A translation that meets the first criterion is said to be a "faithful translation"; a translation that meets the second criterion, an "idiomatic translation." The two qualities are not necessarily mutually exclusive.

The criteria used to judge the faithfulness of a translation vary according to the subject, the precision of the original contents, the type, function and use of the text, its literary qualities, its social or historical context, and so forth.

The criteria for judging the transparency of a translation would appear more straightforward: an unidiomatic translation "sounds wrong," and in the extreme case of word-for-word translations generated by many machine-translation systems, often results in patent nonsense with only a humorous value (see "round-trip translation").

Nevertheless, in certain contexts a translator may consciously strive to produce a literal translation. Literary translators and translators of religious or historic texts often adhere as closely as possible to the source text. In doing so, they often deliberately stretch the boundaries of the target language to produce an unidiomatic text. Similarly, a literary translator may wish to adopt words or expressions from the source language in order to provide "local color" in the translation.

In recent decades, prominent advocates of such "non-transparent" translation have included the French scholar Antoine Berman, who identified twelve deforming tendencies inherent in most prose translations, and the American theorist Lawrence Venuti, who has called upon translators to apply "foreignizing" translation strategies instead of domesticating ones.