What is the difference between a camera dictionary and Google Lens?
Google Lens is a general visual tool. Pointed at a page it will recognise and translate a block of text, which is excellent for a menu, a sign or a paragraph you cannot read at all. It gives you a translation of the passage rather than a definition of a word.
A camera dictionary is narrower on purpose. It identifies a single word and returns its dictionary entry — sense, part of speech, sometimes etymology — in the same language or in yours. If you can already read the sentence and are stuck on one word, that is what you want.
The other differences are practical. Lens needs a connection for most of what it does; some camera dictionaries, Booktionary among them, keep the dictionary on the device and work with no signal. And a translation of a whole region will pull your eye away from the line you were reading, which is the cost the tool was supposed to remove.
Neither replaces the other. For reading a novel in a language you mostly know, the narrow tool wins.