“Il ne faut pas moins que” means precisely that, and would be said “Il faut au moins” in current French, which translates in English to “It requires at least“.

As it is related to metaphysic, let me answer with some rudimentary mathematics:

moins que (less than) : - _(minus)

pas moins que (not less than) : not -, so => _{(greater than or equal to)}

You can understand it as « Il faut au moins (au minimum) la Métaphysique la plus recherchée ».

It means that to answer this question, it will be necessary to make important research in metaphysics.

It’s a complex sentence; I will try to interpret (not translate) as best as I can.

It seems that the reader is astounded to find himself suddenly with the most difficult question that the eye can meet, which the answer requires nonetheless the most advanced metaphysical knowledge.

Translation in this case might confuse you.

A more accurate and native translation than those so far proposed would be:

It appears to me that the Reader is a bit surprised to find (himself/herself) all of a sudden into the most difficult question optics can bring, for which a solution requires nothing less than the most subtle Metaphysics.

It is certainly not yet perfect, since I am not native English (but rather native French), but it should capture the full meaning of the sentence in a way nearly digestible for an English native speaker.

The original:

Il me paroît que le Lecteur est un peu étonné de se trouver tout d’un coup dans la question la plus difficile de l’optique, pour la solution de laquelle il ne faut pas moins que la Métaphysique la plus recherchée.