translating to facilitate the task and transfer cultural elements in the source language to the target language, and thus achieve the same effect on the target receivers as on the original receivers.