Is this a gramatically correct sentence?

"There's a trick you can do with the Nikon 50mm lens."

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I'm talking about a camera lens.

There's this trick you can do with the Nikon 50mm lens… Using "a" makes it sound too general

Yes, but you could improve by saying:
"There's a trick you can perform with the Nikon 50mm lens."

This is a trick you can do with your Nikon 50mm lens: (speech)

A trick you can do with the Nikon 50mm lens is: (writing)

There seems to be a grammatical error at the start of your sentence